Influence of Hydrogen Ion Concentration on the Growth of Higher Plants under Controlled Conditions.

The effects of hydrogen ion concentration on plant growth occupy a prominient position in discussions of the adaptability of plants to their nutrient eiivironmelit. In the last two decades, with the improvements of methods of measurinig pH and particularly with the widespread adoption of the (lass eleetrode, increasing attention has been given to the hydrogen ion coneenitrationi in soils as it may influence their suitability for different crop plants. An uniderstanding 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 knowvledoge of the effects of hydrogen ion concentration on plant growth. There is considerable doubt at present about the precise meaning of a soil pH measuremiienit obtained by standard procedures ineluding the glass electrode.3 But even if some particular pH value were to be assigned to a soil as truly represenltative of the hydrogen ion concentration that happens to characterize the bounidary 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 indepenidenit variable. Is the failure of plants to thrive in an acid soil due to a high hydrogen ioni concentration or to such other unfavorable factors of which a low pH is generally symptomatic; for example, a depletion of calciumi ancd the presence of toxic amounts of aluminum or manganese in the soil soluitioni? 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 additioni to these indirect effects associated with pH which make it difficult to iinterpret the responses of plants grown in a soil, there is a dearth of information about the effects of hydrogeni ion concenitration on plant growth, with other factors of the nutrient environiment held constant. The purpose