Original Articles

Effects of sample drying temperature on indices of available nitrogen in soils

Published in: South African Journal of Plant and Soil
Volume 11, issue 1, 1994 , pages: 34–40
DOI: 10.1080/02571862.1994.10634290
Author(s): G.H. WiltshireDepartment of Soil Science, Republic of South Africa, C.C. du PreezDepartment of Soil Science, Republic of South Africa


During a field trial investigation into the long-term effects of conservation practices on the nitrogen fertility of a soil cropped annually to wheat, it was observed that sample preparation had an effect on some indices of available nitrogen. The trial included 36 treatments, namely, all combinations of straw burning (burnt or unburnt), three primary cultivation methods (ploughing, stubble mulch or no cultivation), two weed control methods (mechanical or chemical) and three levels of nitrogen fertilization (20, 30 or 40 kg N ha−1). Soil from the 30 kg N ha−1 plots was analysed for water content, residual inorganic nitrogen and mineralizable nitrogen before drying (field-moist), after drying at room temperature for one week (air-dried), and after drying for a further week at about 40°C (40°C-dried). The field water content, but not the water content of the dried samples taken in May was reduced by burning straw in December. Ploughing decreased the water content of dried samples from the 0 - 50-mm layer and increased that of samples from the 50 - 150 and 150 - 250-mm layers. There was some loss of residual nitrate and gain of ammonium during air-drying. Residual inorganic and mineralizable nitrogen contents were increased more by 40°C-drying of the 150 - 250-mm layer than of the 0 - 50-mm layer. The changes in residual inorganic and mineralizable nitrogen were not uniform. This resulted in apparent effects of field treatments on residual inorganic nitrogen in dried samples which were not present in field-moist samples, and in effects on mineralizable nitrogen in 40°C-dried samples which were not found in air-dried samples. It is suggested that the extra residual inorganic nitrogen in 40°C-dried samples arises from clay-bound non-exchangeable ammonium or is released from organic matter.

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