Comparing potassium soil tests with a flexible function and a modifier variable

Selecting the appropriate functional form and using modifier variables are key issues in soil test calibration. Flexible functions capture all the information conveyed by the data, except for stochastic aberrations of local maxima, minima, and inflexion points. The objective of this study was to show that a flexible function can be used for comparing soil tests including the sediment volume (SV) as a modifier variable. Data used in this test were collected from 30 irrigated Torrifluvents in which K availability had been evaluated with the Neubauer method. The dependent variable was total K uptake by ryegrass (Lolium perenne L.). Independent soil K variables corresponded to seven soil test methods : NH 4 OAc-exchangeable K ; HNO 3 -extractable K ; H 2 CO 3 -extractable K ; soluble K ; resin-extractable K ; K potential ; and K adsorption ratio. The modifier variable, SV, is a quick, inexpensive, and fairly accurate attribute widely used in western Argentina to appraise soil texture. Data were evaluated using the generalized Cobb-Douglas (GCD) function. Explanation of variability in K uptake using the GCD model, with exchangeable K and SV as independent variables, was highly satisfactory (R 2 = 0.88). The other six methods of soil testing gave R 2 values from 0.55 to 0.76. Contribution of SV as a modifier for the interpretation of K availability was significant (a < 0.01) for all soil K indices. The GCD function captured a positive interaction between the independent variables. This indicates that the higher K buffering capacity of the finer textured soils was an overriding factor that favored K absorption by plants.