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INFLUENCE OF SOIL CHARACTERISTICS AND ENVIRONMENTAL CONDITIONS ON SEASONAL VARIATIONS OF WATER‐SOLUBLE PHOSPHATE IN SOILS

We investigated the seasonal variation of water-soluble phosphate (P) in a Sultan silt loam (Aquic Xerofluvents) with and without wheat or corn residue amendment. The soil was cropped to corn (Zea mays L.) immediately after the residues were incorporated. The P soluble in water and pH for the soil showed a similar annual pattern regardless of residue treatments; they were generally low during the drier and warmer spring and summer, gradually increased in the fall, and reached a maximum during the winter high-rainfall period. High correlations between water-soluble P and soil pH (r = 0.75–0.88, P < 0.01) and between water-soluble P and inorganic N (NH+4-N and NO−3-N) r = −0.65–0.88, P < 0.01, depending on residue treatment) were observed, indicating that the seasonal variations of water-soluble P could largely be explained by these two soil characteristics. Because soil pH and water-soluble P were both negatively correlated with temperature and with NO−3-N levels in the soil and could be further decreased with increasing rates of N application, the biological mineralization of organic N and the transformation of NH+4-N to NO−3-N likely played an active role in the seasonal pattern of water-soluble P in the soil.