Comparison between conventional soil tests and the use of resin capsules for measuring P, K, and N in two soils under two moisture conditions

Abstract

A laboratory incubation study was conducted using sieved (< 2 mm) sandy loam (SL) and silt loam (SiL) soils collected from the plow layer of two upland fields in Hokkaido, Japan. The contents of P, K, and N in saturated (SAT, gravimetric water content=0.65 kg kg-1) and unsaturated (UNSAT, 0.32 kg kg-1) soils were measured using conventional soil tests and resin capsules incubated at 25°C for 1, 7, 14, and 28 d. The amounts of P and K adsorbed on the resin capsules were significantly lower under unsaturated than saturated soil conditions, showing a similarity to the absorption of P and K by plant roots. The two soils had the same P contents according to Bray II method but the content of resin P was much lower in SL than in SiL. The difference in the contents of resin P indicated the negative effect of the coarse texture on P diffusion while the difference between the two methods reflected the difference in the chemical characteristics of the reagents used. Under UNSAT conditions, and with the amount of soil used for incubation (50 g) as a basis for comparison, the content of resin N (predominantly NO3 -) after 28 d was comparable to that of KCl-extractable N in SiL after 7 d of soil incubation. In SL, however, the content of resin N was only 50–60% of that of KCl-extractable N, suggesting a faster diffusion/transport of N to plant roots in the finer-textured soil. Under SAT conditions, the transport of N, primarily as NH4 +, to the resin was faster and more efficient in SL than in SiL. The power function, RAQ i = a i t bi described well the release kinetics of N, P, and K in the two soils under SAT conditions and Nand K under UNSAT conditions. Adsorption of P on the resin under UNSAT conditions was markedly reduced even after 28 d resulting in a low fit to the power function. Resin adsorption quantity (RAQ) is represented by the amount of nutrient i adsorbed per unit surface area of the capsule at time t, and a and b are the rate coefficients used to describe the adsorption kinetics of the ion on the resin. For P, high estimates of b P were obtained under SAT conditions in both soils, suggesting that their P-supplying capacity was high. However, a low value of a P was obtained in SL, indicating the presence of a small amount of readily available P in this soil. Kinetics of resin K showed the presence of a high amount of readily available Kin SiL while a low amount in SL, in conformity with the amounts of NH4OAc-extractable K in the two soils. Resin N (NH4 + + NO3 -) kinetics showed comparatively high RAQN and b N values between SiL under UNSAT conditions and SL under SAT conditions, indicating a similarity in the N-supplying capacity of the two soils although transport and/or loss of N were determined by the moisture and texture. Results of this study confirmed the findings from previous studies showing that the resin capsule method is suitable for assessing the plant-available nutrients in soil and indicated that the differences in nutrient transport to plant roots were due to soil moisture and texture.