{"id":639,"date":"2025-08-07T12:42:00","date_gmt":"2025-08-07T19:42:00","guid":{"rendered":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/?page_id=639"},"modified":"2026-03-10T10:08:01","modified_gmt":"2026-03-10T17:08:01","slug":"chapter-3-soil-science","status":"publish","type":"page","link":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/chapter-3-soil-science\/","title":{"rendered":"Chapter 3: Soil Science"},"content":{"rendered":"<div class=\"wsu-hero wsu-width--full wsu-pattern--wsu-light-radial-left  wsu-hero--style-boxed \">\n\t<div class=\"wsu-hero__background\">\n\t\t<div class=\"wsu-image-frame wsu-image-frame--fill\">\n\t<img decoding=\"async\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/08\/AdobeStock_180233377-scaled.jpg\"\n\t\tsrcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/08\/AdobeStock_180233377-scaled.jpg 2560w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/08\/AdobeStock_180233377-scaled.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/08\/AdobeStock_180233377-scaled.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/08\/AdobeStock_180233377-scaled.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/08\/AdobeStock_180233377-scaled.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/08\/AdobeStock_180233377-scaled.jpg 2048w\"\n\t\tsizes=\"(max-width: 2560px) 100vw, 2560px\"\n\t\talt=\"Gardener digging in a garden with a spade. Man using a big shovel for digging old lawn.\"\n\t\tstyle=\"object-position: 62% 58%\"\n\t\t\/>\n<\/div>\n\t<\/div>\n\t<div class=\"wsu-hero__overlay\">\n\t<\/div>\n\t<div class=\"wsu-hero__content-wrapper\">\n\t\t<div class=\"wsu-hero__inner-content-wrapper\">\n\t\t\t\t\t\t<div class=\"wsu-hero__title-wrapper\">\n\t\t\t\t<h1 class=\"wsu-hero__title\">Soil Science<\/h1>\t\t\t\t<div class=\"wsu-hero__caption\">Chapter 3<\/div>\t\t\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<div class=\"wsu-hero__content\">\n\t\t\t<\/div>\n\t\t<\/div>\n\t<\/div>\n<\/div>\n\n\n\n<p class=\" wsu-max-width--hero\"><strong>Craig Cogger<\/strong>, Soil Scientist Emeritus, Department of Crop and Soil Sciences, Washington State University<\/p>\n\n\n\n<div style=\"height:10px\" aria-hidden=\"true\" class=\"wp-block-spacer\"><\/div>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\" \/>\n\n\n<div class=\"wsu-row wsu-row--sidebar-right\" >\r\n    \n<div class=\"wsu-column\"  style=\"\">\r\n\t\n\n<h2 class=\"wp-block-heading wsu-font-size--xlarge wsu-heading--style-marked wsu-spacing-after--xxmedium\" id=\"learning-objectives\">Learning Objectives<\/h2>\n\n\n\n<ul>\n<li>Know the physical and biological properties of soil and <br>how they relate to plant growth and development.<\/li>\n<\/ul>\n\n<\/div>\r\n\n\n<div class=\"wsu-column\"  style=\"\">\r\n\t\n\n<h2 class=\"wp-block-heading\">Topics Covered<\/h2>\n\n\n\n<ul class=\"wsu-menu--style-sidebar\">\n<li><a href=\"#ch3-soil-components\" data-type=\"internal\" data-id=\"#ch3-soil-components\">Soil Components and Soil Horizons<\/a><\/li>\n\n\n\n<li><a href=\"#ch3-soil-and-water\" data-type=\"internal\" data-id=\"#ch3-soil-and-water\">Soil and Water<\/a><\/li>\n\n\n\n<li><a href=\"#ch3-soil-life\" data-type=\"internal\" data-id=\"#ch3-soil-life\">Soil Life<\/a><\/li>\n\n\n\n<li><a href=\"#ch3-soil-nutrients\" data-type=\"internal\" data-id=\"#ch3-soil-nutrients\">Soil Nutrients<\/a><\/li>\n\n\n\n<li><a href=\"#ch3-soil-pH\" data-type=\"internal\" data-id=\"#ch3-soil-pH\">Soil pH<\/a><\/li>\n\n\n\n<li><a href=\"#ch3-soil-salinity\" data-type=\"internal\" data-id=\"#ch3-soil-salinity\">Soil Salinity<\/a><\/li>\n\n\n\n<li><a href=\"#ch3-soil-tests\" data-type=\"internal\" data-id=\"#ch3-soil-tests\">Soil Tests<\/a><\/li>\n\n\n\n<li><a href=\"#ch3-organic-amendments\" data-type=\"internal\" data-id=\"#ch3-organic-amendments\">Organic Amendments<\/a><\/li>\n\n\n\n<li><a href=\"#ch3-further-reading\" data-type=\"internal\" data-id=\"#ch3-further-reading\">Further Reading<\/a><\/li>\n<\/ul>\n\n<\/div>\r\n\n<\/div>\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\" \/>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-soil-components\">Soil Components and Soil Horizons<\/h2>\n\n\n\n<p><a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Soil<\/strong> (opens in new window)<\/a> is a mixture of weathered rock fragments and organic matter at the earth\u2019s surface. It is biologically active\u2014a home to countless microorganisms, invertebrates, and plant roots. It varies in depth from a few inches to five feet or more. <a rel=\"noreferrer noopener\" href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#n\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#n\" target=\"_blank\"><strong>Native soil <\/strong>(opens in new window)<\/a> is roughly 50 percent pore space. This space forms a complex network of pores of varying sizes, much like those in a sponge. Soil provides nutrients, water, and physical support for plants as well as oxygen for plant roots. Soil organisms are nature\u2019s primary recyclers, turning dead cells and tissue into nutrients, energy, carbon dioxide, and water to fuel new life.<\/p>\n\n\n\n<p>Native soils consist of several layers or <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>soil horizons<\/strong> (opens in new window)<\/a> that were formed by natural geologic and weathering processes. Together, the horizons compose the <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>soil profile <\/strong>(opens in new window)<\/a> (Figure 1). The upper layer is the A horizon, or topsoil. The A horizon is the most biologically active zone, with the greatest abundance of plant roots and organisms. It contains the largest amount of available nutrients and the most organic matter. The organic matter gives the A horizon a dark color. The A horizon typically ranges in depth from a few inches to a foot.<\/p>\n\n\n<div class=\"wp-block-image wsu-spacing-margin-left--large\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-01-688x1024.jpg\" alt=\"A gouge in a field reveals multiple soil layers. A pick axe can be seen toward the bottom of the hole.\" class=\"wp-image-1854\" width=\"516\" height=\"768\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-01-688x1024.jpg 688w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-01-202x300.jpg 202w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-01-768x1143.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-01-1032x1536.jpg 1032w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-01-1376x2048.jpg 1376w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-01-scaled.jpg 1720w\" sizes=\"(max-width: 516px) 100vw, 516px\" \/><figcaption class=\"wp-element-caption\">Figure 1. Soil formed from glacial materials, Mason County, Washington. The dark A horizon contains the most organic matter, and the two B horizons differ from each other by color. Photo by Craig Cogger, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>In most Washington soils, the A horizon is underlain by one or more B horizons (subsoil). The B horizons contain less organic matter than the A horizon and are lighter in color. Their texture can be coarser, finer, or similar to the topsoil. Although subsoils are less biologically active than the A horizon, they are still an important source of water and nutrients for plants. Well-drained subsoils are typically brown or reddish colored, while wet subsoils are usually gray and often flecked with brighter-colored mottles, indicative of poor drainage.<\/p>\n\n\n\n<p>Beneath the B horizon is a relatively unweathered <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#p\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#p\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>parent material<\/strong> (opens in new window)<\/a> or C horizon. The C horizon in most soils contains few roots and has low levels of biological activity and little or no structure.<\/p>\n\n\n\n<p>Soils vary widely across the state (and around the world), based on the parent material (geologic deposits) and the environment where the soil was formed. The environmental factors include topography, climate, and organisms (ecosystem) acting over time.<\/p>\n\n\n\n<p>Many soils in urban and suburban areas have been disturbed by land development or construction and no longer have a naturally developed soil profile. For example, horizons may be mixed by excavation, or the A horizon may have been removed by cutting or buried by filling. Such disturbance often degrades the soil as an environment for plant growth. See Chapter 5: Urban Soil Management for information on identifying and managing disturbed urban soils.<\/p>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-soil-and-water\">Soil and Water<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Soil Pores, Permeability, and Water Supply<\/em><\/h3>\n\n\n\n<p>A productive soil is permeable to water and is able to supply water to plants. A soil\u2019s <a rel=\"noreferrer noopener\" href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#p\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#p\" target=\"_blank\"><strong>permeability<\/strong> (opens in new window)<\/a> and <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#w\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#w\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>water-holding capacity <\/strong>(opens in new window)<\/a> depend on its network of pores.<\/p>\n\n\n\n<p>Large pores (<a rel=\"noreferrer noopener\" href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#m\" target=\"_blank\"><strong>macropores<\/strong> (opens in new window)<\/a>) control a soil\u2019s permeability and <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#a\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#a\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>aeration<\/strong> (opens in new window)<\/a>. Macropores include earthworm and root channels. Because these pores are large, water moves through them rapidly by means of gravity. Thus, rainfall and irrigation infiltrate the soil, and excess water drains through it.<\/p>\n\n\n\n<p>Small or fine pores (<a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#m\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#m\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>micropores<\/strong> (opens in new window)<\/a>) are typically a fraction of a millimeter in diameter. They are responsible for a soil\u2019s water-holding capacity. Like the fine pores in a sponge, micropores hold water (acting against the force of gravity). Much of the water held in micropores is available to plants; however, some water is held so tightly that plant roots cannot extract it.<\/p>\n\n\n\n<p>Soil that has a good balance of macropores and micropores provides adequate permeability and water-holding capacity for good plant growth. Soils that contain mostly macropores drain readily but are often droughty and need more frequent irrigation. Soils that contain mostly micropores have good water-holding capacity but take longer to dry out and warm up in the spring. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#i\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#i\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Infiltration<\/strong> (opens in new window)<\/a> of water during rainfall and irrigation tends to be slower, making runoff more likely from these soils.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Factors That Affect Soil Porosity<\/em><\/h3>\n\n\n\n<p>Soil properties that affect porosity include <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>soil texture<\/strong> (opens in new window)<\/a>, <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>soil structure<\/strong> (opens in new window)<\/a>, compaction, and organic matter. Gardeners can evaluate these properties to understand how well their soil moves, holds, and releases water and what steps they can take to improve the soil.<\/p>\n\n\n\n<h4 class=\"wp-block-heading wsu-spacing-after--small\">Soil Texture<\/h4>\n\n\n\n<p>Texture describes how coarse or fine a soil is. The coarsest soil particles are <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>sand <\/strong>(opens in new window)<\/a>. They are visible to the naked eye and give soil a gritty feel. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Silt <\/strong>(opens in new window)<\/a> particles are smaller than sand\u2014about the size of individual particles of white flour. They give soil a smooth, flour-like feel. On close inspection, sand and silt particles look like tiny rocks. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Clay <\/strong>(opens in new window)<\/a> particles are the smallest\u2014about the size of bacteria and viruses\u2014and can be seen only with a microscope. They typically have a flat shape, similar to a sheet of mica. Soils rich in clay feel very hard when dry, but they are easily shaped and molded when moist. Although all of these particles seem small, the relative difference in their sizes is quite large. Imagine that if a typical clay particle were the size of a penny, then a sand particle would be as large as a house.<\/p>\n\n\n\n<p>Soil texture directly affects porosity. Pores between sand particles tend to be large, while those between silt and clay particles tend to be small. Thus, sandy soils contain mostly macropores and usually have rapid permeability but limited water-holding capacity. Micropores predominate in soils containing mostly silt and clay, creating high water-holding capacity but slow permeability. Particle size also affects the surface area in a volume of soil. Surface area is important because soil surfaces hold plant nutrients, bind contaminants, and provide a space for microorganisms. Clay particles have a large surface area relative to their volume, and a small amount of clay makes a large contribution to a soil\u2019s surface area.<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-02-1024x891.jpg\" alt=\"A triangular soil texture diagram showing the classification of soils based on the relative percentages of sand, silt, and clay. Each side of the triangle is labeled with a percentage scale from 0 to 100: the left axis shows percent clay increasing upward, the base shows percent sand increasing left to right, and the right axis shows percent silt increasing downward from top to bottom. The interior of the triangle is divided into labeled regions, each corresponding to a soil texture class. The bottom left corner (high sand, low silt and clay) is labeled \u201cSand.\u201d Slightly above and to the right are \u201cLoamy sand\u201d and \u201cSandy loam.\u201d The central region is labeled \u201cLoam.\u201d To the upper middle-left are \u201cSandy clay loam\u201d and \u201cSandy clay.\u201d Moving upward, the top center is \u201cClay.\u201d Toward the right side, moving from bottom to top, are \u201cSilt,\u201d \u201cSilt loam,\u201d \u201cSilty clay loam,\u201d and \u201cSilty clay.\u201d At the center-right is \u201cClay loam.\u201d Shaded regions within the triangle visually separate the categories, helping indicate where specific soil mixtures fall depending on their proportions of sand, silt, and clay. For example, a soil with 20% clay, 40% silt, and 40% sand would fall into the \u201cLoam\u201d category.\" class=\"wp-image-1856\" width=\"512\" height=\"446\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-02-1024x891.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-02-300x261.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-02-768x668.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-02-1536x1337.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-02.jpg 1952w\" sizes=\"(max-width: 512px) 100vw, 512px\" \/><figcaption class=\"wp-element-caption\">Figure 2. Soil textural triangle showing soil texture classes and percentages of sand, silt, and clay. <em>Source<\/em>: U.S. Department of Agriculture.<\/figcaption><\/figure><\/div>\n\n\n<p>Nearly all soils have a mixture of particle sizes (Figure 2). A soil with roughly equal influence from sand, silt, and clay particles is called a <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#l\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#l\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>loam <\/strong>(opens in new window)<\/a>. Loams usually make good agricultural and garden soils because they have a good balance of macropores and micropores, meaning they usually have good water-holding capacity and moderate permeability.<\/p>\n\n\n\n<p>A sandy loam is similar to a loam, except that it contains more sand. It feels gritty yet has enough silt and clay to hold together in your hand. Sandy loams usually have low to moderate water-holding capacity and good permeability. Silt loams have more silt and feel smooth rather than gritty. They are pliable when moist but not very sticky. Silt loams usually have high water-holding capacity and low to moderate permeability.<\/p>\n\n\n\n<p>Clays and clay loams are very hard when dry, sticky when wet, and can be molded into wire and ribbon shapes when moist (Figure 3). They generally have high water-holding capacity and low permeability. To learn how to estimate the texture of soil, view the WSU Extension video <em>Determining Soil Texture by Hand<\/em>, which can be accessed in the Further Reading section.<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-03-1024x768.jpg\" alt=\"Someone presses on soil in their hand, creating a ribbon from it.\" class=\"wp-image-1857\" width=\"512\" height=\"384\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-03-1024x768.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-03-300x225.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-03-768x576.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-03-1536x1152.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-03-2048x1536.jpg 2048w\" sizes=\"(max-width: 512px) 100vw, 512px\" \/><figcaption class=\"wp-element-caption\">Figure 3. Estimating soil texture by hand (pictured sample has a high percentage of clay-sized particles). Photo by Andy Bary, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>Soil texture is a fixed property and cannot be changed through management. But soil of almost any texture can be suitable for gardening, as long as gardeners are aware of the soil\u2019s limitations and adjust their management practices to compensate for them. Clay soils hold a lot of water but are hard to dig and dry slowly in the spring. Sandy soils need more frequent watering and lighter, more frequent fertilization, but they can be planted earlier in the spring. Most soils can benefit from additions of organic matter, as described in the section Organic Amendments.<\/p>\n\n\n\n<p>Many soils contain coarse fragments, like gravel and rocks. Coarse fragments do not contribute to a soil\u2019s productivity and can be a nuisance when digging in the soil. However, gardeners should not feel compelled to remove all of them from their garden since coarse fragments are not harmful, and time is better spent on other gardening tasks. The only time rocks are a problem is when the percentage of rocks is so high that it impedes garden management and plant growth. When rocks make up most of the soil profile, water- and nutrient-holding capacities can be so low that it is difficult to grow healthy plants. Raised beds are an alternative strategy for gardening in very rocky soils. For information on raised beds, refer to WSU Extension publication <em>Raised Beds: Deciding If They Benefit Your Vegetable Garden<\/em>, which can be accessed in the Further Reading section.<\/p>\n\n\n\n<h4 class=\"wp-block-heading wsu-spacing-after--small\">Soil Structure<\/h4>\n\n\n<div class=\"wp-block-image wsu-spacing-margin-left--large\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-04-1024x768.jpg\" alt=\"A hand holding granular peds on the left and blocky peds typical of subsoil on the right.\" class=\"wp-image-1861\" width=\"512\" height=\"384\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-04-1024x768.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-04-300x225.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-04-768x576.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-04-1536x1152.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-04.jpg 2048w\" sizes=\"(max-width: 512px) 100vw, 512px\" \/><figcaption class=\"wp-element-caption\">Figure 4. Granular peds typical of topsoil are shown on the left; blocky peds typical of subsoil are shown on the right. Photo by Andy Bary, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>Individual particles of sand, silt, and clay bind together with organic matter, forming aggregates called <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#p\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#p\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>peds<\/strong> (opens in new window)<\/a>, which provide structure to a soil. Dig up a piece of grass sod and examine the soil around the roots. The granules of soil clinging to the roots are examples of peds (Figure 4). <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#a\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Aggregation<\/strong> (opens in new window)<\/a> is a natural process caused largely by biological activity, such as earthworm burrowing, root growth, and microbial production of natural glues that strengthen the peds.<\/p>\n\n\n\n<p>The spaces <em>between<\/em> peds are the soil\u2019s macropores, which improve permeability, drainage, and oxygen levels in the soil profile. The pores <em>within<\/em> peds are predominantly micropores, which contribute to the soil\u2019s water-holding capacity. A well-structured soil is like a sponge, allowing water to enter and soak into the micropores and letting excess water drain downward through the macropores. Good structure is especially important in medium- to fine-textured soils because it increases the soil\u2019s macroporosity, thus improving permeability and drainage.<\/p>\n\n\n\n<h4 class=\"wp-block-heading wsu-spacing-after--small\">Compaction and Loss of Structure<\/h4>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"alignright size-full is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-05.jpg\" alt=\"Heavy machinery tread in compacted soil.\" class=\"wp-image-1864\" width=\"513\" height=\"331\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-05.jpg 805w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-05-300x194.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-05-768x496.jpg 768w\" sizes=\"(max-width: 513px) 100vw, 513px\" \/><figcaption class=\"wp-element-caption\">Figure 5. Soil compaction harms the rooting environment. Photo by Craig Cogger, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>Soil structure is fragile and can be damaged or destroyed by compaction, excessive tillage, or tillage when the soil is too wet. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Compaction <\/strong>(opens in new window)<\/a> squeezes macropores into micropores and creates horizontal aggregates that resist root penetration and water movement (Figure 5).<\/p>\n\n\n\n<p>Soil compaction occurs during land clearing, site preparation, construction, and utility work, creating difficult conditions for establishing plants. Cutting (removing topsoil), levelling, and filling of soils to shape sites for construction also degrade the rooting environment. The removal or burial of topsoil leaves a surface with little structure or organic matter. Cuts also reduce soil depth, while fills create layers that can restrict water movement. These problems are common in urban and suburban landscapes.<\/p>\n\n\n\n<p>Damage to soil structure caused by human activity usually is most severe within the top foot of soil and can be reduced by proper soil management. Adding <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#o\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#o\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>organic amendments <\/strong>(opens in new window)<\/a> at the time of bed preparation, using organic surface mulches and <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>cover crops <\/strong>(opens in new window)<\/a>, and avoiding excess tillage help restore soil structure. For more information, see the section titled Organic Amendments in this chapter.<\/p>\n\n\n\n<p>In some soils, there is deeper compaction caused by the tremendous weight of ancient glaciers. Glacially compacted subsoils (a type of hardpan) are common in the Puget Sound area and in parts of north central Washington, where the compacted layer often begins 18 to 36 inches below the soil surface (Figure 6). The compacted layer may be much closer to the surface where the land surface has been shaped for development. This layer looks like concrete and is so dense and thick that it is nearly impossible to work with. If a garden has a glacially compacted layer close to the soil surface, consider building raised beds to increase soil depth.<\/p>\n\n\n<div class=\"wp-block-image wsu-spacing-after--large\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-06-682x1024.jpg\" alt=\"Various soil layers in cross-section. A narrow shovel is propped against the cutout.\" class=\"wp-image-1865\" width=\"443\" height=\"665\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-06-682x1024.jpg 682w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-06-200x300.jpg 200w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-06-768x1153.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-06-1023x1536.jpg 1023w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-06-1364x2048.jpg 1364w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-06-scaled.jpg 1705w\" sizes=\"(max-width: 443px) 100vw, 443px\" \/><figcaption class=\"wp-element-caption\">Figure 6. Glacially compacted C horizon (gray color) that limits roots and water movement. Pierce County, Washington. Photo by Craig Cogger, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>Tilling a garden bed when soil is too wet also damages soil structure. If a handful of soil can be molded into a wire or worm shape, it is too wet to till. If the soil crumbles when you try to mold it, it is dry enough to till.<\/p>\n\n\n\n<h4 class=\"wp-block-heading wsu-spacing-after--small\">Organic Matter<\/h4>\n\n\n\n<p>Soil organic matter is a mixture of biological materials in varying states of <a rel=\"noreferrer noopener\" href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#d\" target=\"_blank\"><strong>decomposition<\/strong> (opens in new window)<\/a>. <a rel=\"noreferrer noopener\" href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#f\" target=\"_blank\"><strong>Fresh organic matter <\/strong>(opens in new window)<\/a> (such as crop residues, leaves, roots, and manure) is decomposed by soil organisms, releasing nutrients, carbon dioxide, and energy, and is eventually transformed into more stable materials. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Stabilized organic matter<\/strong> (opens in new window)<\/a> consists of complex molecules that decompose slowly, and it contributes to soil structure, water-holding capacity, and slow release of nutrients.<\/p>\n\n\n\n<p>Adding organic amendments is the best way to improve the environment for most types of plants in the wide range of soils found across Washington State. Organic matter helps build and stabilize structure in fine-textured and compacted soils, thus improving permeability and aeration while reducing the risk of runoff and erosion. As microbes break down and stabilize fresh organic matter, they create natural glues that bind and strengthen soil aggregates. Organic matter also helps sandy soils hold water and nutrients.<\/p>\n\n\n\n<p>Although organic matter improves soil physical properties and supplies nutrients, excessive application of organic matter can harm crops and increase the risk of environmental harm through nutrient loss. For more information, refer to the section Nutrient Excesses in this chapter and to OSU Extension publication <em>Nutrient Management for Sustainable Vegetable Production Systems in Western Oregon<\/em>, which can be accessed in the Further Reading section.<\/p>\n\n\n\n<h4 class=\"wp-block-heading wsu-spacing-after--small\">Slope, Aspect, Depth, and Water<\/h4>\n\n\n\n<p>Slope, <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#a\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>aspect <\/strong>(opens in new window)<\/a> (direction of exposure), and soil depth affect water availability. Choose plants that are best suited to a property\u2019s specific conditions. Ridgetops and side slopes shed water, allowing it to run off, while soils at the bottom of slopes and in low areas collect water (Figure 7). Often, soils that collect water have high winter water tables, which can affect the health of some plants. Soils on ridgetops are more likely to be droughty. Site aspect is also important. South- and southwest-facing exposures collect the most heat and increase water demand by plants. For more information on plant selection see Chapter 25: Waterwise Landscaping.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large wsu-spacing-after--large wsu-spacing-before--large\"><img decoding=\"async\" loading=\"lazy\" width=\"1024\" height=\"768\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-07-1024x768.jpg\" alt=\"Grassy slope descending into broad, green valley.\" class=\"wp-image-1866\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-07-1024x768.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-07-300x225.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-07-768x576.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-07-1536x1152.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-07-2048x1536.jpg 2048w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><figcaption class=\"wp-element-caption\">Figure 7. Landscape position affects soil wetness, with drier soils on slopes and wetter soils in low areas. Photo by Craig Cogger, WSU.<\/figcaption><\/figure>\n\n\n\n<p>Soil depth affects water availability by defining the rooting zone. Soil depth is limited by compacted, cemented, or gravelly layers, or by bedrock. Examples include the glacial compaction, described above, and <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>caliche<\/strong> (opens in new window)<\/a> (a cemented layer found in some soils in arid parts of central Washington). A shallow soil has less available water simply because the soil volume available to roots is smaller. The deeper one can dig before hitting a restrictive layer, the greater the soil volume for holding water.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Water Management in Your Garden<\/em><\/h3>\n\n\n\n<h4 class=\"wp-block-heading wsu-spacing-after--small\">Soils and Irrigation<\/h4>\n\n\n\n<p>Most gardens in Washington require summer irrigation. The need for irrigation varies, depending on soil water-holding capacity, season, weather, site aspect, type of plants grown, and plant growth stage. Raised beds usually need more frequent irrigation than native soils. In most cases, the goal of irrigation is to recharge the available water in the top foot or so of soil. For sandy soil, one inch of irrigation water is all that is needed. Any more will <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#l\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>leach<\/strong> (opens in new window)<\/a> (move downward) through the root zone, carrying away important nutrients with it. A silt loam or clay soil can hold more than two inches of water but may need to be irrigated more slowly to prevent runoff.<\/p>\n\n\n\n<p>The two most common types of garden irrigation systems are sprinklers and drip systems. Movable sprinklers are less expensive but also less efficient than drip irrigation. Drip irrigation uses tubing with emitters to direct water to a plant\u2019s root zone (Figure 8). Installing a drip system takes an initial investment of money and effort but results in long-term savings of both water and time. Designing and installing a drip system is simple enough that most gardeners can do it themselves. For more information on designing, installing, and using drip irrigation systems, refer to WSU Extension publication <em>Drip Irrigation for the Yard and Garden<\/em>, which can be accessed in the Further Reading section.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large wsu-spacing-after--large wsu-spacing-before--large\"><img decoding=\"async\" loading=\"lazy\" width=\"1024\" height=\"768\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-08-1024x768.jpg\" alt=\"Drip lines installed in a raised bed.\" class=\"wp-image-1867\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-08-1024x768.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-08-300x225.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-08-768x576.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-08-1536x1152.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-08-2048x1536.jpg 2048w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><figcaption class=\"wp-element-caption\">Figure 8. Drip lines installed in a raised bed. Photo by Craig Cogger, WSU.<\/figcaption><\/figure>\n\n\n\n<p>It is important to know how much water you are applying. For sprinkler systems, place some straight-sided cans around the sprinkler, then measure how much water collects in each can over a set time. You can estimate water application in a drip system using the flow rating (gallons per hour) for the dripline, the length of dripline, and the area irrigated. Refer to drip irrigation suppliers and catalogs for flow information. Once you know how long it takes to deliver the desired amount of water, use an in-line timer to shut off the water when irrigation is complete. Use field observations of plants and soils to adjust water application length and frequency as needed. For guidance on estimating soil moisture, refer to USDA\u2019s <em>Estimating Soil Moisture by Feel and Appearance<\/em>, which can be accessed in the Further Reading section.<\/p>\n\n\n\n<p>When choosing a garden irrigation system, consider the trade-offs in efficiency, cost, complexity, and the time required to move hoses. Sprinkler irrigation or soaker hoses make sense in a temporary garden site or for just a few beds. Drip irrigation will save considerable time and water in larger, permanent gardens and landscapes or with multiple beds.<\/p>\n\n\n\n<h4 class=\"wp-block-heading wsu-spacing-after--small\">Wet Soils<\/h4>\n\n\n\n<p>If soil stays wet in the spring, delay tilling and planting. Working wet soil can damage its structure, and seeds are less likely to germinate in cold, wet soil. Some plants do not grow well in wet soil. Raspberries, for example, often become infected with root diseases in wet soil and thus lose vigor and productivity. Soils may be wet because they are located in a low-lying area that collects runoff from surrounding areas. Soils on level ground may be wet if they have a compacted underlayer that restricts drainage through the soil profile. Either natural or human-caused compaction can restrict drainage.<\/p>\n\n\n\n<p>A soil\u2019s color provides clues to its tendency to stay wet. If a subsoil is brown or reddish, the soil probably is well drained and has few wetness problems. Gray subsoils, especially those with brightly colored mottles, often are wet (Figure 9). If soil is gray and mottled directly beneath the topsoil, it is probably saturated during the wet season.<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-09-1024x688.jpg\" alt=\"Soil cutout showing light gray, orange soil. A folded knife lays to the side.\" class=\"wp-image-1870\" width=\"512\" height=\"344\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-09-1024x688.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-09-300x202.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-09-768x516.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-09.jpg 1500w\" sizes=\"(max-width: 512px) 100vw, 512px\" \/><figcaption class=\"wp-element-caption\">Figure 9. The gray color with red and yellow mottles in this subsoil indicates a seasonal high water table. Pierce County, Washington.<br>Photo by Craig Cogger, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p class=\"wsu-spacing-after--xsmall\">Sometimes, simple actions can reduce soil wetness problems. For example, consider:<\/p>\n\n\n\n<ul>\n<li>Diverting runoff from roof drains away from garden areas.<\/li>\n\n\n\n<li>Avoiding plants that perform poorly in wet conditions.<\/li>\n\n\n\n<li>Using raised beds for perennials that require well-drained soil and for early-season vegetables.<\/li>\n<\/ul>\n\n\n\n<p>A more complex and expensive action is installing subsurface drainage. First investigate whether a drain on a slope will remove excess water. Make sure there is a place to drain the water. Check with local regulatory agencies to determine whether there are restrictions on drainage projects.<\/p>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-soil-life\">Soil Life<\/h2>\n\n\n\n<p class=\"wsu-spacing-after--large\">Soil abounds with life. Besides plant roots, earthworms, insects, and other visible creatures, soil is home to an abundant and diverse population of microorganisms. A single gram of topsoil (about \u00bc teaspoon) can contain as many as a billion microorganisms (Table 1). Microorganisms are most abundant in the <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#r\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>rhizosphere <\/strong>(opens in new window)<\/a>\u2014the thin layer of soil surrounding plant roots.<\/p>\n\n\n<span id=\"tablepress-104-description\" class=\"tablepress-table-description tablepress-table-description-id-104\">Table 1. Approximate abundance of microorganisms in agricultural topsoil.<\/span>\n\n<table id=\"tablepress-104\" class=\"tablepress tablepress-id-104\" aria-describedby=\"tablepress-104-description\">\n<thead>\n<tr class=\"row-1 odd\">\n\t<th class=\"column-1\">Type of organism<\/th><th class=\"column-2\">Number per gram of soil (dry weight basis)<\/th>\n<\/tr>\n<\/thead>\n<tbody class=\"row-hover\">\n<tr class=\"row-2 even\">\n\t<td class=\"column-1\">Bacteria<\/td><td class=\"column-2\">100 million to 1 billion<\/td>\n<\/tr>\n<tr class=\"row-3 odd\">\n\t<td class=\"column-1\">Actinomycetes<\/td><td class=\"column-2\">10 million to 100 million<\/td>\n<\/tr>\n<tr class=\"row-4 even\">\n\t<td class=\"column-1\">Fungi<\/td><td class=\"column-2\">100,000 to 1 million<\/td>\n<\/tr>\n<tr class=\"row-5 odd\">\n\t<td class=\"column-1\">Algae<\/td><td class=\"column-2\">10,000 to 100,000<\/td>\n<\/tr>\n<tr class=\"row-6 even\">\n\t<td class=\"column-1\">Protozoa<\/td><td class=\"column-2\">10,000 to 100,000<\/td>\n<\/tr>\n<tr class=\"row-7 odd\">\n\t<td class=\"column-1\">Nematodes<\/td><td class=\"column-2\">10 to 100<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<!-- #tablepress-104 from cache -->\n\n\n\n<p class=\"wsu-spacing-before--large\">The primary function of soil organisms is to break down the remains of plants and other organisms. This process releases energy, nutrients, and carbon dioxide and creates soil organic matter. Many types of organisms are involved, including tiny bacteria, fungi, nematodes (Figure 10), insects, and earthworms.<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-10-1024x769.png\" alt=\"Slide of microscopic organisms, with \u201c0.1 millimeter\u201d line marked out for scale. Text and arrows point to a predator, bacterial feeders, and a fungal feeder. \" class=\"wp-image-1875\" width=\"520\" height=\"391\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-10-1024x769.png 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-10-300x225.png 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-10-768x577.png 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-10.png 1504w\" sizes=\"(max-width: 520px) 100vw, 520px\" \/><figcaption class=\"wp-element-caption\">Figure 10. Nematodes isolated from agricultural soil. The smallest nematodes feed on bacteria and fungi. The largest ones are predators that feed on other nematodes. Photo by Doug Collins, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>Some soil organisms play other beneficial roles. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#m\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Mycorrhizal fungi <\/strong>(opens in new window)<\/a> inoculate plant roots, resulting in an increase in their ability to take up nutrients (particularly phosphorus) and water from the soil. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#r\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Rhizobia<\/strong> (opens in new window)<\/a> and <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#f\" target=\"_blank\" rel=\"noreferrer noopener\"><strong><em>Frankia<\/em><\/strong> (opens in new window)<\/a> bacteria are responsible for converting atmospheric nitrogen to organic forms, a process known as <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#n\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>nitrogen fixation<\/strong> (opens in new window)<\/a>. Earthworms mix large volumes of soil and create macropore channels that improve soil permeability and aeration. However, not all soil organisms are beneficial to garden plants. Some are <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#p\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>pathogens <\/strong>(opens in new window)<\/a>, which cause diseases, such as root rot in raspberries and scab on potatoes. Moles can damage crops and lawns, and slugs are a serious pest in many Washington gardens.<\/p>\n\n\n\n<p>The activity level of soil organisms depends on soil moisture and temperature, as well as on the soil\u2019s organic matter content and organic inputs. Most microorganisms are most active between 70\u00b0F and 100\u00b0F, while earthworms are most active and abundant at about 50\u00b0F. Most organisms prefer moist soil. Because organic matter is at the base of the soil food web, soils with more organic matter tend to have more organisms.<\/p>\n\n\n\n<p>The relationships between gardening practices, microbial populations, and <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>soil health <\/strong>(opens in new window)<\/a> are complex and often poorly understood. However, almost all gardening activities\u2014including tillage; the use of <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#f\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>fertilizers<\/strong> (opens in new window)<\/a>, manures, and pesticides; and the choice of crop rotations\u2014affect the population and diversity of soil organisms. For example, amending soils with organic matter, mulching, returning crop residues to the soil, reducing tillage, and rotating plantings tend to increase the number and diversity of beneficial organisms.<\/p>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-soil-nutrients\">Soil Nutrients<\/h2>\n\n\n\n<p>Soil supplies 14 essential plant nutrients. Each nutrient plays one or more specific roles in plant growth. Nitrogen, for example, is a component of chlorophyll, amino acids, proteins, DNA, and many plant hormones. It plays a vital role in nearly all aspects of plant growth and development, and plants need a large amount of nitrogen to grow well. In contrast, plants need only a tiny amount of the <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#m\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>micronutrient<\/strong> (opens in new window)<\/a> molybdenum, which is involved in the functioning of only a few plant enzymes. Molybdenum nonetheless is essential, and plant growth is disrupted if it is deficient. Plants also require carbon, hydrogen, and oxygen, which they derive from water and air.<\/p>\n\n\n\n<p class=\"wsu-spacing-after--large\">A soil nutrient is classified as a major nutrient or micronutrient based on the amount plants need for health (Table 2). If a soil\u2019s nutrient supply is deficient, fertilizers can provide the additional nutrients needed for healthy plant growth.<\/p>\n\n\n<span id=\"tablepress-105-description\" class=\"tablepress-table-description tablepress-table-description-id-105\">Table 2. Essential plant nutrients supplied through the soil.<\/span>\n\n<table id=\"tablepress-105\" class=\"tablepress tablepress-id-105\" aria-describedby=\"tablepress-105-description\">\n<thead>\n<tr class=\"row-1 odd\">\n\t<th colspan=\"2\" class=\"column-1\">Major nutrients<\/th><th colspan=\"2\" class=\"column-3\">Micronutrients<\/th>\n<\/tr>\n<\/thead>\n<tbody class=\"row-hover\">\n<tr class=\"row-2 even\">\n\t<td class=\"column-1\">Nitrogen<\/td><td class=\"column-2\">N<\/td><td class=\"column-3\">Zinc<\/td><td class=\"column-4\">Zn<\/td>\n<\/tr>\n<tr class=\"row-3 odd\">\n\t<td class=\"column-1\">Phosphorus<\/td><td class=\"column-2\">P<\/td><td class=\"column-3\">Iron<\/td><td class=\"column-4\">Fe<\/td>\n<\/tr>\n<tr class=\"row-4 even\">\n\t<td class=\"column-1\">Potassium<\/td><td class=\"column-2\">K<\/td><td class=\"column-3\">Copper<\/td><td class=\"column-4\">Cu<\/td>\n<\/tr>\n<tr class=\"row-5 odd\">\n\t<td class=\"column-1\">Sulfur<\/td><td class=\"column-2\">S<\/td><td class=\"column-3\">Manganese<\/td><td class=\"column-4\">Mn<\/td>\n<\/tr>\n<tr class=\"row-6 even\">\n\t<td class=\"column-1\">Calcium<\/td><td class=\"column-2\">Ca<\/td><td class=\"column-3\">Boron<\/td><td class=\"column-4\">B<\/td>\n<\/tr>\n<tr class=\"row-7 odd\">\n\t<td class=\"column-1\">Magnesium<\/td><td class=\"column-2\">Mg<\/td><td class=\"column-3\">Molybdenum<\/td><td class=\"column-4\">Mo<\/td>\n<\/tr>\n<tr class=\"row-8 even\">\n\t<td class=\"column-1\">No Data<\/td><td class=\"column-2\">No Data<\/td><td class=\"column-3\">Chlorine<\/td><td class=\"column-4\">Cl<\/td>\n<\/tr>\n<tr class=\"row-9 odd\">\n\t<td class=\"column-1\">No Data<\/td><td class=\"column-2\">No Data<\/td><td class=\"column-3\">Nickel<\/td><td class=\"column-4\">Ni<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<!-- #tablepress-105 from cache -->\n\n\n\n<h3 class=\"wp-block-heading wsu-spacing-before--large\"><em>Nutrient Deficiencies<\/em><\/h3>\n\n\n\n<p>The most common nutrient deficiencies found in soil are nitrogen (N), phosphorus (P), and potassium (K)\u2014the nutrients that are in largest demand by plants. Sulfur deficiencies are also common in Washington soils. Nearly all soils lack enough available N for ideal plant growth, and phosphorus deficiencies occur in unfertilized soils. Potassium deficiencies are common west of the Cascades, but many soils east of the Cascades have adequate potassium for crop growth. Phosphorus accumulates in soils that have experienced repeated P application, so gardens with a history of P fertilization or manure application are seldom deficient in P.<\/p>\n\n\n\n<p>Calcium and magnesium may be deficient in acidic soils, which are typically found west of the Cascades. Except for boron and zinc, micronutrients are rarely deficient in soils in the Northwest. Boron deficiencies occur most often in soils west of the Cascades, particularly in root crops, brassica crops (for example, broccoli), and caneberries (for example, raspberries). Zinc deficiency is usually associated with high <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#p\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>pH<\/strong> (opens in new window)<\/a> soils found east of the Cascades and most often affects tree fruits.<\/p>\n\n\n\n<p>Each nutrient deficiency causes characteristic symptoms. In addition, affected plants grow more slowly, yield less, and are less healthy than plants with adequate levels of nutrients. For descriptions of nutrient deficiency symptoms, refer to <em>Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota and Identifying Nutrient Deficiencies in Ornamental Plants<\/em>, which can be accessed in the Further Reading section. See also Chapter 4: Plant Mineral Nutrition and Fertilizers for more information on plant nutrient deficiencies.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Nutrient Excesses<\/em><\/h3>\n\n\n\n<p>Excess nutrients can be a significant problem for plants and the environment. Excesses usually result because too much of a nutrient is applied to the soil or because a nutrient is applied at the wrong time. For example, applying too much nitrogen can lead to excessive foliage production, increasing the risk of plant disease and wind damage, and delaying flowering, fruiting, and dormancy. Available nitrogen left in the soil at the end of the growing season can leach into groundwater and threaten drinking water quality. Excess phosphorus can harm water quality if it moves into streams and lakes through runoff and erosion. The key to applying fertilizers is to supply enough nutrients at the right time to meet plant needs without creating excesses that can harm plants or the environment.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Nutrient Availability to Plants<\/em><\/h3>\n\n\n\n<p>Plants can only take up nutrients that are in solution (dissolved in soil water). Most soil nutrients are not in solution; they are tied up in soil mineral and organic matter in insoluble forms. These nutrients become available to plants only after being converted to soluble forms and dissolving into soil water. This process occurs through weathering of mineral matter and biological decomposition of organic matter. Weathering of mineral matter slowly releases small amounts of nutrients into solution. Nutrient release from soil organic matter is somewhat faster and depends on the biological activity in the soil.<\/p>\n\n\n\n<p>Nutrient release from soil organic matter is fastest in warm, moist soil and nearly nonexistent in cold or dry soil. Thus, the seasonal pattern of nutrient release is similar to the pattern of nutrient uptake by plants. Approximately one to four percent of the nutrients in soil organic matter are released in soluble form each year. Soluble, available nutrients are in ionic form. An <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#i\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>ion<\/strong> (opens in new window)<\/a> has either a positive or negative electrical charge. Positively charged ions are called <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>cations<\/strong> (opens in new window)<\/a>, and negatively charged ions are called <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#a\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>anions<\/strong> (opens in new window)<\/a>. Potassium (K<sup>+<\/sup>), calcium (Ca<sup>++<\/sup>), and magnesium (Mg<sup>++<\/sup>) ions are examples of cations. Chloride (Cl<sup>&#8211;<\/sup>) is an example of an anion.<\/p>\n\n\n\n<p>Clay particles and soil organic matter contain negative charges on their surfaces and thus can attract positive charges (cations) (Figure 11). They hold nutrient cations in a form that can be released rapidly into <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>soil solution<\/strong> (opens in new window)<\/a> to replace nutrients taken up by plant roots. This reserve supply of nutrients contributes to soil fertility. A soil\u2019s capacity to hold cations is called its <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>cation exchange capacity<\/strong> (opens in new window)<\/a>, or CEC. CEC is greater in soils with more clay or organic matter and lower in sandy soils with little clay.<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"alignright size-full is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-11.jpg\" alt=\"An illustration of cation exchange. Diagram of various ions, like calcium, magnesium, and potassium, along with the arrows demonstrating movement.\" class=\"wp-image-1880\" width=\"624\" height=\"369\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-11.jpg 820w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-11-300x177.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-11-768x454.jpg 768w\" sizes=\"(max-width: 624px) 100vw, 624px\" \/><figcaption class=\"wp-element-caption\">Figure 11. Simplified diagram of cation exchange showing cations held on a negatively charged clay particle and cations in solution. Illustration by Craig Cogger, WSU.<\/figcaption><\/figure><\/div>\n\n\n<h3 class=\"wp-block-heading\"><em>The Nitrogen Cycle<\/em><\/h3>\n\n\n\n<p>Managing nitrogen is a key part of growing a productive and environmentally friendly garden. Nitrogen is the nutrient needed in the largest amount by plants, but excess nitrogen can harm plants, degrade water quality, or lead to increased emissions of nitrous oxide (N<sub>2<\/sub>O), a greenhouse gas. Understanding how the <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#n\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>nitrogen cycle<\/strong> (opens in new window)<\/a> affects nitrogen availability can help gardeners become better nutrient managers.<\/p>\n\n\n\n<p>Nitrogen is found in four main forms in the soil (Table 3). Only two of the forms\u2014ammonium and nitrate\u2014can be used directly by most plants.<\/p>\n\n\n\n<p class=\"wsu-spacing-after--large\">Most nitrogen in soil is present in soil organic matter. This organic nitrogen is not available to plants. As soil warms in the spring, soil microbes begin breaking down organic matter to obtain energy, releasing some of the nitrogen as ammonium (NH<sub>4<\/sub><sup>+<\/sup>) (Figure 12). Ammonium is a soluble ion that is available to plants and soil microbes. When the soil is warm, a group of microbes called <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#n\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#n\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>nitrifiers<\/strong> (opens in new window)<\/a> use ammonium as an energy source converting it to nitrate (NO<sub>3<\/sub><sup>&#8211;<\/sup>). Nitrate is also soluble and available to plants. The ammonium and nitrate ions released from soil organic matter are the same as the ammonium and nitrate contained in processed fertilizers.<\/p>\n\n\n<span id=\"tablepress-106-description\" class=\"tablepress-table-description tablepress-table-description-id-106\">Table 3. Common forms of nitrogen in soil.<\/span>\n\n<table id=\"tablepress-106\" class=\"tablepress tablepress-id-106\" aria-describedby=\"tablepress-106-description\">\n<thead>\n<tr class=\"row-1 odd\">\n\t<th class=\"column-1\">Form of nitrogen<\/th><th class=\"column-2\">Characteristics<\/th>\n<\/tr>\n<\/thead>\n<tbody class=\"row-hover\">\n<tr class=\"row-2 even\">\n\t<td class=\"column-1\">Organic N<\/td><td class=\"column-2\">Primary form of N in soil. Found in proteins, lignin, etc. Not generally available to plants. Mineralized to ammonium by soil microorganisms.<\/td>\n<\/tr>\n<tr class=\"row-3 odd\">\n\t<td class=\"column-1\">Ammonium (NH<sub>4<\/sub><sup>+<\/sup>)<\/td><td class=\"column-2\">Soluble form. Available to plants. Converted to nitrate by soil microorganisms.<\/td>\n<\/tr>\n<tr class=\"row-4 even\">\n\t<td class=\"column-1\">Nitrate (NO<sub>3<\/sub><sup>\u2212<\/sup>)<\/td><td class=\"column-2\">Soluble form. Available to plants. Can be lost through leaching. Converted to gases in wet soils.<\/td>\n<\/tr>\n<tr class=\"row-5 odd\">\n\t<td class=\"column-1\">Atmospheric N (N<sub>2<\/sub>)<\/td><td class=\"column-2\">Makes up approximately 80 percent of the soil atmosphere. Source of nitrogen for nitrogen-fixing plants.<br \/>\nNot available to other plants.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<!-- #tablepress-106 from cache -->\n\n\n<div class=\"wp-block-image wsu-spacing-before--xlarge wsu-spacing-after--xhero\">\n<figure class=\"alignright size-full is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-12.jpg\" alt=\"Diagram showing the organic nitrogen\u2019s mineralization process into ammonium ion, then nitrification converting to nitrate ion, followed by either leaching or denitrification into gasses like N2 and N2O. During the ammonium ion and nitrate ion phases, two arrows show that these ions can then be taken up by plants and microbes. Additional means of nitrogen fixation occur through uptake of atmospheric gasses by plants and microbes. After plant and microbe breakdown, organic nitrogen is then created through soil organic matter, manure, and plant residues, completing the cycle.\" class=\"wp-image-1886\" width=\"484\" height=\"584\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-12.jpg 701w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-12-249x300.jpg 249w\" sizes=\"(max-width: 484px) 100vw, 484px\" \/><figcaption class=\"wp-element-caption\">Figure 12. The nitrogen cycle. Illustration by Craig Cogger, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p class=\"wsu-spacing-before--large\">Because nitrate has a negative charge, it is not held to the surface of clay or organic matter, so it can be lost readily by leaching. Nitrate remaining in the soil at the end of the growing season will leach out during the fall and winter in all but the driest parts of Washington and may reach groundwater, where it becomes a contaminant. Excess irrigation can leach nitrate during the growing season. In soils that are saturated and depleted of oxygen during the wet season, soil microbes convert nitrate into nitrogen gases through a biological process called <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#d\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>denitrification <\/strong>(opens in new window)<\/a>. Ammonium and nitrate taken up by plants are converted back to organic forms in plant tissue. When plant residues are returned to the soil, they decompose, slowly releasing nitrogen back into plant-available forms.<\/p>\n\n\n\n<p>The nitrogen cycle is a leaky one, with losses to leaching and to the atmosphere. Harvesting crops also removes nitrogen. To maintain an adequate nitrogen supply, nitrogen must be added back into the system through fixation or fertilization.<\/p>\n\n\n\n<p>Nitrogen fixation (Figure 12) is a natural process involving certain plants and nitrogen-fixing bacteria such as <em>Rhizobia<\/em> and <em>Frankia<\/em>. The bacteria form nodules in plant roots (Figure 13). Through these nodules the bacteria are able to take atmospheric nitrogen (N<sub>2<\/sub> gas) from the soil air, convert it to organic N, and supply it to the plant. The plants supply the bacteria with energy and nutrients. Legumes such as peas, beans, clover, and Scotch broom fix nitrogen using <em>Rhizobia<\/em>. Alder trees fix nitrogen with <em>Frankia<\/em>.<\/p>\n\n\n<div class=\"wp-block-image wsu-spacing-after--xlarge\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-13-1024x683.jpg\" alt=\"Close-up of legume (fava bean) roots.\" class=\"wp-image-1887\" width=\"433\" height=\"288\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-13-1024x683.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-13-300x200.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-13-768x512.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-13-1536x1024.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-13-2048x1365.jpg 2048w\" sizes=\"(max-width: 433px) 100vw, 433px\" \/><figcaption class=\"wp-element-caption\">Figure 13. Roots of a legume cover crop (fava bean) with nodules that contain nitrogen-fixing Rhizobia. Photo by Chris Benedict, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>Growing legumes as a cover crop will supply nitrogen to the next season\u2019s garden crops. Inoculating legume seed with appropriate strains of <em>Rhizobia<\/em> helps ensure nitrogen fixation by the legume crop. Inoculants are easy to use and are available through seed catalogs or garden supply stores.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Phosphorus<\/em><\/h3>\n\n\n\n<p>Phosphorus has low solubility in soil water and thus low availability to plants. As a result, many soils without a history of fertilization are deficient in available phosphorus and need phosphorus fertilization to support fast-growing garden crops. Landscape shrubs and trees seldom need P fertilization, even in previously unfertilized soils. Many vegetable crops benefit from phosphorus as a starter fertilizer because it provides adequate P to young plants with small root systems. Repeated applications of phosphorus-containing fertilizers or organic amendments will increase P levels in the soil to the point that little or no additional P fertilizer is needed.<\/p>\n\n\n\n<p>Excess phosphorus accumulation can become a serious environmental problem. Loss of excess P through runoff or erosion into lakes and streams can contribute to <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#e\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#e\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>eutrophication <\/strong>(opens in new window)<\/a> (algae blooms that harm water quality and aquatic ecosystems). Soil testing is a valuable tool for determining if your plants need more phosphorus or if you have an excess amount of P and need to adjust your fertility plan.<\/p>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-soil-pH\">Soil pH<\/h2>\n\n\n\n<p>Soil pH is a measure of the acidity or alkalinity of a soil. At a pH of 7 (neutral), acidity and alkalinity are balanced. Acidity increases by a factor of ten with each one-unit drop in pH below 7. For example, a pH of 5.5 is ten times as acidic as a pH of 6.5.<\/p>\n\n\n\n<p>Native soil pH depends on the minerals present in the soil and on the climate. Soils in arid locations tend to be alkaline, and those in rainy areas tend to be acidic. Gardening practices also affect soil pH. For example, many nitrogen fertilizers reduce pH (acidifying soil), while liming increases pH. Soil pH influences plant growth by affecting the availability of plant nutrients and toxic metals, and by affecting the activity of soil microorganisms, which in turn affects nutrient cycling and plant disease risk.<\/p>\n\n\n\n<p>The availability of phosphorus decreases in acidic soils, while the availability of iron increases. In alkaline soils, the availability of iron and zinc can be quite low, resulting in deficiencies in sensitive plants. Aluminum availability increases in acidic soils. Aluminum is one of the most common elements in soil, but it is not a plant nutrient and is toxic to plants in high concentrations. Very little aluminum is in solution in soils above a pH of 6, so it does not cause problems for plants. However, as pH declines and aluminum availability increases, aluminum toxicity can become a problem.<\/p>\n\n\n\n<p>Microbes are also affected by soil pH. The most numerous and diverse soil microbial populations exist in the middle of the pH range. Fewer organisms are adapted to strongly acidic or strongly alkaline soils. Nutrient cycling is slower in strongly acidic and strongly alkaline soils as a result of reduced microbial populations.<\/p>\n\n\n\n<p>Most garden plants perform best in soil with a pH of 6 to 7.5, but some plants (such as blueberries and rhododendrons) are adapted to more strongly acidic soils (pH 4.5 to 5.5). Before amending soil to adjust pH, it is important to know the preferred pH ranges of your plants.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Increasing Soil pH<\/em><\/h3>\n\n\n\n<p>Many western Washington soils and a few eastern Washington soils are below the ideal pH level for growing vegetable crops. The most common way to increase soil pH is to add <a rel=\"noreferrer noopener\" href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#l\" target=\"_blank\"><strong>lime <\/strong>(opens in new window)<\/a>. Lime is ground limestone, a rock containing calcium carbonate. It is suitable for use by organic gardeners. Lime not only raises the pH of acidic soils but also supplies calcium, an essential nutrient. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#d\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Dolomitic limestone<\/strong> (opens in new window)<\/a> contains magnesium as well as calcium and is a good choice for acid soils that are deficient in magnesium.<\/p>\n\n\n\n<p>Lime is a slow-release material, so apply it in the fall to benefit spring crops. Finely ground lime works more quickly than coarser particles, but it can be dusty. Pelletized lime is more expensive than ground limestone, but many gardeners find it convenient to use.<\/p>\n\n\n\n<p>The amount of lime needed depends on the initial and desired soil pH, soil texture, and organic matter. The best way to determine whether your soil needs lime and how much to apply is to have it tested (see section titled Soil Tests for details). Do not lime areas where you grow acid-loving plants (such as blueberry and rhododendron).<\/p>\n\n\n\n<p>Wood ashes are a readily available source of potassium, calcium, and magnesium. Like lime, they also raise soil pH. High rates of wood ash may cause short-term salt injury, so apply no more than 15 to 25 pounds per 1,000 square feet.<\/p>\n\n\n\n<p>Gypsum (calcium sulfate) is not a substitute for lime. It supplies calcium and sulfur but has little effect on soil pH. Gypsum has been promoted as a soil amendment to improve soil structure, but it has little effect in most cases. Gypsum can improve soil structure when poor structure results from excess sodium in the soil, a rare condition in Northwest gardens. Use organic amendments to improve soil structure, as described in the Organic Amendments section of this chapter and in Chapter 4: Plant Mineral Nutrition and Fertilizers.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Decreasing Soil pH<\/em><\/h3>\n\n\n\n<p>Soil pH east of the Cascades is often too high to grow healthy, acid-loving plants, such as blueberries, rhododendrons, and maple trees. These plants can develop iron deficiency if the pH is too high (Figure 14). Even some soils west of the Cascades are not acidic enough for good blueberry production. Elemental sulfur lowers soil pH. The amount of sulfur needed depends on the soil\u2019s original and desired soil pH and soil texture. Soil testing is the best way to determine whether sulfur is needed and, if so, how much. Applying too much sulfur can cause the pH to drop below desirable levels. Ammonium sulfate fertilizer lowers pH more gradually than sulfur does, and urea also reduces pH slowly, as do some <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#o\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>organic fertilizers<\/strong> (opens in new window)<\/a>. Gypsum does not lower soil pH. See Chapter 4: Plant Mineral Nutrition and Fertilizers for application rates and schedules.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large is-resized wsu-spacing-after--large wsu-spacing-before--xlarge\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-14-1024x768.jpg\" alt=\"A blueberry bush showing iron deficiency symptoms; yellowing around the margins.\" class=\"wp-image-1895\" width=\"1117\" height=\"838\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-14-1024x768.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-14-300x225.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-14-768x576.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-14-1536x1152.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-14-2048x1536.jpg 2048w\" sizes=\"(max-width: 1117px) 100vw, 1117px\" \/><figcaption class=\"wp-element-caption\">Figure 14. Blueberry showing iron deficiency symptoms (yellow leaf tissue surrounding green veins). Reducing soil pH with sulfur additions will increase iron availability in the soil and correct the deficiency. Photo by Craig Cogger, WSU.<\/figcaption><\/figure>\n\n\n\n<p>For additional guidelines on acidifying soils, see OSU Extension publications <em>Acidifying Soils in Landscapes and Gardens East of the Cascades<\/em> or <em>Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden West of the Cascade Mountain Range in Oregon and Washington<\/em>, which can be accessed in the Further Reading section.<\/p>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-soil-salinity\">Soil Salinity<\/h2>\n\n\n\n<p><a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Soil salinity<\/strong> (opens in new window)<\/a> can be a problem, particularly in irrigated soils in arid areas of eastern Washington. <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#s\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Salts <\/strong>(opens in new window)<\/a> from irrigation water, fertilizer, <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#c\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>compost<\/strong> (opens in new window)<\/a>, and manure applications can accumulate to the point where they harm plant growth. Seedlings and transplants are most susceptible to salt injury. In areas with more rainfall, salts are leached from the soil each winter and do not accumulate in the root zone.<\/p>\n\n\n\n<p class=\"wsu-spacing-after--large\">An <a rel=\"noreferrer noopener\" href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#e\" data-type=\"URL\" data-id=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#e\" target=\"_blank\"><strong>electrical conductivity<\/strong> (opens in new window)<\/a> (EC) test measures the level of soil salinity. The more salts dissolved in the soil solution, the higher the EC. Electrical conductivity is measured in units of millimhos per centimeter (mmho\/cm) or deciSiemens per meter (dS\/m). Table 4 compares the relative susceptibility of typical garden vegetable crops to excess soluble salts. For more details, refer to Chapter 4: Plant Mineral Nutrition and Fertilizers or to OSU Extension Service publication <em>Nutrient Management for Sustainable Vegetable Production Systems in Western Oregon<\/em>, which can be accessed in the Further Reading section of this chapter.<\/p>\n\n\n<span id=\"tablepress-107-description\" class=\"tablepress-table-description tablepress-table-description-id-107\">Table 4. Relative sensitivity of vegetable crops to injury and yield loss from excess soluble salts in soil.<\/span>\n\n<table id=\"tablepress-107\" class=\"tablepress tablepress-id-107\" aria-describedby=\"tablepress-107-description\">\n<thead>\n<tr class=\"row-1 odd\">\n\t<th class=\"column-1\">Electrical conductivity<\/th><th class=\"column-2\">Crop sensitivity*<\/th>\n<\/tr>\n<\/thead>\n<tbody class=\"row-hover\">\n<tr class=\"row-2 even\">\n\t<td class=\"column-1\">dS\/m or mmho\/cm\u2020<\/td><td class=\"column-2\"><span class=\"wsu-screen-reader-only\">Not applicable<\/span><\/td>\n<\/tr>\n<tr class=\"row-3 odd\">\n\t<td class=\"column-1\">1\u20132<\/td><td class=\"column-2\">Turnip, carrot, bean, pea, radish, onion, lettuce, pepper<\/td>\n<\/tr>\n<tr class=\"row-4 even\">\n\t<td class=\"column-1\">2\u20134<\/td><td class=\"column-2\">Potato, sweet corn, celery, cabbage, spinach, tomato, cucumber, broccoli<\/td>\n<\/tr>\n<tr class=\"row-5 odd\">\n\t<td class=\"column-1\">4\u20136<\/td><td class=\"column-2\">Beet, zucchini<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<!-- #tablepress-107 from cache -->\n\n\n\n<p class=\"wsu-font-size--small wsu-max-width--xlarge wsu-spacing-after--large\"><em>Source<\/em>: Adapted from <em>Nutrient Management for Sustainable Vegetable Production Systems in Western Oregon<\/em>, which can be accessed in the Further Reading section of this chapter.<br><sup>*<\/sup> Plant injury and yield reduction of 10% or more possible in this EC range.<br><sup>\u2020<\/sup> Measured by saturated paste method. EC measured in a 1:2 soil:water suspension is approximately half of the saturated paste value. Check your lab report to see which method was used.<\/p>\n\n\n\n<p>If you garden in an arid area, be aware that some compost products may increase soil salinity. Yard debris compost generally contains few salts, but manure- or <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#b\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>biosolids <\/strong>(opens in new window)<\/a>-based composts may contain enough salts to be harmful in some environments. If you have problems with salinity in your garden soil, reduce or avoid the use of manure-based compost.<\/p>\n\n\n\n<p>Salts can be leached from soil by applying irrigation water in excess of the water-holding capacity of the soil. The excess water must drain downward through the soil to carry away excess salts. When leaching, apply water slowly enough that it drains freely through the subsoil. Six inches of excess water removes about half of the soluble salts in a soil. A foot of water removes about 80 percent.<\/p>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-soil-tests\">Soil Tests<\/h2>\n\n\n\n<p>A soil test provides information on the amount of nutrients in a soil sample and recommends the amount of fertilizer that should be added based on the test results and the types of plants grown. Soil need not be tested every year; every two to four years is often enough. A basic garden soil test typically includes testing for soil pH and lime requirement, as well as for phosphorus, potassium, calcium, magnesium, and sometimes boron. In arid locations, the basic soil test also includes testing for electrical conductivity, a measure of soluble salts.<\/p>\n\n\n\n<p>Although some soil-testing labs test for nitrate-N, these numbers are seldom useful for predicting crop N requirements, because nitrate levels can change rapidly due to plant uptake, leaching, and microbial activity. There is no rapid and reliable test that can predict nitrogen availability during future growing seasons. However, testing labs will provide a general nitrogen recommendation based on the type of plants grown and on any pertinent information about the soil (for example, whether there is a history of manure applications, which would increase the soil nitrogen supply). WSU, OSU, and Pacific Northwest Extension publications are good sources of information on crop-specific nitrogen requirements.<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"alignright size-large is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-15-768x1024.jpg\" alt=\"Man taking soil sample using a push probe. Someone handles a soil probe while wearing black rubber boots.\" class=\"wp-image-1901\" width=\"384\" height=\"512\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-15-768x1024.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-15-225x300.jpg 225w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-15-1152x1536.jpg 1152w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-15.jpg 1536w\" sizes=\"(max-width: 384px) 100vw, 384px\" \/><figcaption class=\"wp-element-caption\">Figure 15. Soil sampling using a push probe. Photo by Andy Bary, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>To obtain a soil sample, first collect subsamples from at least ten different garden locations. Avoid any atypical areas, such as the site of an old trash dump, burn pile, or rabbit hutch. Sample the top foot of soil (from the soil surface to a 12-inch depth). If the soil is too rocky to get a full 12-inch sample, then sample as deep as possible. Air-dry the samples and mix them together well. Send approximately one cup of the mixed sample to the soil-testing lab. The easiest way to collect samples is with a soil probe (Figure 15), but a trowel or spade can also be used. View the WSU video <em>Collecting a Soil Sample<\/em>, which can be accessed in the Further Reading section, for a demonstration of sample collection using a trowel or a soil probe. Because management and fertilizer recommendations vary for different crops, such as vegetables, lawns, and berries, collect separate samples for each area. For more information on soil sampling, refer to OSU Extension Service publication <em>A Guide to Collecting Soil Samples for Farms and Gardens<\/em>, which is available in the Further Reading section.<\/p>\n\n\n\n<p>Washington State University does not test soils but maintains a list of laboratories that do agricultural and garden soil analyses in the Pacific Northwest. This list can be found at <a href=\"http:\/\/analyticallabs.puyallup.wsu.edu\/analyticallabs\/\" target=\"_blank\" rel=\"noreferrer noopener\">Analytical Laboratories and Consultants Serving Agriculture in the Pacific Northwest (opens in new window)<\/a>. Also, WSU county Extension offices often have lists of analytical labs that are popular locally. Before choosing a lab, make sure they specifically test and make recommendations for garden soils.<\/p>\n\n\n\n<p class=\"wsu-spacing-after--xsmall\">Find out if the lab:<\/p>\n\n\n\n<ul>\n<li>Routinely tests garden soils for plant<\/li>\n\n\n\n<li>nutrients and pH,<\/li>\n\n\n\n<li>Uses WSU or OSU test methods and<\/li>\n\n\n\n<li>fertilizer guides, and<\/li>\n\n\n\n<li>Provides recommendations for garden<\/li>\n\n\n\n<li>fertilizer applications.<\/li>\n<\/ul>\n\n\n\n<p class=\"wsu-spacing-after--xsmall\">Also find out:<\/p>\n\n\n\n<ul>\n<li>What paperwork needs to accompany the soil sample,<\/li>\n\n\n\n<li>How much the test will cost, and<\/li>\n\n\n\n<li>How quickly the test results will be available.<\/li>\n<\/ul>\n\n\n\n<p>See Chapter 4: Plant Mineral Nutrition and Fertilizers for more information on soil tests and an in-depth discussion of fertilizer types, rates, placement, and timing.<\/p>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-organic-amendments\">Organic Amendments<\/h2>\n\n\n\n<p>The best use of specific organic materials varies, depending on their nitrogen concentration. Organic materials that contain greater than 3% total N are rich in nitrogen and are used as organic fertilizers. Examples include poultry manure, feather meal, and fish meal. These materials are a good source of nutrients, but must be used sparingly to avoid overfertilization. Refer to Chapter 4: Plant Mineral Nutrition and Fertilizers to determine how much and when to apply these fertilizers.<\/p>\n\n\n\n<p>Organic materials with intermediate levels of nitrogen (including many composts, leaf mulches, and cover crop residues) have lower nutrient availability and make good organic amendments. They can be added to the soil in large amounts to replenish soil organic matter. Organic matter builds and stabilizes soil structure, thus reducing erosion and improving soil porosity, infiltration, and drainage. It effectively holds water and nutrients for plants and soil organisms.<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"alignright size-full is-resized\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-16.png\" alt=\"Hands holding bark mulch, left hand with a low nitrogen concentration (reddish color) and the right hand holding yard debris compost with a moderate nitrogen concentration (dark brownish color).\" class=\"wp-image-1903\" width=\"537\" height=\"334\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-16.png 979w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-16-300x187.png 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-16-768x479.png 768w\" sizes=\"(max-width: 537px) 100vw, 537px\" \/><figcaption class=\"wp-element-caption\">Figure 16. Bark mulch with a low nitrogen concentration demonstrated on the left and yard debris compost with a moderate nitrogen concentration shown on the right. Photo by Rita Hummel, WSU.<\/figcaption><\/figure><\/div>\n\n\n<p>Materials with low nitrogen concentrations (&lt; 1 to 1.5% total N), such as straw, bark, and sawdust (Figure 16), contain so little nitrogen that they reduce levels of available nitrogen when mixed into the soil. Soil microorganisms use available nitrogen when they break down these materials, leaving little nitrogen for plants. This process is called <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#i\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>immobilization<\/strong> (opens in new window)<\/a> and results in nitrogen deficiency. These materials are suitable as <a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#o\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>organic mulch<\/strong> (opens in new window)<\/a> applied to the soil surface. They do not cause nitrogen immobilization as long as they remain on the surface and are not mixed into the soil.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Compost<\/em><\/h3>\n\n\n\n<p>Compost is an excellent source of organic matter for garden soils. Composting also closes the recycling loop by turning waste materials into a soil amendment. Gardeners can make compost at home or buy commercially prepared composts. For more information on home composting of yard debris and vegetable scraps, refer to Chapter 6: Composting or WSU Extension publication <em>Backyard Composting<\/em>, which can be accessed in the Further Reading section.<\/p>\n\n\n\n<h4 class=\"wp-block-heading wsu-font-size--xxmedium wsu-spacing-after--small\">Commercial Compost<\/h4>\n\n\n\n<p>Many gardeners prefer to use commercial compost or will supplement their home compost with commercial compost. Yard debris (from curbside pickup or landscaper drop off) is the major raw material (feedstock) in most commercial compost sold in Washington. Feedstocks for commercial compost also may include home and commercial food waste, animal manure, biosolids, crop residues, or wood waste. Commercial compost is made on a large scale, under conditions that kill weed seeds, plant pathogens, and human pathogens. Commercial compost is cured and screened to make a product that is uniform and convenient to use.<\/p>\n\n\n\n<h4 class=\"wp-block-heading wsu-font-size--xxmedium wsu-spacing-after--small\">Using Compost<\/h4>\n\n\n\n<p>Add 1 to 3 inches of compost to build soil organic matter when establishing a new garden or landscape bed. For an established garden bed, add about \u00bd inch of compost each year. If soil testing shows very high levels of phosphorus, stop adding compost and grow cover crops instead. Till or dig compost directly into garden soil, or use it as mulch before turning it into the soil. When establishing landscape beds, either incorporate the compost before planting or leave it on the surface. One cubic yard of compost covers approximately 300 square feet of soil at a depth of 1 inch. For more information on how much compost to apply, refer to WSU Extension publication <em>Organic Soil Amendments in Yards and Gardens: How Much Is Enough? <\/em>which can be accessed in the Further Reading section.<\/p>\n\n\n\n<p>In the first or second year after application, partially decomposed woody compost may immobilize some soil nitrogen, resulting in nitrogen deficiency for plants. If plants show signs of nitrogen deficiency (for example, poor growth or yellow leaves), add extra nitrogen fertilizer (either organic or inorganic). In following years, most composts contribute small amounts of available nitrogen to the soil. Composts also supply P, K, and micronutrients.<\/p>\n\n\n\n<p>When gardening in arid locations, be aware that some composts may increase the salinity of the soil. Yard-debris composts generally contain few salts, but manure-based composts may contain enough salts to be harmful. If there are problems with salinity in the garden soil, reduce or avoid the use of manure composts.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Green Manure (Cover Crops)<\/em><\/h3>\n\n\n\n<p class=\"wsu-spacing-after--xsmall\"><a href=\"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/glossary\/#g\" target=\"_blank\" rel=\"noreferrer noopener\"><strong>Green manures<\/strong> (opens in new window)<\/a> are cover crops specifically grown to be tilled or dug into the soil, or left on the soil surface as mulch. Planting green manure cover crops is a way for gardeners to grow their own organic matter (Figure 17). However, the value of cover crops goes beyond their contribution of organic matter. These crops are also able to:<\/p>\n\n\n\n<ul>\n<li>Capture and recycle nutrients that otherwise would be lost by leaching during the winter,<\/li>\n\n\n\n<li>Protect the soil surface from the impact of rainfall,<\/li>\n\n\n\n<li>Reduce runoff and erosion,<\/li>\n\n\n\n<li>Suppress weeds, and<\/li>\n\n\n\n<li>Supply additional nitrogen (legumes only).<\/li>\n<\/ul>\n\n\n\n<figure class=\"wp-block-image size-large is-resized wsu-spacing-after--xlarge wsu-spacing-before--xlarge\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-17-1024x768.jpg\" alt=\"Fava bean planted in raised beds.\" class=\"wp-image-1905\" width=\"1158\" height=\"869\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-17-1024x768.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-17-300x225.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-17-768x576.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-17-1536x1152.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-17-2048x1536.jpg 2048w\" sizes=\"(max-width: 1158px) 100vw, 1158px\" \/><figcaption class=\"wp-element-caption\">Figure 17. Fava bean and rye cover crop planted in raised beds. Hilltop Urban Gardens, Tacoma, Washington. Photo by Craig Cogger, WSU.<\/figcaption><\/figure>\n\n\n\n<p class=\"wsu-spacing-after--large\">However, no one cover crop provides all of these benefits. Deciding which cover crop or crop combination to grow depends on which benefits are most desired and which cover crops best fit into the overall garden plan (Table 5). With the exception of buckwheat, all of the cover crops listed in Table 5 are suitable for fall planting and spring termination. For more information on cover crops, including seeding rates, planting dates, and uses, refer to WSU Extension publications <em>Cover Crops for Home Gardens West of the Cascades<\/em> or <em>Cover Crops for Home Gardens East of the Cascades<\/em>, which can be found in the Further Reading section.<\/p>\n\n\n<span id=\"tablepress-108-description\" class=\"tablepress-table-description tablepress-table-description-id-108\">Table 5. Examples of cover crops grown in Washington and Oregon.<\/span>\n\n<table id=\"tablepress-108\" class=\"tablepress tablepress-id-108\" aria-describedby=\"tablepress-108-description\">\n<thead>\n<tr class=\"row-1 odd\">\n\t<th class=\"column-1\">Cover crop<\/th><th class=\"column-2\">Type<\/th><th class=\"column-3\">Characteristics<\/th>\n<\/tr>\n<\/thead>\n<tbody class=\"row-hover\">\n<tr class=\"row-2 even\">\n\t<td class=\"column-1\">Annual ryegrass<\/td><td class=\"column-2\">Grass<\/td><td class=\"column-3\">Hardy, tolerates wet soils in winter, difficult to till once established<\/td>\n<\/tr>\n<tr class=\"row-3 odd\">\n\t<td class=\"column-1\">Austrian winter pea<\/td><td class=\"column-2\">Legume<\/td><td class=\"column-3\">Fixes nitrogen, does not compete well with winter weeds, not for wet soils<\/td>\n<\/tr>\n<tr class=\"row-4 even\">\n\t<td class=\"column-1\">Barley<\/td><td class=\"column-2\">Cereal<\/td><td class=\"column-3\">Not as hardy as rye, tolerates drought, leafy growth in spring<\/td>\n<\/tr>\n<tr class=\"row-5 odd\">\n\t<td class=\"column-1\">Buckwheat<\/td><td class=\"column-2\">Broadleaf<\/td><td class=\"column-3\">Fast-growing, frost-sensitive, summer cover<\/td>\n<\/tr>\n<tr class=\"row-6 even\">\n\t<td class=\"column-1\">Cereal rye<\/td><td class=\"column-2\">Cereal<\/td><td class=\"column-3\">Very hardy, grows quickly, OK for late planting, matures rapidly in spring<\/td>\n<\/tr>\n<tr class=\"row-7 odd\">\n\t<td class=\"column-1\">Common vetch<\/td><td class=\"column-2\">Legume<\/td><td class=\"column-3\">Fixes nitrogen, similar to hairy vetch but easier to till in spring<\/td>\n<\/tr>\n<tr class=\"row-8 even\">\n\t<td class=\"column-1\">Crimson clover<\/td><td class=\"column-2\">Legume<\/td><td class=\"column-3\">Fixes nitrogen, less biomass than vetches, easy to dig or till into soil in spring, good for raised beds<\/td>\n<\/tr>\n<tr class=\"row-9 odd\">\n\t<td class=\"column-1\">Fava bean<\/td><td class=\"column-2\">Legume<\/td><td class=\"column-3\">Fixes nitrogen, not as winter-hardy as vetches<\/td>\n<\/tr>\n<tr class=\"row-10 even\">\n\t<td class=\"column-1\">Hairy vetch<\/td><td class=\"column-2\">Legume<\/td><td class=\"column-3\">Fixes nitrogen, starts slowly, grows quickly in spring, good companion crop for cereal rye, becomes a weed if allowed to go to seed<\/td>\n<\/tr>\n<tr class=\"row-11 odd\">\n\t<td class=\"column-1\">Oats<\/td><td class=\"column-2\">Cereal<\/td><td class=\"column-3\">Not as winter-hardy as other cereals, leafy, tolerates wet soils<\/td>\n<\/tr>\n<tr class=\"row-12 even\">\n\t<td class=\"column-1\">Triticale<\/td><td class=\"column-2\">Cereal<\/td><td class=\"column-3\">Hardy cross between rye and wheat, leafy<\/td>\n<\/tr>\n<tr class=\"row-13 odd\">\n\t<td class=\"column-1\">Winter wheat<\/td><td class=\"column-2\">Cereal<\/td><td class=\"column-3\">Hardy, leafy<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<!-- #tablepress-108 from cache -->\n\n\n\n<p class=\"wsu-spacing-after--default wsu-spacing-before--large\">Gardeners usually plant cover crops in the fall and terminate them by tilling or cutting them before spring planting begins. The earlier cover crops are planted, the more benefits they provide. Legumes such as vetch and crimson clover need an early start (September planting) to achieve enough growth to cover the soil before cold weather arrives. Cereal rye can be planted into October. Cover crop seedings may need irrigation to get established in arid areas or if fall rains arrive late.<\/p>\n\n\n\n<p>If a garden has crops still growing into late fall, it will not be possible to plant early cover crops over the entire area. In this case, plant cover crops in areas that are harvested early, and use mulch in those areas that are harvested later. For example, plant a cover crop in a sweet corn bed following harvest in September, and mulch a bed of fall greens or carrots after harvest in late fall. Gardeners can also seed cover crops between rows of late crops if space allows.<\/p>\n\n\n\n<p>Till or dig cover crops into the soil before they flower and go to seed. After flowering, plants become woody and decline in quality. Cover crops that set seed can turn into weeds when the seeds germinate. If a cover crop grows too large to till or dig into the soil, cut it off and leave it on the soil surface as mulch or compost it for later use. This will retain the short-term benefit of organic matter from the crowns and roots and from the decomposing mulch.<\/p>\n\n\n\n<p>The benefits of organic matter derived from cover crops last only about one year, so make cover crops an annual part of crop rotation. If cover crops do not fit into an overall garden plan, winter mulches can be used as a substitute. Refer to WSU Extension publication Methods for Successful Cover Crop Management in Your Home Garden for more information on methods of planting and terminating cover crops.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><em>Organic Mulch<\/em><\/h3>\n\n\n\n<p>Some organic materials can be effective mulches. Mulches are applied to the surface of the soil to reduce the loss of water through evaporation, protect the soil surface from erosion, reduce compaction, smother annual weeds, and modify the temperature of the soil. In annual gardens, mulches can be applied after harvest to protect the soil from erosion during the winter, or they can be applied between rows during the growing season to conserve water and reduce weeds. A thin layer of mulch will conserve water, but at least three inches of mulch are needed to smother weeds. Mulches are less effective against perennial weeds, such as horsetail and quackgrass. Straw, leaves, cover crop residues, and compost are effective annual mulches. Since straw and leaves are nitrogen-poor, gardeners may need to add extra N if they dig these mulches into the soil.<\/p>\n\n\n\n<p>Materials such as wood chips, arborist chips, and bark resist decay, so they make effective, long-lasting mulches for permanent landscape beds (Figure 18). As long as these mulches remain on the soil surface, they have little effect on available nutrients in the underlying soil. However, incorporating them into the soil can reduce nitrogen availability for a year or more.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large is-resized wsu-spacing-before--large wsu-spacing-after--large\"><img decoding=\"async\" loading=\"lazy\" src=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-18-1024x768.jpg\" alt=\"Parallel raised garden rows with bark chip paths between them.\" class=\"wp-image-1912\" width=\"1156\" height=\"867\" srcset=\"https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-18-1024x768.jpg 1024w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-18-300x225.jpg 300w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-18-768x576.jpg 768w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-18-1536x1152.jpg 1536w, https:\/\/wpcdn.web.wsu.edu\/extension\/uploads\/sites\/62\/2025\/10\/Figure-18-2048x1536.jpg 2048w\" sizes=\"(max-width: 1156px) 100vw, 1156px\" \/><figcaption class=\"wp-element-caption\">Figure 18. Wood-chip mulch in paths between garden beds helps manage weeds, mud, and dust. Hilltop Urban Gardens, Tacoma, Washington. Photo by Craig Cogger, WSU.<\/figcaption><\/figure>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\" \/>\n\n\n\n<h2 class=\"wp-block-heading  wsu-heading--style-marked\" id=\"ch3-further-reading\">Further Reading<\/h2>\n\n\n\n<p>Bary, A., C. Cogger, and D. Sullivan. 2016. <a href=\"https:\/\/pubs.extension.wsu.edu\/fertilizing-with-manure\" target=\"_blank\" rel=\"noreferrer noopener\">Fertilizing with Manure and Other Organic Amendments (opens in new window)<\/a>. Pacific Northwest Extension Publication PNW533. Washington State University.<\/p>\n\n\n\n<p>Benedict, C., C. Cogger, and N. Andrews. 2014. <a href=\"https:\/\/pubs.extension.wsu.edu\/methods-for-successful-cover-crop-management-in-your-home-garden-home-garden-series\" target=\"_blank\" rel=\"noreferrer noopener\">Methods for Successful Cover Crop Management in Your Home Garden (opens in new window)<\/a>. Washington State University Extension Publication FS119E. Washington State University.<\/p>\n\n\n\n<p>Cogger, C.G. 2010a. <a rel=\"noreferrer noopener\" href=\"https:\/\/puyallup.wsu.edu\/soils\/video_soilsampling\/\" target=\"_blank\">How to Take a Soil Sample (opens in new window)<\/a>, video. Washington State University Extension.<\/p>\n\n\n\n<p>Cogger, C.G. 2010b. <a rel=\"noreferrer noopener\" href=\"https:\/\/puyallup.wsu.edu\/soils\/video_soiltexture\/\" target=\"_blank\">Determining Soil Texture by Hand (opens in new window)<\/a>, video. Washington State University Extension.<\/p>\n\n\n\n<p>Cogger, C.G. 2017. <a href=\"https:\/\/pubs.extension.wsu.edu\/raised-beds-deciding-if-they-benefit-your-vegetable-garden-home-garden-series\" target=\"_blank\" rel=\"noreferrer noopener\">Raised Beds: Deciding If They Benefit Your Vegetable Garden (opens in new window)<\/a>. Washington State University Extension Publication FS075E. Washington State University.<\/p>\n\n\n\n<p>Cogger, C.G. 2019a. <a href=\"https:\/\/pubs.extension.wsu.edu\/home-garden-and-lawn-fertilizer-calculator-home-garden-series\" target=\"_blank\" rel=\"noreferrer noopener\">Garden Fertilizer Calculator (opens in new window)<\/a>. Washington State University Extension Publication FS324E. Washington State University.<\/p>\n\n\n\n<p>Cogger, C.G. 2019b. <a href=\"https:\/\/pubs.extension.wsu.edu\/using-biosolids-in-gardens-and-landscapes-home-garden-series\" target=\"_blank\" rel=\"noreferrer noopener\">Using Biosolids in Gardens and Landscapes (opens in new window)<\/a>. Washington State University Extension Publication FS156E. Washington State University.<\/p>\n\n\n\n<p>Cogger, C.G, C. Benedict, N. Andrews, S. Fransen, and A. McGuire. 2014. <a href=\"https:\/\/pubs.extension.wsu.edu\/cover-crops-for-home-gardens-east-of-the-cascades-home-garden-series\" target=\"_blank\" rel=\"noreferrer noopener\">Cover Crops for Home Gardens East of the Cascades (opens in new window)<\/a>. Washington State University Extension Publication FS117E. Washington State University.<\/p>\n\n\n\n<p>Cogger, C.G., C. Benedict, N. Andrews, and A. McGuire. 2014. <a rel=\"noreferrer noopener\" href=\"https:\/\/pubs.extension.wsu.edu\/cover-crops-for-home-gardens-west-of-the-cascades-home-garden-series\" target=\"_blank\">Cover Crops for Home Gardens West of the Cascades (opens in new window)<\/a>. Washington State University Extension Publication FS111E. Washington State University.<\/p>\n\n\n\n<p>Cogger, C.G., and G. Stahnke. 2019. <a rel=\"noreferrer noopener\" href=\"https:\/\/pubs.extension.wsu.edu\/organic-soil-amendments-in-yards-and-gardens-how-much-is-enough-home-garden-series\" target=\"_blank\">Organic Soil Amendments in Yards and Gardens: How Much Is Enough? (opens in new window)<\/a> Washington State University Extension Publication FS123E. Washington State University.<\/p>\n\n\n\n<p>Cogger, C.G., D.M. Sullivan, and A. Bary. 2017. <a href=\"https:\/\/pubs.extension.wsu.edu\/backyard-composting\" target=\"_blank\" rel=\"noreferrer noopener\">Backyard Composting (opens in new window)<\/a>. Washington State University Extension Publication<em> <\/em>EB1784E. Washington State University.<\/p>\n\n\n\n<p>Fery, M., J. Choate, and E. Murphy. 2018. <a href=\"https:\/\/extension.oregonstate.edu\/catalog\/ec-628-guide-collecting-soil-samples-farms-gardens\" data-type=\"URL\" data-id=\"https:\/\/extension.oregonstate.edu\/catalog\/ec-628-guide-collecting-soil-samples-farms-gardens\" target=\"_blank\" rel=\"noreferrer noopener\">A Guide to Collecting Soil Samples for Farms and Gardens (opens in new window)<\/a>. <em>Oregon State University Extension Service Publication<\/em> EC 628. Oregon State University.<\/p>\n\n\n\n<p>Hart, J., D. Horneck, R. Stevens, N. Bell, and C.G. Cogger. 2003. <a href=\"https:\/\/extension.oregonstate.edu\/sites\/extd8\/files\/documents\/ec1560.pdf\" data-type=\"URL\" data-id=\"https:\/\/extension.oregonstate.edu\/sites\/extd8\/files\/documents\/ec1560.pdf\" target=\"_blank\" rel=\"noreferrer noopener\">Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden West of the Cascade Mountain Range in Oregon and Washington (link to PDF document)<\/a>. <em>Oregon State University Extension Service Publication <\/em> EC 1560-E. Oregon State University.<\/p>\n\n\n\n<p>Horneck, D.A., D.M. Sullivan, J.S. Owen, and J.M. Hart. 2011. <a href=\"https:\/\/extension.oregonstate.edu\/catalog\/ec-1478-soil-test-interpretation-guide\" data-type=\"URL\" data-id=\"https:\/\/extension.oregonstate.edu\/catalog\/ec-1478-soil-test-interpretation-guide\" target=\"_blank\" rel=\"noreferrer noopener\">Soil Test Interpretation Guide (opens in new window)<\/a>. <em>Oregon State University Extension Service Publication <\/em>EC 1478. Oregon State University.<\/p>\n\n\n\n<p>Locke, K., D. Horneck, J. Hart, and R. Stevens. 2006. <a href=\"https:\/\/extension.oregonstate.edu\/catalog\/ec-1585-acidifying-soil-landscapes-gardens-east-cascades\" data-type=\"URL\" data-id=\"https:\/\/extension.oregonstate.edu\/catalog\/ec-1585-acidifying-soil-landscapes-gardens-east-cascades\" target=\"_blank\" rel=\"noreferrer noopener\">Acidifying Soil in Landscapes and Gardens East of the Cascades (opens in new window)<\/a>. <em>Oregon State University Extension Service Publication <\/em>EC 1585-E. Oregon State University.<\/p>\n\n\n\n<p>Miles, C., G. Sterrett, L. Hesnault, C. Benedict, and C. Daniels. 2013. <a href=\"https:\/\/pubs.extension.wsu.edu\/home-vegetable-gardening-in-washington-home-garden-series\" target=\"_blank\" rel=\"noreferrer noopener\">Home Vegetable Gardening in Washington (opens in new window)<\/a>. <em>Washington State University Extension Publication <\/em>EM057E. Washington State University.<\/p>\n\n\n\n<p>Peters, T. 2011. <a href=\"https:\/\/pubs.extension.wsu.edu\/drip-irrigation-for-the-yard-and-garden\" target=\"_blank\" rel=\"noreferrer noopener\">Drip Irrigation for the Yard and Garden (opens in new window)<\/a>. <em>Washington State University Extension Publication<\/em> FS030E. Washington State University.<\/p>\n\n\n\n<p>Rosen, C.J., and R. Eliason. 2005. <a href=\"https:\/\/conservancy.umn.edu\/handle\/11299\/51272\" target=\"_blank\" rel=\"noreferrer noopener\">Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota (opens in new window)<\/a>. <em>University of Minnesota Extension Service<\/em> BU-05886. University of Minnesota.<\/p>\n\n\n\n<p>Shober, A.L., and G.C. Denny. 2017. <a href=\"https:\/\/www.udel.edu\/academics\/colleges\/canr\/cooperative-extension\/fact-sheets\/identifying-nutrient-deficiencies-in-ornamental-plants\/\" target=\"_blank\" rel=\"noreferrer noopener\">Identifying Nutrient Deficiencies in Ornamental Plants (opens in new window)<\/a>. <em>University of Delaware Cooperative Extension<\/em>. University of Delaware.<\/p>\n\n\n\n<p>Sullivan, D.M., E. Peachey, A.L. Heinrich, and L.J. Brewer. 2017. <a href=\"https:\/\/catalog.extension.oregonstate.edu\/em9165\" target=\"_blank\" rel=\"noreferrer noopener\">Nutrient Management for Sustainable Vegetable Production Systems in Western Oregon (opens in new window)<\/a>. <em>Oregon State University Extension<\/em> <em>Publication<\/em> EM 9165. Oregon State University.<\/p>\n\n\n\n<p>USDA NRCS (Natural Resources Conservation Service). 1998. <a href=\"https:\/\/nrcspad.sc.egov.usda.gov\/DistributionCenter\/pdf.aspx?productID=199\" target=\"_blank\" rel=\"noreferrer noopener\">Estimating Soil Moisture by Feel and Appearance (opens in new window)<\/a>. Program Aid 1619.<\/p>\n\n\n\n<p><a href=\"http:\/\/analyticallabs.puyallup.wsu.edu\/analyticallabs\/\" target=\"_blank\" rel=\"noreferrer noopener\">Washington State Pest Management Resource Service (opens in new window)<\/a>. n.d. Analytical Laboratories and Consultants Serving Agriculture in the Pacific Northwest. Washington State University Extension.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Craig Cogger, Soil Scientist Emeritus, Department of Crop and Soil Sciences, Washington State University Soil Components and Soil Horizons Soil (opens in new window) is a mixture of weathered rock fragments and organic matter at the earth\u2019s surface. It is biologically active\u2014a home to countless microorganisms, invertebrates, and plant roots. It varies in depth from [&hellip;]<\/p>\n","protected":false},"author":241,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_wsuwp_accessibility_report":null},"categories":[],"tags":[],"_links":{"self":[{"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/pages\/639"}],"collection":[{"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/users\/241"}],"replies":[{"embeddable":true,"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/comments?post=639"}],"version-history":[{"count":88,"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/pages\/639\/revisions"}],"predecessor-version":[{"id":4784,"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/pages\/639\/revisions\/4784"}],"wp:attachment":[{"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/media?parent=639"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/categories?post=639"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/extension.wsu.edu\/pnw-gardeners-handbook\/wp-json\/wp\/v2\/tags?post=639"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}