The effect of different ammonical nitrogen sources on soil acidification

Consecutive greenhouse cropping of wheat-maize-wheat-maize-wheat grown to maturity in 3 years was conducted to evaluate soil acidification by ammonium sulfate, urea, and ammonium nitrate. Three soils were incorporated with 100 kg N/ha for wheat and 200 kg N/ha for maize and the last crop (wheat). Soil samples of the checks after fourth and last crops were used to estimate relative lime requirement by the (i) Adams-Evans buffer method and (ii) soil extraction with Ca-acetate (pH 8.0) followed by back titration with NaOH to pH 8.0. Another method used two N-treated soil samples after the last crop and involved adding various CaCO(3) rates to soil-water mixtures and shaking for 2 weeks. Soil pH after each crop followed the order of ammonium sulfate < ammonium nitrate = urea < check. Soil acidification decreased with increasing soil clay and organic matter contents. The average relative lime requirements determined by all the methods were 1.6 to 2.3 and 1.4 to 1.9 times of ammonium nitrate and urea, respectively. These values are significantly lower than the Association of Official Agricultural Chemists' value that ammonium sulfate is 3.0 times more acidic than ammonium nitrate and urea.