Nitrogen and Phosphorus Balance Indicators in Organic Soils

106Nutrient losses from agroecosystems depend on the amount of water discharge, soil type, and management practices. This chapter presents N and P indicators of organic soil quality as related to water quality and crop fertilization. Organic soils contain 5 to 27 Mg organic N ha⁻¹ in the arable layer, which could release 800 to 1500 kg NO₃-N ha⁻¹ a⁻¹, depending primarily on C/N ratio and pH. Because 12 to 245 kg NO₃-N ha⁻¹ a⁻¹ could be discharged, and because crop removal cannot account for residual N, most of the N must be denitrified. Organic soils contain in average 700 to 1100 mg P kg⁻¹, of which 67 to 78% is reported to be organic P, inorganic P being more or less chemically sorbed. One to 88 kg P ha⁻¹ would be discharged annually, indicating high potential risk for eutrophication. Due to the predominance of organic forms, N and P microbial turnovers should be diagnosed, possibly using N and P ratios or multiratios (e.g., C/N, N/P, C/N/P, C/N/P/S). An inorganic P sorption or saturation index, as well as N and P turnover attributes as multiratios, should be developed for planning N and P fertilization programs.