Sodium, cation exchange and crop response in acid KwaZulu-Natal soils.

Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2000.In many acid, highly leached South African soils, Na levels are very low. Applications of Na to highly leached soils may be advantageous in some situations as Na has been shown to interact with soil acidity, P availability, and K nutrition of plants. It was found that an increased level of Na at constant anion concentration in the soil solution resulted in lower soil solution pH and Al3 + activity, and improved maize root penetration into an acid subsoil (pot trial). In another pot trial, Na did not improve lucerne root growth into the same subsoil. In a field trial on a limed, low-Na soil with severe subsoil acidity, the effects off our rates of Na, three Na carriers, and three rates of gypsum on maize growth were compared. In three out of four seasons grain yield responses to the Na applications were positive, and of a similar magnitude to the response to gypsum (455-925 kg ha). In another field trial with maize, responses to Na (of up to 1027 kg grain ha) were recorded in two out of three years on plots with topsoil acid saturations of greater than 45%. The reason for the responses to Na was probably improved subsoil rooting as a result of modified soil solution composition in the presence of Na. In a field trial with Italian rye grass on an acid, highly weathered soil, the effect of sodium applications were investigated at different levels of lime and K. In the first season, 200 kg ha Na as NaCl increased dry matter yield from 11289 kg ha to 12815 kg ha , Sodium responses were possibly due to enhanced P uptake, or Na substitution for K, or a combination of the two mechanisms. No consistent lime x Na interaction was observed. Potassium and Na applications affected the mineral balance of the herbage produced. Potassium depressed herbage Ca, Mg and Na concentrations, and Na depressed herbage Ca and Mg where herbage K was low, but had no effect on herbage K. Where soil P and K reserves are marginal, rye grass pastures may benefit from Na applications