Availability and residual effects of phosphate rocks and inorganic P fractionation in a red soil of Central China

The agronomic effectiveness, phosphorus utilization efficiency and inorgani c fractions of phosphorus, applied once to an Ultisol in Manning, Hubei pro vince, and then cropped 9 seasons, were measured. The results showed: (1) f or all crops, the relative agronomic effectiveness (RAP) of three phosphate rocks (PRs), compared with triple superphosphate (TSP), reached 103.1-154. 9% at three rates of P applied; (2) phosphorus utilization ratios during 5 years were 33.7-59.3% with the average of 35.0-52.7% for three PRs, greater than TSP; (3)the total amount of inorganic phosphorus in soils which recei ved phosphate fertilizers were 395.5-492.8 mg/kg, however, it was 387.4 mg/ kg in soil given no phosphorus fertilizer. The most remarkable increases we re occluded P(O-P) and calcium P(Ca-P) fractions at P 35 level (i.e. soil e xtractable P reached 35 mg/kg). (4) after 9 crops were harvested, the balan ce of phosphorus in the soils of control (CK) and all P 15 level treatments was negative, however, for soils treated with P 25 level and P 35 level, t he balance was positive.