PHOSPHORUS ADSORPTION CAPACITY AND AVAILABILITY OF ADDED PHOSPHORUS IN SOILS FROM AQUACULTURE AREAS IN THAILAND

A series of 20 soil samples was collected from aquaculture areas in 14 provinces of Thailand. Samples represented 10 soil suborders and exhi bited wide variation in physical and chemical properties. Soil samples were treated with 0, 25, 50, 100, and 200 ppm phosphorus and incubate d under water-saturated conditions for 1 mo. Amounts of added phosphor us recoverable by water extraction decreased markedly as phosphorus ad sorption capacity (PAC) of samples increased (r = 0.88 to 0.96, P < 0. 01). This suggests that relative abilities of bottom soils to adsorb a nd release phosphorus added to ponds in fertilizers or feeds can be as sessed from PAC data. Because of the importance of phosphorus adsorpti on by soil in regulating phosphorus availability to phytoplankton in p onds, the PAC appears to be a more useful technique than traditional p hosphorus extraction methods as an index of the phosphorus status of a quaculture ponds. The PAC was highly correlated with clay content of s oils (r = 0.957; P < 0.01), and a knowledge of clay content will permi t a rough assessment of phosphorus status.