The effect of water quality on the growth and yield of irrigated crops

English: Salinity is a major limitation and threat sustainable crop production in South Africa and elsewhere in the world. An intensive study by Du Preez et al. (2000) on the water quality of the lower Vaal River system using international salinity indicators were conducted and it was concluded that unacceptable damage to crops would be experienced if the salinity of the irrigation water continued to increase at the projected rates. Following that, intensive germination and glasshouse pot experiments were conducted based on the projected longterm salt accumulation on irrigated soils. Little or no quantitative information on the subject was available for South African conditions. This study was also part of a WRC project titled: The effect of irrigation water salinity on the growth and water use of selected crops. The objective of this study was to quantify the effect of different saline irrigation water levels (electrical conductivity, ECi ) on the germination, growth and yield of selected crops, viz. wheat (Triticum aestivum L. - SST 806.), maize (Zea mays L. - PNR 6335), peas (Pisum sativum L. - Solara) and beans (Phaseolus vulgaris L. - Teebus) at different growth stages. These crops were subjected to five different ECi levels (15 - control, 150, 300, 450, 600 for wheat and maize, and additionally 1200 mSm-1 for wheat, and 15 - control, 75, 150, 225, 300 mS m-1 for peas and beans). Three stages were used for each crop, viz. tillering, flag leaf and maturity (wheat), 2, 4 and 6 wae - weeks after emergence (maize) and 5 wae - five weeks after emergence, flowering and maturity (peas and beans). Various morphological indicators presented on a relative scale were measured to quantify the impact of ECi levels on both below and above ground growth. When subjecting the crops to the different ECi levels, the salt tolerance of these plants was taken into consideration. These crops were affected at varying degrees, depending on the salt sensitivity of the crop involved. Interesting results were obtained for all the crops and based on the findings the study agreed well with the international accepted salinity classification system where wheat is classified as moderately tolerant and peas and beans as sensitive. Maize proved to be sensitive in this study, but it has to be noted that it was only at a very early growth stage. The water use of the pot experiment plants was found to be very high and this was attributed to the smaller volume of soil relative to the canopy. All plant growth indicators proved to be negatively affected over the selected ECi range. The reduction in growth for all crops followed a similar trend with increasing ECi levels. The EC of the saturation soil extract (ECe) was also measured in order to determine the rate of soil salinisation and was found to be 2 to 3 times that of ECi. The study also compared the responses of these crops by using primary growth indicators (leaf area, root mass, biomass and seed yield) and also attempted to determine the salt tolerance values using the regression coefficients. Therefore, all the objectives set for the study were achieved. The effect of salinity on a variety of other crops under South African conditions should be thoroughly and comprehensively investigated in future.