Abstract:
In the tobacco production area of Nelspruit, a problem is experienced in increasing the phosphorus content of tobacco leaves to above0.25% despite of the high phosphorus content of the soil (higher than 30 mg/kg Bray 1). The aim of this study was to investigate the reasons for the low P content in tobacco leaves and how P and Ca content in the soil and soil compaction influence it. During the 1998/99 season, field tests were conducted at 3 different localities in the Nelspruit area to investigate the influence of soil compaction, phosphorus and calcium supply on P uptake. Treatment consisted of 2 calcium treatments, 2 soil treatment actions and 5 phosphorus levels. Leaf samples were taken for chemical analysis, 115 to 120 days after planting and the different treatments were harvested as they ripened physiologically and cured in a tobacco drying oven. This was used to calculate the yield, quality and income per hectare. A pot experiment were done under controlled conditions at the experimental farm of the University of Pretoria with soil from one of the field plots. With the pot experiment different bulk densities were simulated with 3 phosphorus and 2 calcium levels, to evaluate the effect thereof on P uptake. As the leaves ripened physiologically they were picked and dried for chemical analysis. On all the data an analysis of variance was performed. A final conclusion can be made that, high levels of P fertilization are unnecessary and attention must be paid to soil compaction, and that gypsum must be applied with care. Although not investigated in this study the low P content of leaves can be due to the genetic characteristics of the tobacco cultivars used in the Nelspruit area and the high levels of zinc in the soil which can cause P precipitation in roots that could restrict the P transport to the leaves. With the pot experiment it is proved that high P applications lead to a decline of the percentage zinc in the leaves. The conclusion is made in this study that when soil compaction were overcome, high and low dosages of phosphorus and the increase of calcium levels in the soil did not show a constant increase in P values above 0.25%.
