THE ABSORPTION OF PHOSPHATE FROM SOIL AND SOLUTION CULTURES

Abstract

In the study of the phosphate nutrition of plants it is of importance to know what relation exists between the soil solution and the absorption of phosphate by the growing plant, concerning the power to do which, there is a difference of opinion. Some emphasize the effect of carbon dioxide arising from the respiration of the roots ; others suggest an internal factor related to the metabolism of the plant. The question of the rate of supply as well as the effect of concentration is of great importance in the interpretation of results of soil and solution culture experimentation. This has been overlooked by certain of the earlier plant physiologists and has led to an emphasis of the importance of the action of the roots on the soil particles. There is, however, ample experimental evidence to show the ability of plants to absorb phosphate from solutions comparable in phosphate concentration with the soil solution. Thus Birner and Lucanus (4) and Beyer (3) as early as 1870 had shown the ability of oat plants to make normal growth in well water containing 2 ppm. P04 and Schloesing fils (13) and Kossowitch (9) arrived at similar conclusions some thirty years later. Schloesing fils found 1 ppm. P04 adequate for certain plants and concluded that the soil solution supplied at least the major part of the phosphate absorbed from the soil. Kossowitch recognized a difference between flax and mustard and suggested that this was due to effect of the carbon dioxide formed by the roots. Hoagland and Martin (7), using solution cultures, obtained good growth of barley at 0.7 ppm. PO4. Somewhat better growth occurred in similar solutions containing 2.4 ppm. and 9.0 ppm. PO4. The role of calcium carbonate in limiting absorption from rock phosphates has been indicated by Prianischnikow (12), Breazeale (5) and others. Apparently the carbonic acid arising from root respiration is neutralized by the calcium carbonate with the formation of calcium bicarbonate which depresses the solubility of calcium phosphate. It seems that carbonic acid may be an important agent in the solution of soil phosphates, but