Faculty of Natural and Agricultural Sciences
School of Agricultural and Food Sciences
Department of Plant Production and Soil Science
Selected Highlights from Research Findings
It is increasingly recognized that non-point source (NPS) or diffuse pollution plays a major role in the degradation of water quality, specifically with respect to salinity, eutrophication (nutrient enrichment), sediments, pathogens, pesticides and some heavy metals. It is furthermore accepted that water quality cannot be managed effectively without addressing the contribution from non-point sources. It is therefore important to quantify NPS pollution and to identify control measures to mitigate the impact at source. Since most of the land area is utilized for agricultural activities, agriculture has both locally and internationally been implicated as a major source of NPS pollution. In this study, undertaken with the University of the Free State and the consulting engineers Sigma Beta, a closer look is taken at the contribution of agriculture to eutrophication through the nutrients nitrogen (N) and phosphorus (P).
Coarse nutrient balances revealed that, in general on deep soils, N removal by crops exceeded fertilizer N inputs, whilst the opposite is true for P. It appears that dryland cropping on deep soils that provide a buffer, like in most of the maize producing regions, it is unlikely that agriculture is a major contributor to non-point source pollution through nitrate leaching. On the other hand, dryland production on shallow soils - as in the Western Cape - could be problematic, as here more N is applied as fertilizer than is removed by the crop. Crude balances are, however, not sufficient to identify potential problem areas, like the irrigation of high value, high input crops on light soils. The Breede River's Hex sub-catchment for example has a particularly high net nitrogen load of 7 kg ha-1 a-1, and in the Nagel sub-catchment a phosphate load of 0,12 kg ha-1 a--1 has been calculated.
The best way to minimize N and P non-point source pollution from cropping systems, is to develop sound fertilization programmes and focus on practices that favour high crop yields and reduce soil erosion. Nitrogen and phosphorus balances are being built into the Soil Water Balance (SWB) model developed at the University. This should facilitate the identification of potential problem areas, and assist in the design of appropriate mitigation strategies.
Contact person: Prof JG Annandale.
A vegetation screening trial was initiated on the Sasol Secunda coarse ash dump to investigate the possibility of eliminating the need for an expensive engineered soil cap to manage water and salt in the dump. Additionally, the co-disposal of petro chemical sludge with the coarse ash, where the sludge could act as a nutrient source, was investigated. Many grass and trees species established quite easily and the vigorous growth of the vegetation is extremely promising. A larger scale trial is now being initiated in which water and salt balances will be quantified.
Contact person: Prof JG Annandale.
Apart from its competitive ability, the invasiveness of the noxious weed Parthenium hysterophorus L. is thought to be due to a high ability to displace other species by way of allelopathy. Among others, the sesquiterpene lactone parthenin is thought to contribute a major part to the allelochemical system used in the chemical interference of the weed with surrounding plants. By following a novel modeling approach designed by our German collaborators at Hohenheim University we determined the level of involvement of parthenin to overall phytotoxicity of a South African population of P. hysterophorus. Comparison of responses of various test species to pure parthenin with phytotoxic responses of the same species to leaf extracts from the weed allowed for the relative contribution of parthenin to the total allelopathic effect to be quantified. This suggested that the release of parthenin during decomposition of leaf material has a capacity to play a leading role for allelopathy in P. hysterophorus.
Afgesien daarvan dat die uitheemse indringerplant, Parthenium hysterophorus L., kompeterend is, word aanvaar dat die skadelike uitwerking daarvan toegeskryf kan word aan sy sterk-ontwikkelde allelopatiese vermoë. Die sequiterpeen-laktoon, parthenin, is vermoedelik 'n sleutel-allelochemikalie in die chemiese interaksie tussen hierdie onkruid en ander plante. Ons medewerkers aan die Hohenheim Universiteit in Duitsland het 'n innoverende model ontwikkel om die relatiewe bydrae van parthenin tot die totale fitotoksisiteit in 'n Suid-Afrikaanse populasie van P. hysterophorus te bepaal. Analise van parthenin in blaarmateriaal van die onkruid is gedoen en 'n verskeidenheid toetsplante is aan 'n reeks konsentrasies daarvan blootgestel. Toetsplant-reaksie op 'n reeks konsentrasies van suiwer parthenin, en vergelyking van fitotoksisiteit met plantreaksies op blaarekstrakte van die onkruid het dit moontlik gemaak om die relatiewe bydrae van parthenin tot die fitotoksisiteit van die totale allelochemikalie-komponent in blare van P. hysterophorus te kwantifiseer. Hierdeur is vasgestel dat parthenin wel 'n leidende rol speel in die allelopatiese vermoë van die onkruid.
Contact person: Prof CF Reinhardt.
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