Die invloed van variërende vrywatervlakke in grond van die noordwestelike O.V.S. op verbouing van gewasse (veral mielies)

Abstract

Results of research about the origin of water tables in yellow brown apedal sands which occur extensively in the northwestern O.F.S. and its influence on the production of maize are discussed. Morphologic characteristics, typically associated with soils are grey matrix colours, soft plinthite, yellow brown apedal B-horizons and E-horizons. Observations in piezometers, installed at various localities in the research area, suggest that the free water accumulates on paleo-surfaces. The paleo-surfaces, covered by solums derived from aeolian sand, consists of gleyed clayey paleosols or the weathering products of underlying rocks. The free water is phreatic and its presence over long periods can be inferred from the morphology of the soil. High concentrations of cations and anions in the free water reach the root zones and influence the nutrition of plants. The height of capillary rise to the rootzone for different flood densities was calculated to evaluate the influence of fluctuating water tables on the supply of water to plants. The maximum height of capillary rise in clay, calcrete, hard plinthite and sand was measured. A technique was developed for this purpose. Various implements used by farmers in the area for tillage of sandy soils, were evaluated. A threetine ripper gave the best results. Experiments to evaluate the influence of water content and various implements on the compaction of soils were used to set guidelines for tillage-practices in the area investigated. Collapse settlement took place with saturation, causing consolidation which eliminate the positive effect of tillage. In oedometer tests, the internal resistance against compaction exceeded matrix potential at void ratios of 0,71 and less. The pore diameter, where the internal resistance against compression equals matrix potential, was calculated experimentally as 0,0327 mm. Extremely high water tables are unfavourable for the cultivation of maize. Water tables that reach the roots, through capillary action, but fluctuates deeper than the depth of tillage, will supply water to plants during dry periods, without resulting in harmful anaerobic conditions. Soils with water tables fluctuating within this reach during the growth season, could be advantageous for the cultivation of maize. Soil in which water tables are too deep for capillary water to reach the root zone, may regarded be as high risk soils.