Leon D. van Rensburg

SOILS: THE FREE STATE'S AGRICULTURAL BASE

ABSTRACT At least one third of South Africas total grain crop, mainly maize and wheat, is produced in the Free State. Crop production potential and sensitivity to degradation depend largely on the interaction of climate and soil. Climatic conditions vary from arid in the south and southwestern region to nearly sub-humid in the extreme east on the edge of the Drakensberg escarpment. A map of soil distribution in the Province shows groups of land types, each one characterised by a particular soil distribution pattern. The soil components of the map units is also presented. The crop production potential of the Province is presented in the form of a map. Delineations are based on interpretation of land type data and range from arable to non-arable. The total potentially arable area of the Free State is 3.82 million ha. In spite of a detailed understanding of the Free States soils, degradation continues to occur, especially on cultivated land, where the most serious forms are erosion, acidification and organic matter decline.

The role of proline and root traits on selection for drought-stress tolerance in soybeans: a review

Drought stress is a major limiting factor in soybean production. It reduces both the yield and its components. This is dependent on the level and occurrence of drought stress with respect to the growth stages of the plants. Breeding for drought stress tolerance can sustainably reduce the impact of drought stress. However, occurrence of drought stress is irregular over locations and seasons, making selection for drought tolerance challenging. Effort is being put into using proline accumulation as a measure to monitor stress tolerance of higher plants. It is evident that proline accumulates with increased drought stress. Genotypic differences exist in proline accumulation among soybean cultivars of different sensitivities to drought stress. There is a positive correlation between stress-induced proline accumulation and drought tolerance. The capacity of root traits to explore spatial domains with limited water and their ability to dynamically respond to soil-water deficit are also being investigated as indicators for drought stress tolerance. Root traits response to available soil water is a heritable trait and genotypic variation is evident in soybeans. This paper reviews the effects of drought stress and advances in the use of proline accumulation and root traits as selection tools for drought tolerance in soybean.

Precision and accuracy of DFM soil water capacitance probes to measure temperature

The DFM capacitance probes with temperature sensors were evaluated for precision and accuracy.The DFM temperature sensors were able to measure temperature with high precision.The accuracy of temperature measurements of the sensors were improved with calibration procedure. Precision and accuracy of DFM capacitance probes with temperature sensors were evaluated for measuring temperature. Two sets of DFM probes were used in this experiment, one set of 17 for testing the performance of the temperature sensors and the other set of 16 probes to evaluate how to improve the accuracy of measurements of the DFM temperature sensors. Standard deviation (SD) as a measure of precision shows that the range of precision for the all the observations was 0.15-1.08°C. The accuracy of the device, using means of the absolute errors, for measuring temperature was ź1.28°C. The DFM temperature sensors were able to measure temperature with high precision while the accuracy of the measurements was improved with calibration procedure. The accuracy was improved up to ź0.06°C with the calibration procedure. It was concluded from the study that the device is suitable for measuring temperature effectively. Using equation derived during calibration procedure before installation of the device will increase sensors performance in terms of temperature measurements accuracy. The DFM capacitance probe is durable, with easy logging, and data management of high accuracy and precision or repeatability. It can also simultaneously collect data on multiple parameters such as soil water and soil temperature.

Characterisation of the soil pore system in relation to its hydraulic functions in two South African aeolian soil groups

The characterisation of soil pores has a prime importance in soil hydrology particularly for soil water management in both irrigated and dryland agriculture. Hence, this study describes the soil pore system in the Clovelly and Hutton soil forms and its roles in soil hydrology by quantifying the different functional pore classes. The soil pore size–pore volume (PS-PV) response curve, which shows the relationship between cumulative pore volume to the corresponding pore sizes, was estimated from the soil water retention curve (SWRC) by applying the capillary theory. The SWRC was measured in the laboratory using a hanging water column and pressure plate apparatus. Saturated hydraulic conductivity was determined in situ with a constant head permeameter, whereas unsaturated hydraulic conductivity (KL) and diffusivity (Df) were estimated from the SWRC by the van Genuchten conductivity model. The classification of soil pore sizes given in the literature was modified based on the concept of field water capacity. Therefore, pore sizes greater than the drained upper limit (DUL) were classified as transmission pores. Similarly, the pore size between the DUL and the lower limit (LL) were classified as storage pores, and pore sizes smaller than the LL were residuals and bonding pores collectively. Soil water retention increased non-linearly as pore sizes decreased and the opposite was true for KL and Df. Generally, macropores were important for water movement, whereas micropores were responsible for water retention. The findings are important for soil water management of cultivated aeolian soils in semi-arid and arid regions of sub-Saharan Africa. The proposed method to classify soil pores on the basis of their function also gives a consistent and realistic value compared with the fixed boundary classes, although the method requires further development.

The cropping potential of South Africa: land evaluation results obtained during the last 50 years

Reliable detailed information regarding the crop production potential of a country is necessary for the planning and execution of the proactive activities needed to avoid future food shortages due to increases in population and/or adverse climatic conditions. The land-type survey of the Republic of South Africa at a scale of 1:250 000 provides a sound basic framework for the assessment of cropping potential because the delineation criteria incorporate the three natural resource factors that determine cropping potential, namely climate, topography and soil. Assessing cropping potential at regional scales, resulted in estimates of arable area between 16.4 and 28.6 million ha. It is clear from these results that a more detailed countrywide assessment at a much larger scale is essential to provide reliable information. The procedure already demonstrated for an area of 2.7 million ha in the eastern Highveld of South Africa is recommended for this work. Modern soil survey techniques can facilitate this enormous task. Detailed assessment of the potential of the 15 million ha occupied by subsistence farmers needs to receive the highest priority. The ecotope is a suitable land unit for assessment of cropping potential. Information derived from ecotope delineation has potential to be useful in agriculture and forestry.

In situ evaluation of internal drainage in duplex soils of the Tukulu, Sepane and Swartland forms

Internal drainage is a critical hydrological process in soil water conservation. Pedological and physical properties of the South African Tukulu, Sepane and Swartland soil types were studied in respect to soil water storage and hydraulic characteristics during internal drainage. Soil profiles were excavated and classified in situ alongside instantaneous drainage plots and saturated hydraulic conductivity experiments. Laboratory desorption experiments were used to establish the soil water characteristic curves (SWCC) of diagnostic horizons. Each SWCC was then fitted with the van Genuchten 1980 model and matric suction of soil water content corresponding to the 1 200 hours drainage curves were estimated. The pedocutanic B and prismatic C horizons in the Tukulu and Sepane soils had clay plus silt content of more than 40% and were characterised by abrupt transitions, signs of wetness and a sharp drop in hydraulic conductivity at higher soil water content. Consequently, deep drainage from the Tukulu and Sepane was about 40% less than that of the Swartland. The drained upper limit was located when drainage flux rate approached 0.001 mm h−1 irrespective of soil type. At this rate deep drainage losses over the seven-month fallow period are equivalent to 1% annual rainfall.