Land Application of Chemically Treated Sewage Sludge: I. Effects on Crop Yield and Nitrogen Availability

Anaerobically digested sewage sludges resulting from treatment of sewage with Ca(OH)₂, Al₂(SO₄)₃, or FeCl, for phosphorus removal were applied to land on which corn (Zea mays L.) and bromegrass (Bromus inermis Leyess) were grown. The rates of application supplied 200, 400,800, and 1,600 kg N/ha each year for 3 years. Treatments with NH₄NO₃ supplying 0, 100, 200, and 400 kg N/ha were included. Bromegrass yields were increased by sludge application supplying up to 800 kg N/ha and by NH₄NO₃ application supplying up to 400 kg N/ha. There was an appreciable residual effect of sludge nitrogen on the grass yield. The highest rate of sludge produced hazardous levels of NO₃⁻ in the grass in the third year. Corn experiments were conducted in three soils: a loam, a loamy sand, and a clay loam. On the loam and clay loam soils, there was no further increase in the yield of corn with rates of sludge in excess of 200 kg N/ha or NH₄NO₃ in excess of 100 kg N/ha. There was little or no difference in corn yield between the sludges used on the loam soil. On the clay loam, the Casludge appeared to benefit the corn most. There was no yield response to sludge nor to NH₄NO₃ on the loamy sand. Nitrate concentration in corn stover was increased by high rates of either NH₄NO₃ or sludge, and in 1975 on the loamy sand soil nitrate concentrations approached levels hazardous to livestock. In both the corn and grass experiments, 800 kg N/ha from sludge resulted in as much NO₃⁻ in the top 15 cm of soil as 400 kg N/ha from NH₄NO₃. High concentrations of NO₃⁻ were present in the soil to the 90-cm depth in the grass experiment after 3 years of sludge application at rates supplying 800 kg N/ha but not at 400 kg N/ha. With corn on the loam and loamy sand, high NO₃⁻ concentrations were present in the soil to about the 75-cm depth after 3 years of sludge application at rates supplying 400 kg N/ha.