INFLUENCE OF ACIDITY, CALCIUM AND MAGNESIUM ON GROWTH OF XANTHI TOBACCO IN WATER-CULTURE

The influence of acidity on the growth of plants in water-culture has been studied by investigators with varying results. Many have reported a beneficial effect of moderate acidity; whereas others have succeeded in demonstrating equally good growth in solutions of widely different hydrogen-ion concentration. Citations to the literature can be found in a paper by the writer (5) dealing with the effects of acidity on the calcium and magnesium requirements of Xanthi tobacco in aseptic culture. The present experiments are in effect a repetition in the greenhouse of those in aseptic culture at 25? C with about 500 foot-candles of fluorescent light. The yield curves for magnesium remained unaltered in the aseptic cultures with increased acidity, whereas those for calcium did not. An increase in the quantity of calcium needed for maximum growth was apparent at higher acidities. The use in greenhouse experiments of large containers with fixed charges of nutrients, the presence of heavy precipitates, and the need for preparation of the solutions weeks in advance of the growth period all combined to make iron nutrition of the plant unsatisfactory. Preliminary experiments on iron nutrition were therefore necessary and furthermore seemed desirable, since acidity is generally accepted as an important factor in iron nutrition. A factor in acidity studies generally ignored is the effect of acidity on the toxicity of even small quantities of trace-element impurities unavoidably present in the solution. It is well known that the harmful effects of high acidity may be aggravated by the presence of excess aluminum, manganese (1), and vice versa. Work with fungi also has revealed that higher acidity increases the effect of trace-elements on growth (4) even in solutions free of all solid materials. A further factor in studies on iron chlorosis is another effect of the presence of relatively small but still excessive quantities of trace-element impurities. Symptoms of iron chlorosis in plants may be produced by relatively excessive quantities of zinc, copper, manganese, aluminum, cobalt, nickel, etc. (1, 3). These facts may be summarized by the statement that excessive quantities of trace elements relative to iron may lead to toxic responses which are further aggravated by increased acidity. These phenomena were assumed to be the basis for the differences in results reported for studies of acidity and iron nutrition by various workers. Special attention, therefore, was given to the effects of iron salt hydrolysis and traceelement level in these studies.