Development of an alternative to the Olsen test for determining corn plant-available phosphorus in calcareous soils

In this research, several chemical extractants and modified Olsen methods were studied in 25 surface-calcareous soils (0-30 cm) for determining available phosphorus (P) for corn (Zea mays L.) plant. In this experiment, the highest correlation was obtained between extracted P by Olsen and Colwell methods (NaHCO3-extractable P) and dry weight of corn shoot (r =0.67 and 0.75, p<0.01, respectively). Therefore, effects of varying effective conditions of extraction such as the period of shaking, the concentration and pH of NaHCO3 solution on the amount of P extracted from the soil and its correlation with growth parameters of the corn plant was investigated. The results showed that increasing the pH of NaHCO3 solutions: soil ratio, shaking period and concentration of NaHCO3 solution significantly increased the extracted P from the soils, but could not significantly increase the correlation between P extracted from the soil and growth parameters of the corn plant. The highest correlation was obtained between modified Olsen method (with 4-hour shaking period and the solution: soil ratio of 6) and shoot dry weight (r=0.75, p<0.01) which was not significantly different (Δr=0.08) using Olsen’s test (0.5hour shaking time and solution: soil ratio of 20). The results of this study showed that P buffering capacity (PBC) could be the best method for predicting the available P by the corn plant.