Faculty of Natural and Agricultural Sciences
School of Agricultural and Food Sciences
Department of Plant Production and Soil Science
Selected Highlights from Research Findings
Victor A Ochwoh investigated transformations and distribution of applied P to different P pools and P desorption rates in Rustenburg (high P fixing) and Loskop (low P fixing) soils, using sequential and successive extraction procedures. Between 80 and 90% of the added P was transformed into stable P pools within 60 days. This was faster in the Rustenburg soil showing a higher P fixation capacity. All stable soil P pools contributed to the labile P pool, after prolonged successive DMT-HFO extraction, to meet most crops' requirements. Root systems, however, do not exploit soils to the same extent as laboratory methods, which explains the differences in the field. It is foreseen that this method could be used to develop a P desorption model to determine how much extractable P, with a specific extractant, in a particular soil, should be available at the beginning of a growing season to sustain P release rates to meet the requirements of a crop.
Contact person: Prof AS Claassens.
Little research has been done on the phosphate release characteristics of South African soils. Phosphate desorption characterisation of soils under cultivation could be a valuable tool in fertilization programs, because if the time frame of phosphate availability of a soil is known phosphate fertilization programs can be optimised. In a recent study, the phosphate desorption kinetics of an acid sandy clay soil as well as the influence of different phosphate levels on the phosphate desorption kinetics were investigated over a 56-day period. It was found that the long-term phosphate desorption kinetics were described well with a two component first order model. The assumptions made were that two discrete phosphate 'pools', a labile- and a less one, participated in the desorption process and that the two phosphate pools followed pseudo first order kinetics. Applied phosphate had a greater influence on the phosphate desorption rate of the labile pool of the soil than on the less labile pool.
Contact person: Mr PC de Jager.
The use of gypsiferous mine water for irrigation of agricultural crops is a promising technology which could solve problems related to both the shortage of irrigation water and the utilization of mine effluent. Crop response to irrigation with gypsiferous mine water, as well as the impact on soil and groundwater resources were investigated in a three-year field trial set up at Kleinkopje Colliery (Witbank, Mpumalanga Province, South Africa). Sugarbeans, maize and wheat were irrigated with four centre pivots on virgin and rehabilitated land, under three irrigation management regimes using two qualities of mine water. Good crop yields were obtained compared to dry land cropping. Waterlogging in certain areas of the fields indicated that especially rehabilitated land should be properly prepared, and where necessary, waterways built to prevent yield reduction. Soil salinity tended to increase over time mainly due to high concentrations of Ca2+, SO42- and Mg2+ in the irrigation water. Exchangeable Ca2+ and Mg2+ in the soil increased with time, whilst K+ decreased. Plant analyses indicated possible nutrient deficiencies, which should be easily managed through corrective fertilization. The groundwater impact was negligible as no salt breakthrough was observed in monitoring boreholes, indicating the presence of a buffer zone between the cropped soil profile and groundwater. Commercial production of crops under irrigation with gypsiferous mine water is feasible, and the resulting environmental impact is limited.
Technology transfer actions were undertaken to implement the Soil Water Balance (SWB) computer model as a real-time irrigation scheduling tool in order to improve irrigation water use efficiency and reduce agricultural production costs in South Africa. Country-wide courses for irrigation consultants, extension officers and commercial farmers were organized, and improvements to technical aspects and user-friendliness of the model were carried out based on their feedback. A multi-media package, including photographs, video clips, animations and links to the SWB software, is being developed for teaching principles and practices of irrigation management.
Contact person: Dr NZ Jovanovic.
Luke Kanyomeka investigated the differential tolerance of maize genotypes to selected herbicides in a PhD project. The study demonstrated that it is essential to screen all maize genotypes for tolerance to herbicides, in order to reduce the risk of crop damage. It was found that screening under greenhouse conditions could provide valuable information on genotype tolerance that could be applied in practice. The research, through the example of the herbicide metazachlor, also showed that breeding programmes could be used to improve maize tolerance to herbicides. The need for a strategy to screen maize genotypes in terms of their herbicide tolerance was highlighted by the study, and the methodology for doing it economically and quickly is described.
Brian L de Villiers, in his thesis, The influence of salt in carrier water and adjuvants on glyphosate activity, underlined the antagonism of glyphosate by dissolved salts in South African water sources used for herbicide application. This study found reasons for this antagonism and an additive that could effectively neutralize the negative effect was developed for the South African market. If the antagonistic effect of salts in the spray solution cannot be overcome, serious reduction in herbicide efficacy will result. Variable efficacy of glyphosate applied in the presence of various salts contained in the carrier water (spray solution) could be explained by the nature of spray droplet residuals deposited on the leaf surface of treated plants. Scanning electron microscopy of residuals revealed an inverse relationship between the viscosity/crystalline state of the residuals and herbicide effect on sensitive plants. The study highlighted the important impact which water quality has on the efficacy of the herbicide glyphosate in particular, and pesticides in general.
Contact person: Prof CF Reinhardt.
For her PhD studies, Thabsile Balole investigated the seed quality of sixty-five sweet sorghum landraces from Botswana, confirming that seed quality is generally poor. Serial harvesting trials identified the harvesting of seed prior to mass maturity as a major cause of poor seed quality. Ten landraces were characterised with regard to morphologic and agronomic traits, indicating the range of genetic diversity available, and the potential for breeding to improve cultivars. The effects of agronomic practices like planting date, spacing, nitrogen fertilisation and plant manipulation on stem yield and juice quality were determined in field trials. The results demonstrated the agronomic potential of sweet sorghum as a cash crop for small-scale farmers. Economic viability will depend on price elasticity in the supply-demand function.
Plectranthus esculentus is an endemic African plant with the potential for commercialisation. It has been used as food by indigenous peoples for generations, although to a lesser extent lately due to destruction of natural habitats and loss of planting material. It produces edible tubers comparable to potato in flavour and texture, with potential niches in rural agriculture, but also in the sophisticated vegetable market. Botanical information is scanty and production requirements non-existent. Dr James Allemann (ARC) and Professors Hannes Robbertse and Piet Hammes (UP) confirmed that the storage organs are true stem tubers (like potato), based on morphological and anatomical studies. Tuber induction is controlled by photoperiod, with temperature playing a minor role. Tuberisation is a short-day process, and tubers will be initiated if the dark period is 12.5 hours or longer. A tissue culture multiplication procedure has been used successfully. Research information on the reaction of P. esculentus to environmental factors is presently utilized to develop production programmes for this potential crop.
Contact person: Prof PS Hammes.
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