Changes in the organic matter and nutrient contents of some South African irrigated soils

Abstract

Cultivation caused, without exception, a decline in the organic matter content of dryland soils in the central regions of South Africa. The question arose as to how cultivation affects the organic matter content of irrigated soils from these regions. The main aim, therefore, was to quantify the effect of cultivation on the organic matter content of soils from three irrigation schemes with virgin soils serving as reference. Virgin and cultivated topsoils (0–200 mm) were sampled from seven sites at Riet River, eight sites at Ramah and six sites at Vaalharts. The sites at each irrigation scheme represented varying periods of cultivation. Soil samples were analysed for particle size distribution, pH, exchangeable Na, K, Mg and Ca, extractable P and Zn, CEC, organic C and total N. The K, P and Zn contents differed significantly between virgin and cultivated soils: K decreased, while P and Zn increased with cultivation and irrigation. Organic C and total N significantly increased at seven sites and decreased at eight sites as a result of cultivation and irrigation. At another three sites only one of organic C or total N changed significantly and at the remaining three sites cultivation and irrigation had no significant effect. Neither cultivation and irrigation history, nor soil properties provided a clear explanation for these contrasting findings. The gain of soil organic matter at five sites from Ramah and two sites from Vaalharts could possibly be attributed to enhanced biomass turnover on these irrigated lands