Losses and Utilization of Nitrogen by Sorghum as Affected by the Depth of a Swelling Clay Soil

Chromusterts and Pellusterts located in India represent 20 % of the 257 million ha of Vertisols found worldwide. These soils are commonly associated with soils that exhibit vertick characteristics, such as Ustochrepts and Ustropepts, but that are too shallow (< 50 cm) to be classified as Vertisols; India has 20 million ha of such soils. Nitrogen is generally deficient in these soils, irrespective of their depth. N response of sorghum on the Vertisols is good even under rainfed conditions. Over a period of 5 years (1982, 83, 84, 85, and 87) the responses of sorghum to applied N on a deep Pellustert and a shallow vertic Ustochrept were compared and, with the use of labelled fertilizer, the fate of N was traced through the soil-plant system. It is clear from the results that, when weather conditions were favorable, response to applied N was excellent and losses were low, irrespective of soil depth. However, when periods of excessive rainfall occurred, the loss of native and applied N apparently increased rapidly, particularly in the shallow Ustochrept where rooting is largely limited to the top 50 cm. Losses of N from applied urea were up to threefold higher in the shallow Ustochrept than in the deep Pellustert; these losses, together with the rapid loss of applied NO₃, suggest that leaching is the predominant cause of N loss. Nitrogen application rates should be adjusted on the basis of rainfall during the early part of the growing season. Due to the unpredictability of the rainfall pattern following fertilizer application, NO₃ sources should be avoided and nitrogen split as frequently as possible on the shallow vertic Ustochrepts of the Indian semi-arid tropics.