• Authors:
    • Barfoot, P.
    • Brookes, G.
  • Year: 2008
  • Authors:
    • Schmidt, C. J. J.
    • Adriaanse, F. G.
    • Preez, C. C. du
  • Source: South African Journal of Plant and Soil
  • Volume: 24
  • Issue: 1
  • Year: 2007
  • Summary: The principle objective of this study was to establish P fertilizer guidelines for dryland maize on the South African Highveld according to the sufficiency concept of soil extractable P. Data sets from nine different P fertilizer trials at various localities in the Free State, Gauteng, Mpumalanga and North West provinces were used. Different P treatments were applied for all trials in order to establish differences in extractable soil P levels, which were expected to have corresponding effects on maize yield. Long-term rainfall varied from 765 mm per annum for the Dirkiesdorp trial in the east to 494 mm per annum for the Wolmaransstad trial in the west. The duration of trials varied between one and nine seasons. The clay content of the top 150 mm soil at these localities ranged between 8.4 and 47%. Extractable P threshold values with varying R 2 values were derived for all localities. These values were related to soil properties and it was shown that the degree of leaching and silt-plus-clay content were the parameters that explained most of the variation. However, it was decided only to explore relationships between threshold P values and silt-plus-clay contents in more detail. By excluding data from two localities of which the topsoil contained free lime, the R 2 values of the mentioned relationships improved substantially so that P threshold values could be derived from the silt-plus-clay content range of the other seven localities. The extractable soil P threshold concentrations based on Bray 1 for the top 150 mm soil layer, to obtain 90% relative yield varied from 33.5 mg kg -1 at 13% silt-plus-clay to 14.6 mg kg -1 at 60% silt-plus-clay. These P thresholds were much higher on the sandy soils than the value of 19 mg P kg -1 (Bray 1) for 95% relative yield according to the ARC-Grain Crops Institute (1994) guidelines over all soils. This may not necessarily imply that overall more P fertilizers should be applied, since the corresponding soil sampling procedure also measures residual P from enriched zones over rows where P fertilizer was band-placed. Research results used to establish the ARC-Grain Crops Institute (1994) guidelines excluded sampling from enriched zones over rows.
  • Authors:
    • Agenbag, G. A.
    • Louw, P. J. E.
    • Fourie, J. C.
  • Source: South African Journal of Enology and Viticulture
  • Volume: 28
  • Issue: 2
  • Year: 2007
  • Summary: The trial was conducted over a period of ten years (1993/94 to 2002/03) on a sandy soil in a Sauvignon blanc/Ramsey vineyard near Lutzville (31degrees35′S, 18degrees52′E), situated in the semi-arid Olifants River Valley of the Western Cape. Fourteen treatments, consisting of three grain species and four legumes, managed according to two cover crop management practices, were included. One management practice consisted of cover crops which were sown annually and full surface, post-emergence chemical control which was applied before bud break and when the berries reached pea size (BB). The second management practice consisted of cover crops which were sown biennially. Post-emergence chemical control was applied to the vine row before bud break and full surface when the berries reached pea size (AB). From 1999/2000 to 2002/03 the cover crops were sown annually, while the full surface post-emergence control applied at the end of November was advanced to mid-October. Two treatments in which Avena sativa L. v. Saia ('Saia' oats) and Vicia dasycarpa Ten. (grazing vetch) were sown annually, controlled mechanically in the work row and chemically in the vine row from bud break to harvest (MC), were also applied. These treatments were compared to a control, in which no cover crop was sown and MC was applied. A treatment in which no cover crop was sown and BB was applied (weedchem), was also included. During the third growing season of the vines (1994/95), the grapevine shoot mass of the BB treatments of grazing vetch and Medicago truncatula Gaertn. v. Paraggio ('Paraggio' medic) was significantly more than that of the AB and MC treatments, with the exception of Secale cereale L. v. Henog (AB) and grazing vetch (MC). The first harvest (1994/95) from the grapevines in the BB treatments was significantly higher than that of weedchem and the MC treatments. The grape yield of the BB treatments, grazing vetch (AB) and Ornithopus sativus L. v. Emena (pink Seradella) (AB) was significantly more than that of weedchem and the control during the 1997/98 season. The NO 3-N concentration in the leaf petioles in all the cover crop treatments was, with the exception of the AB treatments of rye, M. truncatula Gaertn. v. Parabinga ('Parabinga' medic) and grazing vetch, significantly higher than that in weedchem and the control, as measured during the 1994/95 season. The NO 3-N concentration in the leaf petioles of the BB and AB treatment of a species differed significantly. The N concentration in the juice of the cover crop treatments during the 1995/96 season was, with the exception of 'Saia' oats (MC) and 'Parabinga' medic (AB), significantly higher than that of weedchem and the control. During the 1998/99 season, the N concentration of the juice in the BB and AB treatments of grazing vetch and pink Seradella was significantly higher than that of the MC treatments, two rye treatments, weedchem and the AB treatments of the other cover crops. The concentration of Ca in the juice of the cover crop treatments was, with the exception of the pink Seradella treatments, significantly higher than that of weedchem and the control. Wine quality did not differ between treatments.
  • Authors:
    • Marais, G. F.
    • Pakendorf, K. W.
    • Pretorius, Z. A.
    • Prins, R.
    • Komen, J. S.
  • Source: Australian Journal of Agricultural Research
  • Volume: 58
  • Issue: 6
  • Year: 2007
  • Summary: The cultivation of small grain cereals was introduced to South Africa by Dutch settlers in the 17th Century. According to historical records the first documented epidemic of wheat stem rust occurred in the south-western parts of the current Western Cape in 1726. Recurring stem and leaf rust epidemics were associated with expanding wheat production and became particularly severe in the winter-rainfall regions of the Western and Eastern Cape, as well as in the summer-rainfall regions of the Free State. The wheat stripe rust pathogen was first detected in South Africa in 1996. Due to susceptibility of cultivars at the time of this exotic introduction, stripe rust has caused significant losses in commercial wheat production over the past 10 years. Pathotype surveys of Puccinia graminis and P. triticina were initiated in the 1920s, but were discontinued until research on wheat stem rust was resumed in the 1960s. Recent evidence has shown that P. graminis f. sp. tritici continues to evolve. In addition, the annual number of wheat stem rust collections is increasing, emphasising the sustained threat of this damaging pathogen. A stem rust pathotype first detected in 2000, with newly acquired virulence for Sr8b and Sr38, currently constitutes more than 80% of all collections. Leaf and stem rust diseases also occur on barley, oat, triticale, and rye and are important production constraints in several regions. Some studies have described variability in these pathogens but long-term records of pathogenicity changes in barley and oat rust are not available. Cereal rust diseases have clearly played an important role in South African agriculture and many production regions remain favourable for rust development. Current expertise in cereal rusts covers most technologies necessary to study the respective host-pathogen systems. However, a general lack of capacity and fragmentation of research groups prevent a unified approach and remain a challenge for sustainable cereal rust control in South Africa. A national strategy for cereal rust control, with particular emphasis on pathogen and host resources, and breeding for resistance, is urgently needed.
  • Authors:
    • Lal, R.
  • Source: Critical Reviews in Plant Sciences
  • Volume: 22
  • Issue: 2
  • Year: 2003
  • Summary: An increase in atmospheric concentration of CO2 from 280 ppmv in 1750 to 367 ppmv in 1999 is attributed to emissions from fossil fuel combustion estimated at 270 +/- 30 Pg C and land use change at 136 +/- 55 Pg. Of the emissions from land use change, 78 +/- 12 Pg is estimated from depletion of soil organic carbon (SOC) pool. Most agricultural soils have lost 50 to 70% of their original SOC pool, and the depletion is exacerbated by further soil degradation and desertification. The restoration of degraded soils, conversion of agriculturally marginal lands to appropriate land use, and the adoption of recommended management practices on agricultural soils can reverse degradative trends and lead to SOC sequestration. Technological options for SOC sequestration on agricultural soils include adoption of conservation tillage, use of manures, and compost as per integrated nutrient management and precision fanning strategies, conversion of monoculture to complex diverse cropping systems, meadow-based rotations and winter cover crops, and establishing perennial vegetation on contours and steep slopes. The global potential of SOC sequestration and restoration of degraded/desertified soils is estimated at 0.6 to 1.2 Pg C/y for about 50 years with a cumulative sink capacity of 30 to 60 Pg. The SOC sequestration is a cost-effective strategy of mitigating the climate change during the first 2 to 3 decades of the 21(st) century. While improving soil quality, biomass productivity and enhanced environment quality, the strategy of SOC sequestration also buys us time during which the non-carbon fuel alternatives can take effect.
  • Authors:
    • Yang, H.
    • Walters, D. T.
    • Dobermann, A.
    • Cassman, K. G.
  • Source: Annual Review of Environment and Resources
  • Volume: 28
  • Issue: 1
  • Year: 2003
  • Summary: Agriculture is a resource-intensive enterprise. The manner in which food production systems utilize resources has a large influence on environmental quality. To evaluate prospects for conserving natural resources while meeting increased demand for cereals, we interpret recent trends and future trajectories in crop yields, land and nitrogen fertilizer use, carbon sequestration, and greenhouse gas emissions to identify key issues and challenges. Based on this assessment, we conclude that avoiding expansion of cultivation into natural ecosystems, increased nitrogen use efficiency, and improved soil quality are pivotal components of a sustainable agriculture that meets human needs and protects natural resources. To achieve this outcome will depend on raising the yield potential and closing existing yield gaps of the major cereal crops to avoid yield stagnation in some of the world's most productive systems. Recent trends suggest, however, that increasing crop yield potential is a formidable scientific challenge that has proven to be an elusive goal.