Variations in potassium content of alfalfa due to stage of growth and soil type and the relationship of potassium and calcium in plants grown upon different soil types

The stems and leaves and the juice of stems and leaves of alfalfa plants grown upon different soil types were analyzed for potassium at a number of stages during the growth period. Variations in the potassium content of the alfalfa plant at different hours of the day were noted as was also the relationship of potassium to the specific gravity of the expressed juice of the plant parts. The relative distribution of potassium in the sap and in the woody tissue of the plants was determined. A consideration of the physiological relationship between the elements potassium and calcium was made. Soil type differences were responsible for marked variations in the potassium content of alfalfa stems and leaves and of their juice. The greatest percentages of potassium were present in the plants grown upon the light sandy soils, medium amounts were present in the plants obtained from the very heavy soils, and the smallest amounts in the plants from the sandy loam soils. The potassium content of the plants was not an exact function of soil texture. Greater amounts of potassium were present in alfalfa stems than in the leaves, while about equal amounts were present in the expressed juice of the two plant parts. There was a decided decrease in the percentage of potassium present in the stems and leaves and in their juice as the growth period advanced. About equal amounts of potassium were present in the plants of the first and second crops and apparently the potassium content of the soil was not depleted at any time to the extent that it controlled the percentage of potassium contained in the plants. Potassium evidently was not an important cation in determining the specific gravity of the expressed juice of alfalfa sterns and leaves, an increase in the specific gravity usually occurring in spite of a decrease in the concentration of potassium in the juice. The potassium present in the green material of the alfalfa stems and leaves existed largely in solution in the plant sap, very little of it being held intimately in the woody tissue. About equal amounts of potassium were present in the woody tissue of the stems and leaves, although slightly more was deposited in the woody tissue of the leaves by the full bloom stage. In the plants obtained from some soil types increasing amounts of potassium were deposited in the woody tissue, while in the plants from the other soil types a decrease occurred during the growth period. A physiological balance appeared to exist in the alfalfa plants between the elements potassium and calcium. An inverse correlation was established between these two elements in the plants obtained from the different soil types, an increased calcium content being found to be accompanied by a decreased potassium content, both at different stages of growth and in the different plant parts. Potassium and calcium appeared to be interchangeable in the plants, although not necessarily alike functionally. The potassium content of the plants depended upon the calcium content, which in its turn appeared to be dependent upon the available supply of this element in the soil, although this last cannot be stated definitely until further data are obtained.