Potassium relationships in selected eutrophic hutton soils of irrigation areas
The relationship between red leaf disease and soil fertility status was studied in the glasshouse. For red leaf disease affected plants grown on Mangano soils, no effective fertilizer treatment could be found. However, yield could be predicted more accurately by soil parameters other than the traditional ammonium acetate extractable potassium or exchangeable potassium. These parameters were the ratio (Ca+Mg)/K in the soil and exchangeable potassium percentage. These initial findings were tested in fifteen field trials on Mangano soils with variable clay contents and exchangeable potassium contents at Vaalharts. A large matrix of data resulted from the field investigation. However, only statistical procedures relating soil and leaf-K parameters to yield and leaf-K were investigated. The data were statistically analised by means of analysis of variance, correlation and multiple regression on an IBM 900 computer. For each soil and leaf potassium parameter relationships were established with yield and leaf-K content for individual trials, pooled data and grouped data. Attention is drawn to the fact that ammonium acetate extractable potassium and exchangeable potassium are not reliable predictors of cotton yield for this soil/crop relationship. These two parameters should rather be considered together with the ratio (Ca + Mg)/K in the soil, exchangeable potassium percentage and leaf-K content when cotton yield is to be predicted on Mangano soils at Vaalharts. The initial results from one pot experiment and fifteen field trials were largely substantiated by two additional pot experiments. In order to find an explanation for the lack of response to applied potassium, potassium fixation and K fractionation were also investigated. All the soils tested fixed appreciable quantities of K. Total K amounted to approximately one percent. Recommendations are made to clarify the complex nature of potassium reactions in the soil/crop relationship.