Abstract
The use of high rates of nitrogen fertilizer for three years caused significant changes in soil chemical properties to a depth of 35cm in a leached ferruginous soil at Samaru and to at least 50cm in a sandy ferrisol at Mokwa. At Samaru only ammonium sulphate gave serious acidification below the topsoil, and this could be controlled by annual liming; but at Mokwa calcium ammonium nitrate and urea also acidified the subsoil, and annual dressings of lime gave little or no control except in the top soil. Acidification was accompanied by losses of exchangeable calcium and magnesium, but those of magnesium were the more serious; at Mokwa the magnitude of the loss depended on rate of nitrogen application irrespective of form, and amounted to at least half the exchangeable magnesium content of the top 50 cm. Yields of a cotton crop grown in the fourth season to test residual soil effects were much reduced by premature moisture stress, but the adverse effects of acidification, and especially subsoil acidification, could still be demonstrated.
