Direct application of phosphate rocks to rice on acid sulfate soils in South China

Acid sulfate soil is characterized with low pH, low phosphorus (P) availability, and excessive salt accumulation. Good fertilization (especially P) and water management will improve the productivity of this problem soil. In this study, field trials were conducted on rice with four phosphate rocks (PRs) from Morocco (Calcine PR and Brut PR), Algeria, and China, respectively, and one local single superphosphate (SSP) and one local triple superphosphate (TSP). The trials were arranged in both reclaimed and unreclaimed fields at two locations from 1998 to 2001. The phosphate fertilizers were applied to each crop before transplanting during the first three seasons. From the year 2000 on, the plots were split into two subplots with one receiving phosphate application every season and another receiving a single phosphate application with the amount for three seasons. The effectiveness of these phosphate fertilizers on rice growth was determined in terms of grain yield. Soil P status was also monitored. The present results from the first 3 years indicated that all phosphate fertilizers could increase rice grain yield significantly in all fields except the reclaimed field at Zhanjiang. The high soil available P in this field may explain this exception. No significant difference was observed among phosphate sources. A comparison of the cumulative and residual effects (split and single P application) indicated no significant difference between the two application methods. The rice yield on acid sulfate soils was much lower than that of rice grown on normal paddy soils. This indicates that there are more factors than P limiting rice growth on this type of soil.