MAKING COMPOSTS WITH LOWER WATER-SOLUBLE P AND HIGHER NUTRIENT DENSITY

Authors: Chaney, Rufus; Sikora, Lawrence; DAVIS, ALLEN - UMD, COLLEGE PARK, MD; KIM, HUNHO - UMD, COLLEGE PARK, MD; Codling, Eton; Novak, Jeffrey; TYLER, ROD - GREEN HORIZONS, OH

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/20/2004
Publication Date: 1/24/2005
Citation: Chaney, R.L., Sikora, L.J., Davis, A.P., Kim, H., Codling, E.E., Novak, J.M., Tyler, R. 2005. Making composts with lower water-soluble p and higher nutrient density. Proceedings of the US Composting Council Annual Meeting. p. 1-15.

Interpretive Summary: Land application of manures and composts as N fertilizers adds much more phosphorus than plants require, so P accumulates in amended soils. Because P runoff or leaching to surface water contributes to potential for eutrophication, limits on soil P are being developed. On the other hand, application of composts at high rates offer the ability to remediate infertile, disturbed, and phytotoxic soils. Thus there is need to determine how compost manufacturers should consider management of P. Composting can hydrolyze phytate in manure, increasing the solubility of manure P, so other solutions are needed to reduce P solubility during composting. Studies of biosolids and biosolids composts showed that high Fe in some biosolids could remarkably increase P adsorption by the biosolids. Both Fe and Al oxides increase adsorption of P, and these can be added as chemical amendments or by mixing byproducts rich in Fe or Al with the manure or biosolids before composting. The many factors which affect the potential for P runoff or leaching are discussed, with emphasis on the method used to analyze soluble P. Although some have recommended using soil test methods to evaluate composts and soils, it is evident that higher P amounts are extracted with these methods than is extracted by water. And water extractable P is better related to the potential for manure or compost P to enter runoff or leaching. Further, the addition of Fe and Mn rich byproducts during composting can allow production of composts with reduced P solubility and higher Fe and Mn fertility. When Fe is added to reduce P solubility, Mn should be added to assure that application does not induce Mn deficiency in Coastal Plain soils. Because Coastal Plain soils are so low in Fe and Al, P and trace elements often leach down the soil; thus application of manure or composts with added Fe, Al, and Mn could substantially reduce the potential release of P from these soils.

Technical Abstract: Limits on soluble soil phosphate and soluble P in manure, composts, and by-products to protect against eutrophication of surface waters will require the composting industry to come to grips with the low N to P ratio of composts. Because the N:P ratio is lower for manure and composts than crops or chemical fertilizers, soil P accumulates more rapidly and would bring prohibition of all fertilizer-P additions until available soil P has declined. On the other hand, high levels of slowly soluble N and P are important benefits of compost products that make composts especially valuable in remediation of infertile or contaminated soils. One approach to aid compliance with expected P limits would be to incorporate P adsorbents into composts so the water solubility of compost-P would be much lower. Composts made entirely from plant debris have low levels of P-adsorbents. Inclusion of feedstocks that add hydrous Fe, Al and Mn oxides can yield compost products with both the normal benefits of composts, but lower soluble P, and fertilizer value of Fe and Mn. Other approaches to limit water soluble P are noted, including new High Available Phosphate grains that can be used to prepare feeds that do not require P supplements, thus lowering total P in manure or composts. Addition of phytase enzyme to feeds can make organic phytate-P more bioavailable for the livestock and more soluble in manure; both HAP grains and addition of phytase may reduce the levels of Cu and Zn addition to non-ruminant feeds. All alternatives may influence manure and compost management to comply with soluble P limits in soils. High Fe composts may even support edge of field filtration to remove phosphate from runoff water. In existing papers, we have shown that adding certain agricultural and industrial byproducts to manures can keep P solubility low and reduce the bioavailability of Pb in soil ingested by children. Composts rich in Fe and P aid formation of insoluble Pb pyromorphite in soil, while strong turfgrass limits soil ingestion by young children. Central to development and commercialization of this concept is better information on which byproducts are most cost-effective for addition during composting to lower compost P solubility and produce higher value composts. We have been evaluating levels and forms of Fe and Mn in by-products and evaluating their ability to reduce P solubility and increase heavy metal adsorption by organic amendments. Al addition can also reduce P solubility, but Al is not a nutrient addition. Fe-rich low P-solubility compost products need to have some Mn along with Fe so that soil fertility is maintained or improved. Trace element limits for byproducts to be included in mixed compost feedstocks may require monitoring or management decisions to maintain production of high quality Tailor Made Composts which are especially useful in soil remediation.