Evaluating Plant-Available Potassium in Different Soils Using a Modified Sodium Tetraphenylboron Method

Finding a uniform method to evaluate plant-available potassium (K) in a variety of soils has been a challenge. In this study, the sodium tetraphenylboron (NaBPh4) method was modified and compared with the conventional ammonium acetate (NH4OAc) method to evaluate K availability to perennial ryegrass (Lolium perenne L.) in soils with different K fertilities. The amount of K extracted using NaBPh4 was influenced by extraction time and concentrations of NaCl and NaBPh4. Without NaCl, the NaBPh4 method with low concentrations of NaBPh4 (0.001 and 0.003 mol L-1) could only extract soluble and exchangeable soil K, equivalent to three sequential extractions using the NH4OAc method, whereas the NaBPh4 method with a high NaBPh4 concentration (0.2 mol L-1) could extract all the NH4OAc-extractable K and some easily released nonexchangeable K (NEK) in soils. Easily released NEK contributed significantly to K uptake by ryegrass. Soil K availability estimated using the 60-min (r(2) = 0.83-0.92) and 120-min (r(2) = 0.84-0.94) modified NaBPh4 methods correlated well to the K removed by one to eight crops of ryegrass. The 60-min modified NaBPh4 method without NaCl (0.2 mol L-1 NaBPh4 + 0.01 mol L-1 EDTA) was suitable for evaluating K availability to plants in a variety of soils, whereas the NH4OAc method was only suitable for evaluating K availability in soils of the same type or with similar K-buffering capacities, but not in soils with variable K buffer capacities or in which the NEK contribution to plant K uptake varied.