A Field Study of the Agricultural Use of Sewage Sludge: II. Effect on Soil N and P

Liquid digested sewage sludge applied at rates from 3.75 to 60 metric ton/ha significantly increased the concentrations of inorganic N, organic N, and Bray P 1 extractable P in a sandy loam and a silt loam soil in south‐central Wisconsin. Most of the applied inorganic N was as NH 4 ‐N and was nitrified rapidly. At sludge rates of 30 metric ton/ha or more, substantial losses of sludge‐applied N occurred by leaching. Soil solution NO 3 ‐N concentrations exceeded 100 mg/liter at the 120‐ to 150‐cm depth 10 weeks after the first sludge application on the permeable sandy loam, and after about 1 year on the silt loam. The N balance indicated that considerable amounts of N may have been lost by denitrification, volatilization, or both, where > 30 metric ton/ha were applied. Mineralization of organic N was more rapid than previously reported. Up to 50% of the applied organic N was mineralized within 3 weeks after the last sludge application. During the remaining 25 months of the experiment, the mineralization rate was essentially constant at about 250 mg organic N/kg of soil/year at the 60 metric ton/ha sludge rate, and 180 mg organic N/kg of soil/year at the 30 metric ton/ha rate. Sludge applications resulted in immediate marked increases in Bray P 1 extractable P. However, the extractable P levels decreased with time after sludge application, probably as a result of P fixation. Significant amounts of P remained available for at least 2 years at the higher treatment rates.