Jeffrey Hughes

The effects of addition of bauxite red mud to soil on P uptake by plants

The chemical properties of red mud, a byproduct of Bayer process refining of bauxite to alumina, make disposal of the material problematic. It is very alkaline (pH > 11), contains a large amount of sesquioxides, and thus has a very high P retention capacity. These characteristics have encouraged its use as a soil amendment to enhance P retention in sandy soils. A glasshouse experiment was carried out to investigate the effect of red mud on plant-available P. Leached red mud (LRM) (pH 7.24) was mixed at rates of 0, 5, 10, and 20 t/ha with a very sandy soil, provided with a full basal fertiliser and various rates of phosphate, and then sown with perennial ryegrass. Five harvests were obtained over a period of 245 days. At the end of the experiment the highest rate of addition of LRM gave an increase in soil pH of less than one unit and the electrical conductivity had not changed substantially or systematically. Bicarbonate-extractable P (bic-P) had decreased considerably from the initial values. However, there were no significant differences between bic-P values at the different levels of red mud application for the same rate of P application. Plant yield was not significantly different between treatments. Addition of red mud (a) decreased the P concentration of plants for the same amount of P applied; and (b) required a larger amount of bic-P to maintain a constant level of P in the plant. The red mud had adsorbed both applied and existing P and reduced the plant availability of bic-P. The economic impact of these processes needs evaluation.

Soil mineralogy research in South Africa, 1978 to 2002 - a review

Over 160 publications that contain mineralogical information were produced in the last 25 years of which about half were published in national and international journals. In many articles mineralogical data were presented but not discussed. Most studies were concerned with the clay fraction and X-ray diffraction was the dominant technique used for mineral identification. The investigations covered a very wide range of topics such as inheritance of clays from parent materials, and broad regional studies at Lusikisiki and in the Western Cape Province. Studies of mineralogy in relation to soil properties such as erodibility, the susceptibility of minerals to dispersion, and the importance of the mineral fraction for K-fixation were undertaken. Pedological research Included the red-black sequences of the Springbok Flats and Kruger Park, podzols of the Cape Province and major mineralogical investigations of melanic, vertic and humic horizons. Other aspects reviewed are concerned with environmental history, engineering, rehabilitation and archaeology. The future of mineralogy in South Africa appears bleak due to the lack of mineralogical training being offered at tertiary education establishments. It is hoped that this. review will rekindle an interest in soil mineralogy as a topic that impacts on an ever-widening range of soilrelated topics.

Effect of tillage systems and nitrogen application rates on selected physical and biological properties of a clay loam soil in KwaZulu-Natal, South Africa

The effects of agricultural management practices on soil physical and biological parameters provide fundamental information for assessing sustainability. Tillage (annual conventional tillage and no-till) and nitrogen fertiliser (limestone ammonium nitrate at rates of 0, 100 and 200 kg N ha−1) were evaluated for their effects on bulk density, water retention characteristics, saturated hydraulic conductivity, soil penetration resistance and microbiological activity (hydrolytic and cellulolytic) on a clay loam soil (Typic Haplustox) at a field experiment initiated in 2003 and planted to dryland maize. Tillage regime and nitrogen application rate significantly (p < 0.05) affected soil physical properties and microbial activity in the 0–5 cm layer. Under no-till the saturated water content and saturated hydraulic conductivity were significantly lower than under annual conventional tillage while plant available water was greater under no-till. Penetration resistance, bulk density and microbial activity were greater under no-till than annual conventional tillage. At 200 kg N ha−1 microbial activity, bulk density, water retention and saturated hydraulic conductivity were significantly lower than at the lower rates of application, especially under no-till. These results indicate that no-till improved soil physical properties and suggest there is a need to re-evaluate the sustainability of using high rates of nitrogen to increase maize yields, especially under a no-till system.

Characterisation of alkaline tailings from a lead/zinc mine in South Africa and evaluation of their revegetation potential using five indigenous grass species

Tailings from a lead/zinc (Pb/Zn) mine were characterised and their revegetation potential investigated under glasshouse conditions using five grass species with three rates of inorganic fertiliser. The tailings were alkaline with low nutrient concentrations but high total and extractable Zn. The yield of all grass species increased with an increase in fertiliser rate. The yield of Cenchrus ciliaris at the full fertiliser application rate was significantly higher than the other species tested, followed by Digitaria eriantha. Cymbopogon plurinodis was the third-highest-yielding species, whereas yields of Eragrostis superba and Fingeruthia africana were similar. Concentrations of Zn in the foliage tended to be over the reported grass foliage ranges, whereas Pb concentrations were within typical norms. It is recommended that C. ciliaris, D. eriantha and E. superba be used for initial revegetation, with other species used to improve biodiversity after initial cover has been established.

Mineralogy of volcanically derived alluvial soils at Moshi, Tanzania

Despite intensive commercial agriculture in the rift zone of Tanzania, mineralogical studies on the soils influenced by volcanic parent materials are scarce. A mineralogical investigation of the soils and two buried ash layers from an irrigated sugar estate was undertaken using X-ray diffraction, transmission electron microscopy and measurements of extractable iron, aluminium and silicon and the specific surface area (SSA) of the clay fraction. The dominant mineral in the sand and silt fractions was sanidine. The clay fractions contained mainly high-defect kaolin, illite and K-feldspar, with small amounts of calcite, talc and gibbsite in some samples. Electron microscopy revealed the presence of tubular and spheroidal halloysite and <0.5µm kaolinite crystals, as well as nanocrystalline material, probably allophane, and volcanic glass. The amounts of ferrihydrite (0.34–1.84%) and allophane (0.52–6.84%) were low in the soils but higher in the buried ash layers. The surface areas of the clay fractions were high (up to 145m2g–1) and it was calculated that 5% allophane constituted 22% of the total SSA. Although all soils were dominated by halloysite and small kaolinite particles, it is likely that the small amounts of allophane with high SSA has a strong effect on their physical and chemical properties.