The ability of chemical extraction methods to estimate plant-available soil P and a better understanding of P availability of fertilized andosols by using isotopic methods.
1995
Kato N. | Zapata F. | Fardeau J.C.
Plant-available P content in soils differing in P fertilization history was determined by both the isotopic dilution method (pot experiment) and isotopic exchange method (laboratory test) using phosphate ions labelled with 32P in order to evaluate the residual effect of P fertilizers applied to the soils. Another isotopic technique was also used to evaluate the ability of chemical extractions, Troug and Bray No.2 methods, to extract plant-available soil P. Four soil samples of a Humic Andosol taken from experimental plots, which did not receive fertilizer (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. In a pot experiment, maize (Zea mays) was grown on the soil where available P was labelled with 32P. P uptake and specific radioactivity in shoot were measured. The fraction of P in plant derived from fertilizer (%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. The highest value of %Pdff in plant was also observed in treatment J3. More than 50% of P in the applied FMP remained in the soil as isotopically exchangeable P. Three parameters of soil P status, intensity, quantity, and capacity factors, were examined by a laboratory test using the isotopic exchange method. Quantity factor, which expresses the amount of isotopically exchangeable soil P (E1-value), was highest in the J3 treatment, followed by the J4, J2, J1 treatments. These results agreed 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 than that of the J3 soil. These data were not in agreement with plant growth and P uptake.
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