THE EFFECT OF TIME ON PHOSPHORUS AVAILABILITY ON TWO SOILS AS DETERMINED BY SOIL ANALYSIS AND PLANT UPTAKE

Laboratory analysis and greenhouse experiments were conducted to investigate if standard soil tests for P adequately evaluate the availability of applied fertilizer, the nature and availability of residual P, and evaluation of the applicability of Langmuir and Freundlich adsorption equations in describing P sorption by a calcareous Uly silt loam subsoil and an acidic Thurman loamy fine sand topsoil. Monopotassium phosphate (KH(,2)PO(,4)) at 0, 10, 20, 40, and 80 mg P kg(\u27-1)was mixed with soils and incubated at field capacity for 20 months. Phosphorus was extracted with Bray P-1, NaHCO(,3), Mehlich No. 2, North Carolina, and Truog\u27s extractants. Oats (Avena sativa) were grown on the soils after 4, 8, and 20 months. The A and L values were also determined using (\u2732)p. Highly significant (P \u3c 0.01) regression coefficients were obtained between extractable P with A and L values. The results suggest that Bray P-1, NaHCO(,3), and Mehlich No. 2 extractants were better indicators of plant available P as compared to North Carolina and Truog extractants. However, it was apparent that as soils were fertilized with P. all three extractants underestimated plant available P. Maximum dry matter yield and total P uptake was obtained at \u3e40 mg P kg(\u27-1). Generally, there was a decrease in extractable P with time using the five soil tests in both soils. However, there was high correlation (r \u3e 0.88) among amounts of Bray P-1, NaHCO(,3), and Mehlich No. 2 extracted phosphorus. The phosphate potential studies indicated that fertilizer reaction products are P rate dependent. Uly soil was undersaturated with octocalcium phosphate (OCP) \u3c20 mg P kg(\u27-1), whereas the soil was at equilibrium or OCP was precipitated at \u3e40 mg P kg(\u27-1). The pH and phosphate potentials suggest amorphous Al-P and Fe-P were the dominant reaction products in Thurman soil. The sorption data indicated two adsorption surfaces in Uly soil and one adsorption surface in Thurman soil