y 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 plants 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.