Experiments were carried out on an acidic, clay loam soil (Ragály) to study the release of potassium into the soil solution as affected by soil acidification and soil water content. Two replicates of air-dried samples were acidified with HCl solutions to various water contents: soil suspensions (at 1:10, 1:5, 1:2.5 and 1:1 soil:water ratios) and wet soil samples having water potentials of -0.1 kPa, -20 kPa and -100 kPa were prepared. Constant acid loads, corresponding to 0, 5, 12.5, 25, 37.5, 50 and 62.5 mmol H+/kg soil were applied to each soil water content series. At field capacity acid loads of 75, 87.5 and 100 mmol H+/kg soil were also applied. After one week of incubation the liquid phases were extracted by centrifugation with a rotor speed corresponding to -1500 kPa (equal to the conventional wilting point of plants). At constant soil water content, the potassium concentration in the liquid phase of the soil (cK) increased with decreasing pH according to an exponential relationship (cK = a e-bpH). The slope (b) was higher at low soil water contents. At constant acid load, the potassium concentration in the liquid phase increased with decreasing soil water content (q) according to a hyperbolic relationship cK = a' + b' {1 / (qq-qq')}, where q' denotes the gravimetric soil water content at -1500 kPa water potential. The slope (b') was higher at lower pH values. The combined effect of the matrix of changing acid load and soil water content gave a three-dimensional surface characterizing the plant available potassium concentration over a wide range of these parameters: ln cK (mg/L) = 4.79 - 0.66 pH + 9.79 {1/(qq-qq'); R2 = 0.87. A finely ground (<100 mmm) feldspar mixture (80% orthoclase + 20% albite) was added as potassium source to the air-dried samples of a slightly acidic sandy soil in 0:1, 1:3 and 1:1 feldspar:soil ratios (Somogysárd). Two replicates of the control and feldspar-enriched soil samples were moistened to field capacity with HNO3 solutions of 0, 0.25, 0.50, 0.75 and 1.0 mol/L concentrations (equal to acid loads of 0, 50, 100, 150 and 200 mmol H+/kg soil). The soil solution was extracted with the above centrifugation method. After feldspar application, the potassium concentration in the soil solution increased many times as compared with the control. Due to acid treatment the soil pH decreased by three units and the potassium concentration in the soil solution increased according to a saturation curve. Due to a two-unit decrease in soil pH, the potassium concentration increased threefold in the control and sixfold in feldspar-enriched (1:3) soil. This decrease in pH may take place due to root activity, promoting the dissolution of potassium minerals, and increasing potassium availability in the rhizosphere. The impact of drying-rewetting was also studied at the above feldspar:soil ratios. After one week of incubation the samples were kept in open vessels for one year, irrigated weekly with distilled water to field capacity, then the soil solution was extracted by centrifugation. The concentrations were compared to those measured in a soil solution obtained from soil not subjected to the drying-rewetting procedure. The potassium concentration decreased in the liquid phase of the soil with no added feldspar: presumably it entered more strongly bounded forms during the drying-rewetting cycles. In the feldspar-enriched soil, however, the potassium concentration in the soil solution increased, which may be the consequence of the slow dissolution of the feldspar mineral.
Water contents of glasshouse soils at field capacity and at saturation. 1. Relationships between water contents.