Soil P status, inorganic P fractions, and P sorption properties were studied using sandy fluvo-aquic horticultural soils,which are high in organic matter content for vegetable production in comparison with a soil used for grain crop production in Zhengzhou, Henan Province, China. P fractions, Olsen-P, and OM were determined at different depths in the soil profile and sorption isotherm experiments were performed. Most P in excess of plant requirements accumulated in the topsoil and decreased with soil depth. Total P, inorganic P, and OM concentrations increased with continued horticultural use. Olsen-P concentrations in the 0-20 cm depth of horticultural soils were 9 to 25 times higher than those of the grain crop soil. A linear transformation of the Langmuir equation showed that the P adsorption maximum (491.3 mg P kg-i)and the maximum phosphate buffering capacity (162.1 L kg-1) for 80-100 cm were greater in the grain crop soil than the horticultural soils. Thus, the most immediate concern with excess P were in areas where heavy P fertilizer was used for vegetable crops and where soil P sorption capacities were low due to sandy soils and high organic matter content.
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