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- Author or Editor: Rufus L. Chaney x
Depending on the materials used to produce a compost, it will contain lower or higher levels of nutrients and metals. If composts have been appropriately matured, nutrients are in plant-available forms for crop production, and the compost pH will be near neutral. After 25 years of research and development of regulations and advice for biosolids and compost utilization, pretreatment of industrial wastes allows biosolids composts, and composts prepared from biosolids mixed with municipal solid wastes or yard debris to contain levels of microelements needed for plant nutrition but not high levels that could cause phytotoxicity. Composts can supply N, P, K, Ca, Mg, Fe, Zn, Cu, Mn, B, Mo, and Se required by plants or animals. When used in potting media, supplemental N fertilization is usually required, depending on crop requirements. Use of compost can replace other forms of microelements used as fertilizers in media or fields. Detailed evaluation of potential food chain transfer of Cd, Pb, and other elements in composts clearly shows that consumption of 60% of garden foods produced on pH 5.5 soils with 1000 t compost/ha would not comprise risk over a lifetime of consumption, nor would ingesting the composts at 200 mg/day for 5 years. Potentially toxic organic compounds are either destroyed during composting, or bound very strongly by the compost so that plant uptake is trivial. Compost use can be a safe and wise choice for both home and commercial use to replace peat or uncomposted manures, etc. Many states have developed regulatory controls to assure that pathogenic organisms are killed during composting, and that product quality standards are attained that allow marketing for general use in the community.
Marigold was grown on 4 media treatments with different levels of composted digested sewage sludge. On each medium, a number of fertilizer amendment treatments were studied to evaluate compost as a replacement for the amendments ordinarily required for complete media. Media contained 0, 33, 67, and 100% sludge compost (by volume); the remainder was unfertilized Cornell mix (equal volume mixture of sphagnum peat moss and vermiculite). The recommended amendments for Cornell mix (N, P, limestone, and trace elements) were deleted one at a time, all, or all but N. Severe P deficiency of −P media was fully corrected by compost at all rates. Compost provided only part of the N requirement at 33%. All trace element requirements were supplied by compost, yet toxicities did not occur; compost-amended media pH was ≥ 6.7, which limited metal availability to plants. Soluble salts limited yield somewhat on all treatments containing 67 or 100% compost. Fertilization of 33% compost media with only KNO3 (recommended rate for Cornell mix) allowed plant performance equal to the complete Cornell mix. Compost supplied much higher amounts of Fe, Zn, and Cu than are ordinarily added to media, and corrected an apparent marginal Cu deficiency-stress of complete Cornell mix. Composted digested sewage sludge was found to be an effective ingredient for potting media for marigolds when N is supplied from chemical sources.
When plant nutrition problems are observed in the field, one is faced with the question “What is the best and most economical way to solve this problem?” Traditionally, workers in both agronomy and horticulture have used soil amendments to correct deficiencies of macro- and micronutrients, and to correct soil pH to avoid Al or Mn toxicity. Horticulturists have had few economic limitations in solving plant nutrition problems because they work with crops with higher production costs and potential profit. Philosophically, we must recognize that some nutrients are removed from soils by cropping, and these must be replaced eventually. We can remove stored nutrients from the soil, but this reduces soil fertility. For elements such as Zn, Cu, Mn, B, and Co, addition of elemental fertilizers is both effective and inexpensive in nearly all cases. Boron, Cu, and Zn fertilization are normal horticultural management practices. Soil testing or plant analysis can identify potential microelement fertility problems and deficiencies can be avoided by timely fertilizer application. Similarly, the pH of the surface soil can be economically raised by limestone to reduce the availability of some toxic ions such as Al and Mn. This approach has been called “Change the soil”.