Soil Test Correlation Problems with Two Phosphorus Methods on Similar Soils
1981
Survey data on the fertility status of soils in western Nebraska, on which sugarbeets (Beta vulgaris) were grown, indicated that 78% of the fields tested were high in Bray P-1 P. These results seemed questionable, especially when compared to the results being obtained on samples from the area by the NaHCO₃ method. A study was then initiated to correlate and calibrate the Bray P-1 and NaHCO₃ methods on the soils in the area for sugarbeets. Observations from the first year of research indicated that a separation of the sites according to soil series, Mitchell or Tripp, despite their similarities in pH, CaCO₃ content, organic matter, and texture, greatly improved correlation. Therefore, an additional objective was added to compare extractable P by both methods with sugarbeet response to P fertilization on each of the series. Twenty-two field experiments were conducted from 1974 through 1976 on these soil series. The experiments consisted of six rates (0, 23, 45, 67, 112, and 224 kg/ha) of P on sugarbeets with four replications. Yields were determined from samples harvested in the fall of each year. Correlation of crop response with soil-test P levels by either soil test method was very poor when used across all locations. However, separation by soil series and the use of the Bray P-1 and NaHCO₃ methods on the Tripp and Mitchell series respectively, greatly improved the correlation. This improvement was obtained by using available soil classification data, which had already separated these soils, despite their similarity in many physical and chemical properties generally used to explain the problem.
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