THE ABILITY OF CHEMICAL-EXTRACTION METHODS TO ESTIMATE PLANE-AVAILABLE SOIL P AND A BETTER UNDERSTANDING OF P AVAILABILITY OF FERTILIZED ANDOSOLS BY USING ISOTOPIC METHODS

Plant-available P content in soils differing in P fertilization histor y was determined by both the isotopic dilution method (pot experiment) and isotopic exchange method (laboratory test) using phosphate ions l abelled with P-32 in order to evaluate the residual effect of P fertil izers applied to the soils. Another isotopic technique was also used t o evaluate the ability of chemical extractions, Truog and Bray No. 2 m ethods, to extract plant-available soil P. Four soil samples of a Humi c Andosol taken from experimental plots, which did not receive fertili zer (J1 treatment), or received a readily soluble fertilizer (RSF) (J2 treatment), RSF plus a fused magnesium phosphate (FMP) (J3 treatment) , and RSF plus Florida phosphate rock (J4 treatment), were analyzed. I n a pot experiment, maize (Zea mays) was grown on the soils where avai lable P was labelled with P-32. P uptake and specific radioactivity in shoot were measured. The fraction of P in plant derived from fertiliz er (%Pdff in plant) and pool size of isotopically exchangeable soil P (L-value) were determined. P uptake increased in the order: J1 < J2 = J4 < J3. The L-value became larger in the order, J1 = J2 < J4 < J3. Th e highest value of %Pdff in plant was also observed in treatment J3. M ore than 50% of P in the applied FMP remained in the soil as isotopica lly exchangeable P. Three parameters of soil P status, intensity, quan tity, and capacity factors, were examined by a laboratory test using t he isotopic exchange method. Quantity factor, which expresses the amou nt of isotopically exchangeable soil P (E(1)-value), was highest in th e J3 treatment, followed by the J4, J2, J1 treatments. These results a greed with the results of P uptake and L-value. The P availability of the J4 soil measured by the Truog and Bray No. 2 methods was higher th an that of the J3 soil. These data were not in agreement with plant gr owth and P uptake. It was found by another isotopic approach that not only plant-available P but also unavailable P was extracted from the s oils by the Bray No. 2 method. The Truog method also overestimated ava ilable P in the soils amended with P fertilizers but gave an accurate result for the unfertilized soil.