The Mineralization of the Organic Phosphorus of Various Compounds in Soil

R. W. PEARSON, A. G. NORMAN, AND CHUNG Ho THE amounts of phosphorus present in many soils in .forms available to higher plants and the value of green and farm manures as phosphorus carriers depend to a large extent upon the rate of liberation of phosphoric acid from organic combination. Although recent work (13, 17) has shown that some plants may absorb certain organic phosphorus compounds directly, it is generally believed that the bulk of the organic phosphorus in soils must be converted to inorganic form before it can be absorbed by plants. This process of mineralization is particularly important in soils high in organic matter, such as those of the Prairie group in the surface layers of which the phosphorus in organic combination often amounts to 50% or more of the total phosphorus (12). It is also of great importance in regions where commercial phosphate fertilizer cannot be obtained cheaply. This applies especially to such countries as China, India, and Japan, where phosphorus fertilization depends almost entirely upon the addition of organic phosphorus-bearing materials such as animal manure, green manure, and various, plant products. In spite of the importance of the problem to soil fertility very little is known about the rates of mineralization, or conversely, immobilization, of phosphorus by microorganisms in the soil, and the influence of soil reaction on these transformations. Vincent (16), however, has reported that the mineralization of the organic phosphorus compounds present in acid soils is favored by liming. Although several investigations (2, 6) have been made of the changes in the relative proportions of organic and inorganic phosphorus in decomposing organic materials, no similar study has been reported of the changes occurring in organic materials added to the soil. During the course of this investigation, Dyer and Wrenshall (3) reported a study of the susceptibility of phytin and nucleic acid to decomposition after addition to soil. However, their conclusions were based on changes in dilute acid-soluble inorganic phosphorus which may not provide a wholly accurate measure of organic phosphorus decomposition. The rate of decomposition of these two compounds has a direct bearing on the organic-inorganic phosphorus relationship in soils since they form the bulk of the organic phosphorus of plant tissue and appear to constitute a large part of the organic phosphorus of soils (14, 18, 19). The studies to be described in this paper fall into two groups. The first includes investigations of (a) the mineralization of the organic phosphorus of various plant materials and the influence of liming on this process, and (b) the release of phosphorus from phytin and nucleic acid in soil. The second group includes (a) an examination of the comparative rates of mineralization of the organic phosphorus from nucleic acid, its component nucleotides, and from samples of microbial tissues, and (b) an attempt to obtain information as to the possible extent of immobilization accompanying the decomposition of nucleic acid.