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Harry A. J. Hoitink, The Ohio State University, Ohio Agricultural Research and Development Center, Plant Pathology; Daniel A. Herms, Pierluigi Bonello, |
Fertility Values of Composts
Fertility affects disease severity. The emphasis here is on nitrogen availability because it varies most severely among nutrients released from composts and mulches and has the greatest impact on plant disease severity. Composted biosolids, cow and swine manures, typically contain 1.7 to 2.5% total nitrogen (dry wt.), or even more. Typically, 20 to 25% of the total nitrogen in these materials is released within the first three months after utilization. The remainder is released over a long period of time. The fertility values of these products obviously must be considered.
Composted biosolids or manures should be incorporated into container media at not more than 8 to 12% by volume. At higher levels, they can have toxic effects. Composted poultry manure should be applied as a top dressing. It may contain 3 to 5% nitrogen. An inch of composted biosolids tilled into the top four inches of soil prepares an ideal seed bed for turf grasses. Low fertility crops such as rhododendrons should be treated with very low quantities of this type of compost. If excessive quantities are applied, then the plant becomes too succulent and this aggravates foliar and dieback diseases.
Leaves composted passively in piles for two years with infrequent turning tend to yield a product similar to that composted in windrows for one year with frequent turning. Passively composted leaves applied three inches deep on a sandy soil, followed by reapplications of one inch per year, provide many nutrients required for most crops. The concentration of nitrate in ground water under these plots remains below 10 ppm. It is clear, however, that compost loading rates need to be reduced after several years of applications due to release of nitrate and other nutrients from the more resistant to decomposition organic matter which accumulates in treated soils over time.
The concentration of total nitrogen in composted yard wastes may range from 0.7% to 2.0%. Fresh wood and bark contain only 0.1 to 0.3% N. Composted yard waste with a nitrogen concentration lower than 1.4% does not release significant quantities of nitrogen immediately after its incorporation. Batches of composts even lower in N should only be used as mulches or composted further to reduce the carbon content and thus the C:N ratio. Composted yard wastes with a total N concentration of 1.9% have provided excellent growth and suppress root diseases and in some instances also foliar diseases, whether incorporated into soil or used as mulches.