GROWTH AND NUTRITION OF YOUNG BEAN PLANTS UNDER HIGH ALKALINITY AS AFFECTED BY MIXTURES OF AMMONIUM, POTASSIUM, AND SODIUM

High concentrations of bicarbonate (HCO-3) cause alkalinity of irrigation water and are associated with suppression in plant growth and micronutrient deficiencies, such as iron (Fe) and zinc (Zn). Because reports indicate that the deleterious effects of alkalinity may be counteracted partially by supplementary potassium (K+) or ammonium (NH4+) an experiment was designed to evaluate the response of bean plants (Phaseolus vulgaris L.) grown in high alkalinity conditions to varying proportions of NH4+, K+, or sodium (Na+) (as a potential substitute for K+). Plants established in a growth chamber were grown in hydroponics for 21 days in solutions containing 5 mM HCO-3 and a total of 5 mM of a mixture of NH4+, K+, and Na+. The proportions of NH4+, K+, and Na+ were designed according to mixture experiment methodology. Total N in all the mixture treatments was maintained at 10 mM by using nitrate (NO-3)-N, thus the NH4+:NO-3 ratio varied according to the proportion of NH4+ in the mixtures. Alkalinity caused suppression in plant growth and chlorophyll concentration in the younger leaves, whereas excessive NH4+ was associated with leaf scorching and decreased leaf expansion. High proportions of K+ alleviated alkalinity symptoms and produced higher shoot and root dry mass provided that NH4+ was included in the mixture. However, a proportion of NH4+ higher than 0.333 in the mixture (1.66 mM NH4+) induced toxicity. The highest shoot dry mass occurred if the NH4+:NO-3 ratio was 0.19:0.81 and the NH4+:K+:Na+ proportion was 0.38:0.38:0.24 (1.9 mM NH4+ + 1.9 mM K+ + 1.2 mM Na+). Thus, an improvement in plant growth is achieved when NH4+, K+, and Na+ are blended together, in spite of the high alkalinity treatment imposed. Optimum NH4+ was associated with a decrease in solution pH and an increase in shoot Fe and Zn concentration.