Desorption of cations from scottish soils by electro‐ultrafiltration

Soil cations were desorbed by electro‐ultrafiltration (EUF), i.e. by applying an external electric field to aqueous suspensions, and collecting the cations by ultrafiltration. Three different field strengths were used to obtain several cation fractions in a single extraction run lasting 35 min. Potassium desorbed in 10 min (K₁₀) was closely correlated with the equilibrium activity ratio (AR₀) of soil K measured in 0.01M CaCl₂, for all but the sandy loam soils, while that desorbed in 35 min (K₃₅) was less than the exchangeable K, but correlated closely with it. EUF values compared favourably with other soil K parameters, determined by conventional methods, for predicting uptake of K by rye‐grass from ten soils during intensive cropping in the glasshouse. Sodium desorbed in 35 min (Na₃₅) approximated to the exchangeable Na, although most of it was desorbed in the first 15 min. Na₁₀ and Na₃₅ accounted for about 56% of the variation in Na uptake by ryegrass at the first cut, and 68% of the variation in the cumulative uptake at the sixth cut. Between 9–24% of the exchangeable calcium was desorbed in 35 min, and magnesium desorbed was closely correlated with both the Mg concentration of the soil saturation extract and the exchangeable Mg. EUF extraction of Mg is less satisfactory than for K, Na and Ca owing to the precipitation of Mg(OH)₂ on the cathode filter.