Aspects of Mineral Nutrient Balance as Related to Sap Hydrion Concentration.

Abstract

Fluctuations in amounts of mineral nutrients found in plants is known to exert a profound effect on their metabolism. Results of recent experiments with grain plants grown on a number of acidic, humus soils deficient in potash disclose striking effects of certain mineral nutrients.

The accompanying chart (Fig. 1), typical of conditions in a number of grains, depicts schematically the chemical analyses of 3 oat crops (Avena sativa) grown on a soil variously treated with mineral

fertilizers. Variation in the amount of potassium is readily apparent. Correlated with decreases in potash is a fall, often greater than 3 pH units, in a sap hydrion concentration, a variation of sufficient magnitude to distinctly alter the solubility and, hence, also the mobility of essential nutrients in plants. The effects of impaired translocation are thus apt to be superimposed upon those of potassium insufficiency. External symptoms of injury and the results of tissue analyses support this view.

Potash starvation is known to interfere with carbohydrate storage, a process of fundamental significance in plants. Concomitant with decreases in potassium there was observed a decrease in hydrolyzable carbohydrates and an increase in nitrates. The great increase in soluble nitrates during potash starvation suggests a carbohydrate insufficiency severe enough to impede protein synthesis. This is true even though allowance is made for nitrates which accumulate in soils in the presence of lime. Consideration of this phenomenon is especially pertinent because lime was abundant in soils and tissues low in potash. In fact, the data (Fig. 1) show that potassium on the one hand is rather delicately balanced by calcium and magnesium on the other, an increase in the former entailing a diminution in the latter and vice versa.

Effects of potash insufficiency were accentuated by depressed iron mobility in plants whose sap had a low hydrogen-ion concentration. Inability to translocate insoluble forms of iron from roots to foliage interferes with the photosynthetic mechanism of the plant by impairing chlorophyll formation. Although analyses disclosed little variation in iron content of entire plants, iron was found massed in roots of specimens whose sap pH was near 7 while foliage of these same plants suffered from iron chlorosis. Plants from acid soils had their iron more evenly distributed between roots and tops.