INFLUENCE OF HYDROGEN ION CONCENTRATION ON THE GROWTH OF HIGHER PLANTS UNDER CONTROLLED CONDITIONS

Abstract

The effects of hydrogen ion concentration on plant growth occupy a prominent position in discussions of the adaptability of plants to their nutrient environment. In the last two decades, with the improvements of methods of measuring pH and particularly with the widespread adoption of the glass electrode, increasing attention has been given to the hydrogen ion concentration in soils as it may influence their suitability for different crop plants. An understanding of the responses of plants to soil reaction is dependent : first, on the determination of the true pH of the soil ; and secondly, on a knowledge of the effects of hydrogen ion concentration on plant growth. There is considerable doubt at present about the precise meaning of a soil pH measurement obtained by standard procedures including the glass electrode.3 But even if some particular pH value were to be assigned to a soil as truly representative of the hydrogen ion concentration that happens to characterize the boundary between the soil and the plant root, there still would remain the difficulty of interpreting a soil pH value in terms of an isolated and independent variable. Is the failure of plants to thrive in an acid soil due to a high hydrogen ion concentration or to such other unfavorable factors of which a lowT pH is generally symptomatic ; for example, a depletion of calcium and the presence of toxic amounts of aluminum or manganese in the soil solution? On the other extreme, there is the question whether poor plant growth in a soil characterized by a relatively high pH is to be attributed to a high hydroxyl ion concentration or to the unavailability of such plant nutrients as phosphate, iron, and manganese. In addition to these indirect effects associated with pH which make it difficult to interpret the responses of plants grown in a soil, there is a dearth of information about the effects of hydrogen ion concentration on plant growth, with other factors of the nutrient environment held constant. The purpose