Khwam samphan rawang kha wikhrao phosphorus nai din doi withi khemi tang-tang kap kan charoen toepto phonphalit lae poriman phosphorus thi khao-phot dut chak chanit tang-tang.

A pot experiment was conducted to compare the reliability of various indexes for soil phosphorus, namely, phosphorus extracted by Brays II method, Olsen method, anion exchange resin water and P concentration in soil solution and P concentration in equilibrium solution plus buffering capacity. The critical level was also examined. Results showed that relative reliability of different P soil test values varied when different data from plant were used as reference. However, when P uptake was used as reference, correlation coefficients were found in a decreasing order of P concentration in equilibrium solution plus buffering capacity, amount of P extracted with anion exchange resin method, amount of P extracted with Olsen's method, amount of P extracted with Bray's II method, and amount of P extracted with water, respectively. Therefore, P concentration in equilibrium solution plus buffering capacity was the best index for soil phosphorus and better than amounts of P extracted with anion exchange resin method, Olsen's method, Bray's II method, respectively. But P concentration in equilibrium solution plus buffering capacity and anion exchange resin method were not practical for routine service, because of tediousness in obtaining the indexes. The Olsen's and Bray's II methods seem to be more suitable because they were less tedious and gave appreciably high correlation coefficient. Critical level of soil phosphorus for corn had 95% of the maximum possible yield, was 22 ppm P for Olsen's method, 27 ppm P for Bray's II method, and 4 ppm P for concentration in soil solution. However, it must be noted that these values were from the pot experiment and, thus, would tend to be higher than those from the field experiment.