A potential method to incorporate quantity/intensity into routine soil test interpretations

Abstract

Exchangeable potassium (K), extracted with 1M ammonium acetate, widely used as the measure for plant available K does not estimate the K supplying capacity of the soil. This research was undertaken to quantity the K supplying capacities of some Missouri soils and evaluate a modified K Quantity‐Intensity (Q/I) approach for adaptation into routine soil analysis. The K supplying capacities as analyzed by the modified Q/I and plant removal were measured on the 0–20 cm and 20–40 cm depths of Broseley loamy fine sand (loamy, mixed, thermic Arenic Hapludalf), Mexico silt loam, (fine, montmorillonitic, mesic Udollic Ochraqualf), Waldron clay loam [fine, montmorillonitic (calcareous) mesic Arenic Fluvaquemt] and the 0–20 cm of Sharkey clay (very fine, montmorillonitic, nonacid, thermic Vertic Haplaquet). The dominant clays in the clay fractions were identified by X‐ray diffraction to be composed of montmorillonite and illite. The high linear coefficient of correlation (R₂ = 0.92**) between the potassium buffer capacity index (KBC Index) and cation exchange capacity [CEC, sum of exchangeable Ca, Mg, K, Na, and Neutralizable Acidity (NA)] suggests that KBC Index of soils with similar clay mineral compositions, but different clay quantities, could be estimated from the measured CEC. Plant K uptake was highly linearly correlated with K quantity (Kq, R₂ =0.98**) as measured by the modified Kq/i method. It appears to be possible, using the KBC Index derived from the CEC, to calculate a Kq/i value with a single measurement of the K in equilibrium with 0.01 M CaCl₂. With the modified Kq/i method, and using this approach, a total plant available K model is presented. This modified Kq/i approach offers the opportunity for soil testing laboratories to complete a calculation for total plant available K with only one additional measurement of the equilibrium K in 0.01 M CaCl2. This would facilitate the fine tuning of soil test interpretation by basing K fertilizer recommendations on the quantity of plant‐available K in the soil.