To study spatio-temporal variation of soil available phosphorus in farmland soils may provide some scientific basis for rationalizing fertilization and identifying key regions for control of agricultural non-point phosphorus pollution.Rugao County,Jiangsu Province was chosen as an object for a case study to analyze spatio-temporal variation of soil available P at three time periods.Results show that spatial distribution of soil available P varied remarkably in the 20 years' period from 1982 to 1997 and to 2002,on the whole showing a continuous rising trend,which,however,differed from soil series to soil series.In the Loam Aquatic cambosols (Yinfang) in the southern part of the area,soil available P increased in recent years after experiencing a decrease in the middle of the 1990s,and in the other soils it showed a rising trend,different in extent within each time period.In the sandy Aquatic Cambosols (Motou and Taoyuan) derived from river alluvium in the mid-western part of the area it increased more slowly than in the other soils in recent years,meanwhile in the loam Aquatic Cambosols (Dongchen),also developed from river alluvium,in the northeastern part and the loam Aquatic Cambosols (Banjing) derived from lacustrine sediment in the northwestern part of the area it increased very fast.Fertilization,land utilization and soil properties were found to be the main factors affecting spatio-temporal variation of soil available P.Based on the data of soil available P figured out with a certain fitting equation,it is anticipated that soil available P in the area may reach the critical value undermining the water bodies in the region in another 24 years.