Refinement of Quicktest Methodology for Improved Potassium Fertilizer Recommendations1

Abstract

AbstractPotassium fertilizer recommendations that can be applied to soils that vary widely in K‐fixation tendencies, and that also provide adequate levels of K nutrition without costly excesses, are needed. The purpose of this study was to refine the soil quicktest methodology and to describe how the refined procedure can be used to make fertilizer recommendations that are based on the K‐fixation tendencies of individual soils. The fixation tendency is determined via a 2‐h equilibration of the soil with a 200 mg kg 1 KCl solution, followed by extraction with neutral molar NH4OAc. An initial recommendation is computed, based on the amount of the added K fixed, which is mathematically adjusted to provide a K fertilizer recommendation based on a longer (60‐day) equilibration. Using a population of 100 soils, preliminary observations that the 2‐h adjusted quicktest methodology provides similar K recommendations as those obtained by a 60‐day equilibration, were confirmed. A subsample of 20 soils selected on the basis of (i) K‐fixation tendency, (ii) soil test K level, and (iii) fertilizer K required for near maximum yield, was found to represent the 100‐soil population accurately. A regression equation relating quicktest and 60‐day equilibration data for the 20 soils was used to predict K requirement for additional soils collected in a field study. Potassium recommendations were considerably improved over those computed by the currently‐used procedure which assumes that a constant recovery of 60% of added K applies to all soils. A 16‐h (overnight) equilibration involving the same population of 100 soils also accurately predicted estimates of fertilizer K needs. This alternative procedure provides greater flexibility in use of laboratory space.