RELATIONSHIPS BETWEEN LABILE PHOSPHORUS AND OLSEN PHOSPHORUS TEST VALUES OF ALKALINE SOILS

Abstract

For the development of soil and plant phosphate (P) model, the soil labile-P values are required in addition to fertilizer P availability indices. This study was conducted to determine the relationship of labile-P (IRA-400 anion exchange resin, NaHCOs form) with a routine Olsen-P (NaHCOi extractant) method in order to develop predictive equations for labile-P. Nineteen alkaline surface soils representing a wide range of properties with initial P treatment at the rate of 0, 30, 00, and 90 mg P kg'1 soil were used in laboratory experiment for a 7- day and a 6-month incubation period. Labile-P (Pa) and Olsen-P (OP) increased with increased P rates. Pa related linearly to OP values at all Prates, showing a decreasing trend in correlation coefficient (r = 0.877" - 0.751" with a 7-day incubation; r - 0.856" - 0.643" with a 6-month incubation) with increased P rates. After a 7-day incubation, about 77% of variation in Pu were explained by the regression equation and it decreased to 50% at higher P rate. Both incubation periods showed similar relationships , indicating the feasibility of a 7-day incubation period for such studies. Using mean values of all P treatments of a 7-day incubation (76 soil P test values), the relationship was further improved (r = 0.92", P = 0.01) by a workable predictive equation of labile-P (Pa) from Olsen-P (OP): Pa = 7.936 + 1.385 OP,