How Much Water Does Your Soil Hold?
Estimated By Texture
Soil holds much more water than the amounts listed below. Best tree growth and fruit development occurs when the orchard is irrigated before the soil dries to 50% of its water holding capacity. The first 50% of the stored water in the soil can be considered “usable” by the trees. Water stress becomes excessive if more than that amount is used between irrigations. The amount of water listed below is an approximation of the “usable water” in soil. Use this estimated amount as a starting point in your irrigation scheduling. The actual amount of water stored in your orchard soil depends on many factors, including compaction, soil layers and the percentage of stones and gravel.
Assume about 2 feet of root zone in young trees or those with very dwarfing rootstocks. Older trees with vigorous rootstocks concentrate most of their roots in the top 3 feet of soil.
Approximate “Usable Water” in your orchard soil:
|SOIL TEXTURE Per Foot 3 ft. Root Zone|
|Coarse Sand .46 inches 1.38 inches|
|Sand .62 in. 1.88 in.|
|Fine Sand .70 in. 2.10 in.|
|Loamy Sand .72 in. 2.15 in.|
|Loamy Fine Sand .75 in. 2.24 in.|
|Sandy Loam .77 in. 2.32 in.|
|Fine Sandy Loam .95 in. 2.83 in.|
|V Fine Sandy Loam 1.00 in. 3.00 in.|
|Loam 1.10 in. 3.31 in.|
|Silt Loam 1.22 in. 3.65 in.|
|Silty Clay Loam 1.11 in. 3.33 in.|
|Sandy Clay Loam 1.02 in. 3.07 in.|
|Clay Loamy 1.02 in. 3.08 in.|
|Silty Clay, Clay .95 in. 2.83 in.|
Estimated “Useable Water” In Orchards
by Soil Series
|SOIL SERIES TOP THREE FEET|
|Quincy Coarse Sand 1.4|
|Quincy Fine Sand 1.6|
|Quincy Loamy Fine Sand 2.2|
|*Chelan Gravelly Sandy Loam 1.75|
|Pogue Fine Sandy Loam 2.0|
|Cashmont Sandy Loam 2.3|
|Cashmere Sandy Loam 2.7|
|Burch Fine Sandy Loam 3.1|
|Burch Loam 3.5|
|Ellisforde Silt Loam 3.5|
|Stemilt Silt Loam 3.0|
It is difficult to determine the “usable water” in an orchard based on soil texture alone. The amount of water held in the soil may be affected by other physical soil factors, such as the percentage of gravel or larger stones, the degree of compaction, soil layers that restrict root growth, or radical changes in soil texture by depth. Each area of soil has its’ unique character, and variable ability to hold water for tree use. There is a strong tendency for finer textured soils to hold more water than those with sandy, coarse texture. However, a deep, uncompacted sand may provide more available water to the tree than a shallow, rocky loam.
The actual amount of water stored in your soil must be determined over time, by observation. Your calculated usable water estimate should results in proper soil moisture remaining in the soil on the day before you calculate the need for irrigation. In most soils, your observation of the soil brought up from the top three feet with a probe will provide an adequate evaluation of it’s readiness for irrigation. In mature, deep rooted orchards, it’s time to irrigate when the top foot near the tree is moderately dry, the second foot is moderately moist (about 50% field capacity), and the third foot is moist.
Average Water Use per Day by Orchards in North Central Washington:
|EARLY APRIL||.04 Inch /Day||.05
To determine the approximate number of days that may pass between irrigation sets:
Divide the number from the table that relates to the time of season and the weather conditions into the number of acre inches of usable water stored in your orchards soil, or the net water applied per set, whichever is least. EXAMPLE: Your orchard soil has the ability to store 2.4 acre inches of water in the trees effective root zone. You have evaluated the irrigation system, and have determined that it applies a net 2.2 acre inches of water per irrigation set. It is early June, the weather is warmer than normal, and you want to determine the approximate number of days that should pass between sets in your orchard. Look to the table under the Early June-warm column. The average daily use under these conditions is about .26 inches per day. Divide that value into the net 2.2 inches of water you apply per set ( which is less than your soils’ usable water holding capacity). You determine that about 8.5 days should pass between irrigation sets in that block. Start irrigating as soon as 9 days have passed since the day you last started. Worksheet:
For each irrigation unit:
Step 1: What is your soils’ estimated “Usable water”? —-> __________
What is your “Net applied water per set”? ___________
Which of these two amounts is the least? A.__________
Step 2: Determine the set interval.
Do you think the weather over the next several days will be cooler than average, average, or warmer than average? What is the average “Water use per day” for this time of season under these general weather conditions?
Put the daily use rate from the proper column here:—B._________
Divide the amount in space “A” by the decimal in space “B” to determine the approximate number of days that may pass between sets in that irrigation unit.
Example: It is late May, and the weather is cooler than average. Your estimated usable water is 1.9 inches. Your system applies 2.1 inches per set. Referring to the Average water use chart, you see that about .17 acre inches of water are used by the orchard daily under the present weather conditions. You divide .17 into 1.9 usable inches of water, (which is less than the 2.1″ net applied water) and determine that about 11 days may pass between sets.
You probe the soil after 11 days and feel that it is too moist to require irrigation for another day or two. You irrigate when ready, but add another 10% to your usable water estimate, bringing it up to 2.1 inches. It is now early June, and the weather is about normal. Divide .23 ( the daily use rate for average-early June) into your new estimate of 2.1 inches of usable water. You find that about 9 days can pass between sets. After 9 days, you check the soil prior to irrigating, and find it in the moisture condition you prefer. You irrigate, and continue to use 2.1 inches as your usable water estimate during the rest of the season. Your set interval becomes shorter during July, then lengthens as Fall approaches, but you feel that the soil is staying adequately moist between sets.