EFFECT OF WEATHERING ON THE TRANSFORMATION AND AVAILABILITY OF PHOSPHORUS IN DIFFERENT FRACTIONS OF PARTICLE SIZE OF MAIN SOILS OF CHINA
Gu Yichu, Jiang Baifan and Lu Rukun(Institute of Soil Science, Academia Sinica, Nanjing)
The present paper deals with the effect of the intensity of weathering process during soil formation on the composition, distribution and transformation of phosphorus of various soil types and fractions of soil particle sizes in main soil types of China. Studies indicated that the higher the weathering degree of the soil, the greater the relative contents of total phosphorus, available phosphorus and organic phosphorus in clay. For example, the total phosphorus in clay fraction was five times as much as that in sand fraction of latosol, while this ratio fell below two in dark loessial soil which was more weakly weathered compared with the former. It was also showed that the forms and distribution of inorganic phosphate were also affected by the weathering process. In dark loessial soil the occluded phosphate might be up 47% of total inorganic phosphate in clay fraction, far higher than that in the original soil. On the contrary, the content of occluded phosphate in latosol was very high, but no significant differences in the content of occluded phosphatewere observed among different faction of narticle sizes. Calcium phosphate was found to be rich in sand, especially in calcareous soils. Aluminium phosphate was characterized by a transitional product of weathering process was accumulated only in transitional soil. Obviously, the distribution of various inorganic phosphate forms in different fractions of particle sizes in these soils is closely related to the weathering degree during soil formation. It is considered that the transformation of inorganic phosphate during soil formation may be as follows: firstly the primary mineral apatite is weathered into aluminium phosphate, and finally it is converted into iron phosphate and occluded phosphate. Correlation between content of available phosphorus and content of various forms of soil inorganic phosphorus in different fractions of particle sizes demonstrats that aluminium phosphate in soils is likely to be the source of available phosphorus to crops. And in neutral or acidic soils, the source of available phosphorus also includes a part of unoccluded iron phosphate and calcium phosphate.