Effectiveness of standard soil tests for assessing potassium availability in sand rootzones

Potassium (K) is the most abundant cation in the cytoplasm, and large amounts are needed for optimum plant growth. In sand rootzones with low cation exchange capacity (CEC), commonly used for putting greens, K availability may be limited, especially when the sands are calcareous. Tools are needed to assess K availability and to direct K management for such calcareous sands. The objectives of this study were to: (i) evaluate six extraction methods (1 N NH4OAc, Mehlich 3, Morgan, 0.01 M CaCl2, 0.01 M SrCl2, and water) for their effectiveness in quantifying soil extractable K following K fertilizer applications; and (ii) to ascertain how the extractable K was related to tissue K concentrations. Potassium was applied as K2SO4 at six rates (0, 3, 6, 13, 19, and 25 g K m 56 days) to a creeping bentgrass [Agrostis stolonifera var. palustris (Huds.) Farw.] putting green grown on a calcareous sand rootzone. The potassium was applied at 2-week intervals (4 applications within the 56-day period) and repeated during two different irrigation regimes. Soils were sampled and analyzed after the fourth application. Each extraction method detected an increase in soil extractable K following K fertilizer application. The K concentrations were lowest in the water extracts and highest in the Mehlich 3 extracts, irrespective of irrigation regime, but less K was recovered under higher irrigation intensity. Creeping bentgrass tissue K content increased with K application rate. However, maximum tissue K content was obtained at lower soil K levels under the high irrigation intensity than under the lower irrigation intensity. Our results show that each extraction method could be used to detect an increase in extractable K and to predict an increase in tissue K content. Nevertheless, factors other than soil K concentration also affected the tissue K content, which suggests that soil K concentrations may not be a reliable predictor of tissue K content in this sand rootzone.