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Evaluation of Residual Effects of Phosphate Fertilization by Laboratory and Plant-Response Methods¹

ABSTRACT

Samples of soil were collected in 1955 from three field experiments in which 0, 13.1, 26.2, 52.3, and 104.7 pounds of P per acre had been applied as superphosphate to one series of plots in 1953 and to a second series in 1954. In a greenhouse experiment conducted in 1957, sorghum was grown on quantities of these samples without further phosphate fertilization and on quantities of the samples from the control plots after treatment with superphosphate at rates equivalent to those applied in the field. Conditions in the greenhouse experiment were such that the yield of P in the test crop increased linearly with the quantity of P applied to each soil in each year. Samples of soil from all phosphate levels and years of application were analyzed in the laboratory for labile inorganic P by the 0.025N HCl, 0.03N NH₄F method of Bray and Kurtz, the anion-exchange resin method of Amer et al., and the NaHCO₃ method of Olsen et al. The quantity of P extracted with each method increased linearly with the quantity of P applied to each soil in each year. The ratios of the slopes of the linear plots of yield of P obtained with the 1953 and 1954 applications to the 1957 applications were calculated for samples of soil from each of the three field experiments, and these were designated as availability-coefficient ratios of residual and currently-applied P. Corresponding slope ratios calculated from the quantities of labile P extracted were designated as extractability-coefficient ratios of residual and currently-applied P. The six availability-coefficient ratios thus obtained from the two residual years in the three experiments were correlated with the corresponding extractability-coefficient ratios, the best correlation (r = 0.98) being obtained with extractability-coefficient ratios derived from the 0.025N HCl, 0.03N NH₄F method, second best (r = 0.90) with the anion-exchange resin method, and third best (r = 0.51) with the NaHCO₃ method. These results show that appropriate laboratory tests provided an estimate of the relative residual value of fertilizer P that was largely independent of the nature of the soil.

¹ Journal Paper No. J-3833 of the Iowa Agr. and Home Econ. Exp. Sta., Ames. Projects 1183 and 1189. Contribution of the Department of Agronomy. This work was done in cooperation with the Division of Agricultural Relations, Tennessee Valley Authority.

² Research Associate, Associate Professor, Professor, former Research Associate (currently Soil Scientist, Agricultural Research Service, USDA, Pendleton, Ore.), and Professor, respectively.

Received for publication April 2, 1960. Accepted for publication May 13, 1960.