Progress in regard to soil erosion, soil conservation, land suitability evaluation and land use planning in South Africa during the 25 year period 1978-2003 is reviewed, with special emphasis on the role of soil scientists. An attempt has been made not only to give a list of the work done, but to present an interpretive review that could hopefully serve as a reference source for future soil scientists.

Many scheduling approaches have been developed with Water Research Commission funding over the past 4 decades and deployed with varying levels of success; 2 approaches have won prestigious international awards. Soil-based approaches which include measurement of matric potential (tensiometry), water content (neutron probes, capacitance sensors) and depth of wetting (wetting front detectors) have been relatively well accepted by farmers. Atmospheric-based approaches apply, through biophysical modelling of the soil-crop-atmosphere system, thermodynamic limits to the amount of water that can evaporate from a cropped surface under particular environmental conditions. Modelling approaches have been quite empirical or somewhat more mechanistic, generic or crop specific, with pre-programmed (e.g. irrigation calendars) or real-time output. Novel mechanisms have been developed to deliver recommendations to farmers, including resource-poor irrigators. Although general adoption of objective irrigation scheduling in South Africa is still low, the high cost of electricity and nitrogen, and scarcity of water is reviving the interest of consultants and irrigators in the application of these tools to use water more efficiently. Where adoption has been relatively high, intensive support and farmer-researcher-consultant interactions have been key contributing factors. We propose 4 avenues in the R&D domain to ensure responsible water utilisation. Firstly, there is a need to continue to advance existing soil-water measurement technology; and secondly, to further develop new and emerging technologies, like the use of remote sensing. Thirdly, the user-friendliness should be improved as should systems that support existing scheduling tools; and finally, we need to appreciate that farmers are intuitively adaptive managers, and we need to develop simple monitoring tools and conceptual frameworks that enable structured learning.

The general perception that livestock is a major contributor to global warming resulted mainly from the FAO publication, Livestock's Long Shadow, in 2006, which indicated that livestock is responsible for 18% of the world's greenhouse gas (GHG) emissions. This figure has since been proved to be an overestimation, since it includes deforestation and other indirect contributions. The most recent figure is in the order of 5% -10%. Although only ruminants can convert the world's high-fibre vegetation into highquality protein sources for human consumption, ruminant production systems are targeted as they are perceived to produce large quantities of GHG. Livestock is also accused of using large quantities of water, an allegation that is based on questionable assumptions and the perception that all sources of food production require a similar and equal quantity and quality of water. In the case of ruminants, extensive systems are usually found to have a lower per-area carbon footprint than grain-fed systems, but a higher footprint if expressed in terms of kg product. Feedlots maximize efficiency of meat production, resulting in a lower carbon footprint, whereas organic production systems consume more energy and have a bigger carbon footprint than conventional production systems. Cows on pastures produce more methane than cows on high concentrate diets. In South Africa, as in most of the countries in the sub-tropics, livestock production is the only option on about 70% of the agricultural land, since the marginal soils and rainfall do not allow for crop production and the utilization of green water. An effective way to reduce the carbon and water footprint of livestock is to decrease livestock numbers and increase production per animal, thereby improving their efficiency.________________________________________________________________________________

No abstract

Conflicting information is available on the propensity different soil clays exhibit to dispersion. We therefore assessed the relative stability of the clay components of 12 soil samples, from various parts of South Africa, to predominantly physical disruption by a mild disruptive treatment [mechanical shaking for 5 min in distilled water (DW), and after the addition of phosphogypsum (PG; 2% by weight) and polyacrylamide (PAM; 0.04% by weight)]. The soils differed markedly in their physical and chemical properties. Clay fractions were of mixed mineralogy and dominated by kaolinite, illite, or smectite. Comparison of the clay mineral composition of the disaggregated clay with that of the original <2 �m fraction indicated that the disaggregated clay composition depended on the amendment. In DW, clay-sized quartz and feldspar were disaggregated preferentially over layer silicates. Within the phyllosilicate fraction, the 2:1 clay minerals (mica, smectite) were on average slightly more easily disaggregated than kaolinite. Goethite was the least easily detached clay component in DW. The increase in quartz and feldspar proportions relative to the other components of the clay fraction was dramatically more pronounced when the soils were mixed with PG. With PAM, however, differences in the nature of the clay fraction between original and disaggregated clay were only marginal. Disruption was not particle-size related, as the minerals of the fine-clay fraction showed no selective increase in any of the treatments. These findings indicate that the most inert members of the clay fraction are most actively involved in the process of disaggregation. PG influenced disaggregation in a manner markedly different from that of PAM. Gypsum preferentially stabilised components with a net negative charge over uncharged species. PAM, in contrast, seemed to affect all clay components equally, independent of charge characteristics.