Effect of soil forms on soil moisture and dryland cotton production
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Cotton (Gossypium hirsutum) production in South Africa relies on rainfed cultivation, and as such the availability of water is a key factor in the production of the crop. Cotton production in South Africa is declining gradual annually. The decline is marked by the competition with traditional crops, high input costs, international market prices and a lack of technical production information. The cotton produced in the 2019/2020 season consisted of 27 850 hectares in total from which 15 966 hectares is produced under dryland conditions, and 11 884 hectares under irrigated conditions. Dryland cotton production has been becoming more prominent, which establishes the need for technical information in the production under these conditions. Technical knowledge regarding soil moisture regimes is unknown in the industry and this uncertainty prevents the continued cultivation of the crop. This study aims to quantify the effect of the different soil forms and their associated water regimes on the rainfed cotton seed yield and quantity. The effect of the water table was duly visible as bedrock interflow soils exhibited a higher amount of soil water throughout the production season in comparison with sandy clay soils. It was expected that the sandy clay soils would indicate higher soil water contents throughout the production season, but certain interflow soils had a substantial amount of soil water more during the both the productions seasons. To determine the soil water regime the relationship between the soil water and the soil matric potential was established. The soil matric potential was determined by means of undisturbed core sampling and suction under pressure by using a pressure plate apparatus. By determining the soil water capacity at 33 and 1500 kPa, the drained upper limit (DUL) and lower limit (LL) was established for each of the various soil forms. Through the utilization of these parameters, the amount of planting-available water was determined which is critical in the production of cotton. Considering the DUL and LL the state of the soil water content throughout the production season was assessed for each of the soil forms. It was evident that sandy clay loam soils tend to be above DUL during a high rainfall period, but due to the drainage capacity of the soils, the period above DUL does not occur over a long period. The sandy clay soils indicated the opposite effect by being above DUL during the entire production season. The cotton quality was assessed with the High-Volume Instrument (HVI) system and the focus was on the main quality parameters such as length, strength and micronaire. It was evident that the sandy clay soils produced the best quality cotton in comparison with the sandy clay loam soils. Even though these soil’s soil water content was above DUL it produced thicker and stronger fibres with an optimal spinning consistency in comparison with the sandy clay loam soils. The cotton yield maps for each of the sites was obtained and the yield per soil form was extracted for the specific sites of measurement. Although no statistical differences occurred between the different soil forms it was evident that sandy clay loam soils produced a higher yield in comparison with the sandy clay soils. The ability of cotton roots to shoot in sandy clay loam soils with a less dense compaction, in association with the ability of soil water to be drained enabled the cotton to produce more cotton bolls, resulting in a higher yield. It was evident that the amount of soil water did vary between the various soil forms due to the variation in the soil physical properties such as the soil profile depth, clay percentage of the soil horizons, and soil moisture retention capabilities. Sandy clay soils tend to remain above DUL for longer periods than sandy clay loam soils, which causes anoxic conditions within the soil profile. These anoxic conditions result in stress induced effects in the rooting system which causes a lowered yield in comparison with the sandy clay loam soils which provides an oxidised environments for the rooting system, resulting in increased yields. Sandy clay loam soils tend to drain freely due to the large amount of macro pores, which in turn creates an oxidized environment for roots to develop and extract soil water. It is recommended that the sites for both seasons remains the same to ensure consistent data with less variation. In association with the sites the same cultivar should be produced over two cropping seasons to ensure that all possible variation is excluded from the results. By including a weather system that measures the evaporative demand, the soil water balance can be defined more accurately.