Loss of nitrogen fertility and its restoration in some Orange Free State soils.

The effects of present or past cultivation on nitrogen fertility in some central Orange Free State soils were investigated by the comparison of paired samples of cultivated or reverted soils with uncultivated soils situated as closely as possible to one another and belonging to the same soil form. The parameters measured were texture, pH, organic carbon, total nitrogen content and net nitrogen mineralization under uniform controlled conditions in the laboratory. Paired samples were taken at selected sites at depths of 0–0,15; 0,15–0,50 and 0,50–1,00 m. Effects of cultivation were small downward displacements of silt and clay, slight acidification and large losses of organic carbon and nitrogen from the surface layer. The easily mineralized fraction of nitrogen decreased more than the more resistant fractions. The rate of depletion decreased as the period of cropping increased but data were insufficient to show whether a new equilibrium had been reached. Reversion to pasture appeared to restore fertility w...