Comparison of bromide and nitrate transport in the Bainsvlei soil of South Africa under natural rainfall

Abstract

Bromide is commonly used to simulate the movement of nitrate fertilisers through the soil profile. However, there exists no comparative evaluation of the leaching properties of Br and NO3--N under local soil and rainfall conditions at Bloemfontein. The purpose of this work was to conduct a field experiment to evaluate the leaching behaviour of Br in comparison with NO3--N on the Bainsvlei soil of South Africa under natural rainfall conditions. For this purpose, KBr and KNO 3 solutions were applied to a 2.45 x 2.45 m2 plot at rates of 13.5 g Br·m-2 and 20 g N·-2 respectively. The subsequent movement of the solutes through the soil was investigated through studies of the water and mass balances, determined from soil samples taken from a 1600 mm deep soil profile during the period October 2000 to May 2001, the rainy season in Bloemfontein. The results were also analysed with the one-dimensional convective dispersive equation and stream tube models. Two important results were derived from the study: Br can be used with confidence as a substitute for NO3--N in studies of the movement of the latter through soils, and it is more economical and environmentally friendly to distribute the application of nitrate over the growing season of a crop, instead of applying it as a batch at the time of planting.