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.

A greenhouse pot experiment in a Hanford soil (Typic Xerorthent) with rates of feedlot manure at 0, 20, 40, and 60 metric tons/ha and sewage sludge at rates of 0, 10, 20, and 30 metric tons/ha combined factorially with 0, 50, 100, 150, and 200 mg N/kg of soil as (NH4)2SO4 was conducted using barley (Hordeum vulgare L.) and sudangrass (Sorghum sudanense (Piper) Stapf.) as test crops. The objectives were to measure the net mineralization of N in the organic materials, to compare the availability of N in the organic materials with the N in (NH4)2SO4 and to determine the critical levels for N and NO3 in barley. The available N from the organic materials was estimated from the amounts required to equal the N in (NH4)2SO4 in terms of barley forage production. Net mineralization was determined by soil analyses. Yields of barley forage were increased by additions of organic materials and (NH4)2SO4. The addition of either manure or sludge decreased the response to (NH4)2SO4. In a 2.5‐month period after application, the manure gave an average of 0.65 kg of available N/ton, whereas the sludge gave 7.05 kg of available N/ton. This represents 4.2% of the total N from the manure and 17.0% of the total N in the sludge. In a 10‐month period only 17.2 and 40.9% of the N in manure and sludge, respectively, had mineralized. The estimated critical levels of total N and NO3−‐N in barley forage were 2.0 and 0.12%, respectively. These values were independent of the N source. The minimum content of total N in barley forage at which NO3−‐N began to increase was about 2.0%. The level of NO3−‐N in plant tissue was more sensitive than total N to changes in N supply.