J. C. Hughes

Some physical and chemical properties of sediments exposed in a gully (donga) in northern KwaZulu-Natal, South Africa and their relationship to the erodibility of the colluvial layers.

Abstract Gullies (locally termed dongas) are a characteristic feature of the landscape in central KwaZulu-Natal and are highly associated with widespread, stratified colluvial sediments and the buried palaeosols that have formed in this material. Research objectives were to find out which sediment property or properties contribute most to the vulnerability of the colluvium to gully erosion; to study the erosion susceptibility of the sequence of buried stratigraphic layers and palaeosols; and to improve understanding of the relationship between erosion susceptibility of the colluvial layers and palaeosols and their disposition in the landscape. Materials from several colluvial layers and palaeosols exposed in the donga sidewall of the Dabekazi donga, 13 km north of Nqutu in central KwaZulu-Natal, were subjected to chemical and mineralogical analyses, several dispersion and erodibility tests and were characterized physically. Exchangeable sodium percentages (ESP) ranged from 0% to 23% and the dispersivity and erodibility tests revealed a range of dispersion potential and erodibility across the sequence of colluvial layers and palaeosols. However, a poor correlation was found between the results of different dispersion tests, as well as between dispersion tests and dispersion-related properties. Dispersion tests showed significant correlation with silt content. An erodibility test using a flume showed very strong correlation with electrical conductivity (EC) and sodium adsorption ratio (SAR) of the saturated paste extract and ESP values. The plasticity index ranged from 0% to 0.23 and estimated hydraulic conductivities ranged from 33 to 0.7 m/day. High correlation with ESP and EC suggests that dispersion plays an important role in the erodibility of the materials under conditions of short duration, turbulent water flow. However, colluvium in which deep gullying occurs is not necessarily strongly sodic. Also, the degree of erodibility of the materials differs depending on the test used, illustrating the complexity of the interactions that govern dispersion and erosion. The erodibility of the colluvial layers and palaeosols from the Dabekazi donga, and properties with relevance to dispersion, differ considerably down the profile, indicating that geomorphic threshold conditions and position in the landscape with respect to surface runoff hydrology also codetermine donga erosion. Hydraulic conductivity estimates of the investigated materials are relatively high and do not differ sufficiently to enable lateral subsurface flow through, and piping of, these materials to be the main causes of donga formation.

The effects of composts produced by a simple composting procedure on the yields of Swiss chard (Beta vulgaris L. var. flavescens) and common bean (Phaseolus vulgaris L. var. nanus)

Abstract The effect of four composts, produced by a simple process, on the yield of Swiss chard and common bean was studied in two glasshouse pot experiments. The composts which differed in raw material (garden refuse, market and garden refuse) and turning frequency (0×, 6×) were applied at three rates (0, 25, 50% (m/m)) to a sandy soil with and without low applications of inorganic fertiliser. In both the experiments, the soil–compost mixtures out-performed the soil alone irrespective of the amount of fertiliser added. Swiss chard produced the highest total leaf fresh mass on composts made from market and garden refuse. The yield was further significantly improved when the composts had been turned or when the proportion of compost was increased from 25 to 50%, but the addition of fertiliser had no significant effect on the total yield. In contrast to the results for Swiss chard, the application of compost produced from garden and market refuse which had undergone turning had a negative effect on the yield of common bean, especially when the proportion of compost was increased to 50% and when fertiliser was added. Common bean performed the best when grown on soil mixed with compost made exclusively from garden refuse. With the exception of the treatments which contained compost made from turned market and garden refuse, adding fertiliser and increasing the proportion of compost generally led to higher numbers of pods and seeds and increased total seed dry mass. The contrasting responses of the two crops to the applied composts are believed to be caused by differences in the crops’ nutrient requirements and salinity tolerances. In order to maximise yields, compost raw materials and turning frequency need to be carefully chosen to produce composts particularly suited to specific crop requirements.

Improved criteria for classifying hydric soils in South Africa

There is an increasing awareness of the ecological and agricultural importance of hydric (wetland) soils. Because of the scarcity of information in South Africa, this study aimed to critically examine systems currently being applied to the hydric soils of South Africa, recommend the best criteria to be used from a management point of view, and suggest future research. Soil taxonomy (Soil Survey Staff, 1975) was found to be superior to the South African soil classification system for describing hydric soils, primarily because it accounts for depth of waterlogging. Nevertheless, Soil taxonomy does not account adequately for degree of wetness, with some series including a wide range of water regimes. A review of soil morphology/water-regime studies revealed that along the continuum from temporarily wet to permanently wet areas: matrix chroma decreases; the most intensively mottled zone becomes progressively shallower; mottle abundance increases then decreases; and soil organic matter increases. Based on thes...

Extraction of chromium, nickel and iron and the availability of chromium and nickel to plants from some serpentinite‐derived soils from the eastern Transvaal as revealed by various single and sequential extraction techniques

Abstract A distinct feature of certain plant species growing on serpentinite derived soils is their ability to hyperaccumulate heavy metals such as nickel (Ni) and chromium (Cr). This study was conducted to determine the availability of these heavy metals and their ease of extraction with various reagents. Various standard chemical reagents were used to extract Cr and Ni from nine different soils originating from two sites in the eastern Transvaal, South Africa. In addition, the sequential extraction of Cr, Ni, and iron (Fe) was undertaken. Of all the extradants used 4M nitric acid was the most efficient. Acetic acid appeared to be the best single extradant for estimating plant available Cr on the basis of leaf concentration. Acetic acid, ammonium acetate, copper (II) chloride, EDTA and aluminium chloride appeared to be equally efficient for estimating plant available Ni. Oxidation of organic matter resulted in a higher recovery of Cr and Ni thus highlighting the importance of this fraction in immobilizin...

A simple test to determine cellulolytic activity as indicator of compost maturity

A simple assay, based on soil burial tests, was developed to determine the activity of cellulolytic microorganisms in compost. The assay used filter paper as the cellulose substrate. The filter paper was buried in compost and incubated in the dark for 10 days at 30°C. The amount of cellulose degraded was determined by the difference in dry mass of the filter paper, before and after burial, and the cellulolytic activity was expressed as percentage cellulose degraded in 10 days. The assay was applied to visually mature composts from various sources that differed in raw material composition and treatment. The results were compared with those obtained from a cress (Lepidium sativum, L.) germination and growth experiment carried out with the same composts and with the microbial activities of the composts determined by a fluorescein diacetate method. The fresh mass of the cress roots was negatively correlated with the cellulolytic activity of the composts. A positive correlation existed between the cellulolytic activity and the microbial activity of the composts. It is suggested that cellulose degradation could be one of the factors responsible for ‘latent toxicity’ exhibited by some visually mature composts.

Sequential fractionation of chromium and nickel from some serpentinite‐derived soils from the eastern Transvaal

Abstract Soils derived from ultramafic serpentinitic rocks are inherently infertile. These soils support plant species that are able to hyperaccumulate both chromium (Cr) and nickel (Ni). This study was conducted to determine the efficacy of a sequential extraction technique in explaining sources of Cr and Ni that are taken up by plant species growing on these soils. The sequential extraction of soil samples obtained from the eastern Transvaal involved the following reagents: H2O and 0.5M KNO3, 0.5M NaOH, 0.05M Na2EDTA, and 4M HNO3. More than 95% of the total Cr was extracted by HNO3 while the remaining extractants fell into the order NaOH > EDTA ≫ KNO3 + H2O. There would appear to be a loose correlation between easily soluble Cr (KNO3 + H2O) and the uptake of Cr by the plant. A somewhat higher proportion of Ni was extracted prior to the HNO3 treatment although amounts removed by KNO3 + H2O were all less than 1% of the total. It would appear that plant species growing on these soils are able to accumulate...

Changes in chemical properties and temperature during the degradation of organic wastes subjected to simple composting protocols suitable for small-scale farming, and \ quality of the mature compost

Despite the high cost of chemical fertilizers and pesticides, composting is not widely practiced in many rural communities in South Africa. To demonstrate the simplicity and benefit of composting, four composting protocols were selected, based on technology and resources available to rural households and small-scale farmers, and their effect on the degrading material and on the quality and yield of the resulting compost was investigated. The protocols varied in raw material choice (garden refuse with and without market refuse) and turning frequency (6×, 0×). Changes in chemical properties during composting were in accordance with those generally observed and were alternately affected by raw material and turning frequency. The highest temperature measured in compost made from market and garden refuse was 69°C whereas in compost made purely from garden refuse the highest temperature was 53°C. Turned compost heaps yielded the highest volumes of mature compost. All composting protocols produced composts with C/N ratios and pH values suitable for horticultural use. Nutrient concentrations were in the range generally exhibited by composts. The incorporation of market refuse and the turning were conducive to the production of a nutrient-rich compost. However, turning also significantly increased the electrical conductivity of the composts and application rates should be reduced accordingly when turned compost is used for crops with a low resistance to osmotic stress.

Use of solid waste bagasse from a paper mill as an organic amendment and its effect on some soil properties: a case study

At a paper mill in KwaZulu-Natal, South Africa bagasse waste was disposed of by burying it in a layer resulting in variable sugar cane growth. The waste was mostly organic fibres, had a high water holding capacity, and contained small amounts of Ca, K, P and N. The soils were sandy with low water holding capacities and organic matter contents. Soil temperatures up to 38°C were measured in treated soils, compared to about 22°C in untreated soils. Treated soils contained more total cations, and variations in the sodium adsorption ratio and electrical conductivity of soil saturation extracts were not great or consistent across sites, suggesting neither excessive soil salinity nor sodicity. Field observations showed that the best cane growth was on soils that had been treated with the waste. Poor growth was found, either on recently treated sites or, on untreated areas within a larger treated area owing to poor distribution of the waste. Changing the method of application to mix the waste into the soil would facilitate decomposition, and avoid the problems of high soil temperatures and the creation of anaerobic conditions which discouraged root growth.

The effects of leather tannery sludge on the leachates from soil columns

Leather tannery sludge from a chrome-tanning plant was added to the surface horizon of a typic haplustox from South Africa at rates equivalent to 0, 10 and 100 t ha−1. Leaching columns were set up with 100 mm of the topsoil over 200 or 300 mm of subsoil material. The columns were watered intermittently and the leachates collected and analysed. The results show that, of the major cations, sodium leached fastest, followed by potassium and then magnesium and calcium. The anion analyses revealed that chloride predominated in the early leachates with nitrate and latterly sulphate becoming more important with increasing volumes of leachate. Moderate to high sodium absorption ratios of the leachate were counterbalanced by high electrical conductivity so that, for this soil, the use of such a sludge posed no real threat to either soil structure or groundwater quality; extrapolation to the field situation would require caution, however. It is probable that the recycling of chromium and washing of the sludge at the...

Assessment of phosphorus availability in soils fertilized with Langebaan rock phosphate or superphosphate

Problems currently exist in the interpretation of soil test values for soils fertilized with rock phosphate. This study was undertaken to compare the ability of four extradants (Ambic, Bray 1, Mehlich 3 and modified Truog) to assess plant available phosphorus in soils fertilized with superphosphate or Langebaan rock phosphate (Langfos). A Balmoral clay (Typic Haplorthox) and an Avalon sandy loam (Plinthic Paleudult), adjusted to three levels of acidity and fertilized with superphosphate or Langfos, were consecutively cropped three times with maize (Zea mays L.) in a glasshouse. Soil samples were taken after each harvest and analysed for available phosphorus using the four extradants. Inorganic phosphorus in samples, taken after the third crop, was fractionated into aluminium-, iron- and calcium-bound phosphorus. Ambic, Bray 1 and Mehlich 3 were more sensitive than Truog in reflecting the favourable effect of acidity and cropping on the dissolution of Langfos. In spite of large differences in the quantity ...