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Washington State University

Can Compost Damage Plants?

Reprinted with permission from BIOCYCLE, September 2000, Volume 41, No. 9

Q: If compost damages plants, what are the possible reasons and symptoms? Are herbicide residues in compost a problem? (Editors note: This question complements the article about herbicide residues in compost on page 42.)

A: Given the multitude of benefits that compost provides to plants and soils, it is hard to accept that the use of compost can result in plant damage. Although rare, it is possible, especially if compost is not properly prepared or it is mismatched to its application. The risk of damage increases if the compost is used at high rates or with sensitive plants, such as seedlings. When compost injures plants, a gut reaction is to blame a contaminant, like an herbicide. However, other causes are more likely at work. The following are other factors that can turn a particular compost from a benefit into a bane. They are divided into those factors that produce acute and chronic effects.

Acute Effects — Nearly immediate response by plants

High Salinity (Salts) — High salinity may be the most frequent cause of compost problems. Plants obtain nutrients from soils in the form of dissolved salts, but too high of a concentration injures plant roots. Symptoms of salt damage include rapid wilting of the plant and yellowing of leaves. High salinity weakens plants and often leads to root rot, particularly those caused by Phytophthora and Pythium. Soybeans, rhododendrons and many other plants may be affected. Although some types or salts are more damaging, it is the total salt concentration that is critical. Because plants differ greatly in sensitivity to salinity, it is best to follow guidelines for each crop. In general, however, a conductivity value in compost-amended substrates (soil or potting media) that exceeds 3-4 mmhos/cm (based on the saturated paste method) leads to phytotoxicity unless the substrate is leached first. Salt levels in compost vary greatly with the feedstocks and tend to be highest in manure-derived composts. In general, most feedstocks produce compost with salinity levels greater than 4 mmhos/cm. Salinity can be lowered by mixing compost into the soil or with other low salinity materials (including other types of composts such as tree bark) or by leaching with water. Compost with high salts levels must be applied well ahead of planting (fall or mid winter) to allow for leaching and avoid root diseases. Potting mixes must be prepared to correct specifications because leaching before planting typically is not possible. In general, do not use more composted manure than required to supply adequate concentrations of available essential nutrients. Salinity should be a standard quality control parameter. It is easy to measure in compost with conductivity meters.

Ammonia Toxicity — Although ammonia is a source of plant nitrogen, high concentrations are phytotoxic. Affected plants die overnight if exposed to high levels. Lower levels typically cause burning on the margins of young leaves. On less sensitive plants, root tips turn brown. Azaleas are very sensitive indicator plants. The risk of ammonia toxicity is greatest with composts that have a high nitrogen content (low C:N ratio), especially if the nitrogen is in an available form. The remedy, and wise practice generally, is to apply compost at or below agronomic rates (i.e. no more nitrogen than the plant can use).

Organic (Fermentation) Acids — Organic acids occur in compost that is immature (not sufficiently decomposed) or that has been processed or stored under anaerobic conditions (fermentation), even for a short time. Many organic acids are phytotoxic, often producing nearly immediate injury to plants that shows white or chlorotic (bleached) leaves. In composts with a high C:N ratio (>40:1), acetic acid is the primary organic acid formed (along with some alcohols). In composts with low C:N ratios, anaerobic conditions yield ammoniacal products (putrid odors) that cause the toxicity. A drop or increase in pH often is not a reliable indicator of the toxicity problem because these compounds decompose quickly (24 to 48 hr) after causing injury. The first line of defense is to use mature compost. However, even mature compost products can develop organic acids if stored or shipped under anaerobic conditions (the likelihood decreases with maturity). Many organic acids have a sharp odor that signals their presence. Tests that indicate maturity and bioassays can be used to detect the presence of organic acids also. However, in a relatively mature compost, the organic acids will decompose or dissipate if the sample is aerated. As a precaution, allow compost stored in piles or in a sealed truck to aerate for several days prior to use or sale. Also, inform customers about proper storage.

Chronic Effects — Symptoms slower to develop and often complicated by interactions with other factors

High C:N Ratio — A compost that has a high carbon to nitrogen (70:1) ratio can cause nitrogen immobilization and therefore nitrogen deficiency in plants. The microorganisms decomposing the excess carbon monopolize the available nitrogen, leaving the plants in short supply. Symptoms typical of nitrogen deficiency are yellow leaf color in the new growth and slow growth. High C:N ratio composts should be used as mulches and supplemental N may have to be provided to overcome short-term N immobilization. While nitrogen is nearly always the limiting nutrient, a relatively large proportion of available C may also affect the availability of other nutrients if they are present at limiting levels.

Phytotoxicity From Excess Minerals — Certain feedstocks can supply compost with an extra heavy load of certain minerals. Beneficial in low concentrations, some minerals cause toxicity on plants at high concentrations. They are detrimental not by their presence, but because they interfere with the plant’s ability to access other nutrients. Symptoms tend to be subtle and show themselves over a longer time period. They vary with the plant species and specific mineral in excess. Boron toxicity is the most widely known example that can cause crop losses. Excessive boron can be introduced with feedstocks like coal ash and cellulose insulation, etc. Much already has been done in the U.S. to prevent these toxicity problems. Compost standards will resolve these issues. Compost should be tested for specific minerals if the mix of feedstocks indicates that phytotoxic concentrations may result.

High pH — Perhaps the worst chronic problems are caused by lime-stabilized composted sewage sludges or compost products that have been treated with minerals that gradually increase the pH of treated soils. These problems often are not detected in blends of composts containing materials that gradually raise the pH. Although many agricultural crops benefit from liming, most landscape plants do not and prefer a soil pH of 5-6. Lime stabilized products should be applied at rates that do not exceed the need for the crop and this frequently is not realized until it is too late. It is all but impossible to lower the pH of some affected soils and crop failure is the result. In summary, only purchase composts if performance data are supplied as well.

Answer by: Harry Hoitink, professor of plant pathology, Ohio State University, and Robert Rynk, BioCycle.