EFFECT OF RAINFALL ON PLANT RESPONSE TO PHOSPHORUS

Abstract

Measurements of the phosphorus content of wheat at intervals during the growing season (1958–1962) indicated that percentage phosphorus in the growing plant was closely related to the amount of rainfall and to frequency of irrigation. The percentage phosphorus decreased during dry years or when irrigation was applied infrequently. This was attributed to the vertical stratification of available phosphorus in the soil since moisture use data indicated that a larger proportion of the active roots zone of the crop was in the phosphorus-deficient subsoil during dry periods. This relationship between percentage plant phosphorus and moisture supply was found with fertilized and unfertilized crops alike. However, frequent watering prior to the 5-leaf stage increased the difference in percentage plant phosphorus between fertilized and unfertilized wheat.Yearly variation (1936–1952) in the degree of plant response to phosphorus on a chernozemic soil was high. The degree of plant response was positively correlated with May rainfall and negatively correlated with June rainfall. It is proposed that this relationship existed because during years of above-normal May rainfall, phosphorus fertilizer was more efficiently utilized and with below-normal June rainfall a higher proportion of root development occurred in the phosphorus-deficient subsoil which increased phosphorus stress in the crop.