A greenhouse pot experiment was conducted with various rates of manure (0, 20, 40, and 60 metric tons/ha) and sludge (0, 10, 20, and 30 metric tons/ha) combined factorially with 0, 50, 100, 150, and 200 mg N/kg soil as (NH4)2SO4 labeled with 15N, using barley and sudangrass as test crops. Most of the N from (NH4)2SO4 was recovered by the first cutting of barley forage (from 37.2 to 70.2%); the N from (NH4)2SO4 recovered by sudangrass, the last crop of the cropping sequence, ranged from 0.7 to 8.9%. No significant effect of both wastes and (NH4)2SO4 rates on the overall inorganic N fertilizer recovered by the crops was found. The difference method underestimated the (NH4)2SO4-N recoveries by the crops under sludge treatments. The depression in the mineralization rate of sludge by the (NH4)2SO4 applied was proposed to account for these discrepancies.
A fertilizer N balance, in which the sum of (NH4)2SO4-N removed by the crops plus the N accumulated in the soil was subtracted from the (NH4)2SO4-N input, was used to estimate the fertilizer N losses. These losses were 7.8, 9.1, and 13.1%, respectively, for control (unamended), manure and sludge treatments.
With increasing rates of (NH4)2SO4, the nitrogen “A” values defined as the N available in the soil in terms of N from (NH4)2SO4, increased in the control, remained almost constant with manure, and decreased with sludge treatments.