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
