Rainfall intensity effects on crusting and mode of seedling emergence in some quartz-dominated South African soils

Predicted changes in rainfall intensity due to climate change are likely to influence key soil health parameters, especially structural attributes and crop growth. Variations in rainfall intensity will impact crop production negatively. It is therefore imperative to investigate the interaction between predicted increases in rainfall intensity and key soil health parameters, particularly in relation to soil structural attributes and plant growth. The objectives of this study were to determine the effects of rainfall intensity on soil crust formation and mode of seedling emergence in soils dominated by primary minerals. Soil samples were collected from the top 200 mm, air dried and then packed uniformly into plastic pots, which were perforated at the bottom. Three maize seeds of equal size were planted in a triangular pattern in each pot at a depth of 30 mm, after which the pots were pre-wetted by capillary. The samples were then subjected to simulated rainfall at 3 intensities, i.e., 30, 45 and 60 mm/h, for 5 min. Rainfall intensity significantly (P < 0.05) affected crust strength and mean emergence day (MED), but not emergence percentage (EMP) and shoot length (P > 0.05). The 60 mm/h rainfall intensity resulted in the highest crust strength and MED. The strength of crust for all three rainfall intensities was influenced by quartz content, soil organic matter, clay and hematite. Most seedlings emerged through cracks, which resulted in rainfall intensity having no significant effects on seedling EMP and shoot length. We concluded that any increase in rainfall intensity is likely to increase the severity of crusting in these soils. However, soils with extensive cracking are likely to have higher EMP and lower MED and more vigorous seedlings despite the strength of the crust. As a result, post-planting tillage methods that enhance crust cracking may be employed to enhance seedling emergence and growth in these soils.Keywords: climate change, crusting, mineralogy, penetration resistance, soil organic matte

Effects of water stress at different growth stages on yield and yield components of common bean (Phaseolus vulgaris)

Common bean (Phaseolus vulgaris L.) has great potential for improving human nutrition due to its high protein content. However, water stress was found to be one of the major constraints to common bean production in Zimbabwe. A field experiment was therefore carried out at Agricultural Research Trust farm in early summer of 2004, and repeated at the same time in 2005, to determine the effect of water stress at different growth stages on grain yield and yield components of three common bean varieties. The experimental design was a 3×5 split-plot in a Randomised Complete Block Design; with water stresses as the main plot factor (withholding irrigation for a period of twenty-one days at; two weeks after emergence (WAE), four WAE, at flowering and two weeks after flowering (WAF) as well as non-stressed control) and the varieties in the subplot factor (Onyx, Pan 329 and Dark Red Kidney). Water stress significantly reduced grain yield (p<0.05) with flowering and two WAF stages being the most sensitive, and vegetative stage being the least sensitive. Water stress also produced significant effects (p<0.05) on yield components. Water stress during flowering and after flowering significantly reduced number of seed per pod and seed weight. For maximum yield, water stress must be avoided during flowering and pod-filling stages, so early planting is important under dry land planting