Potassium fixation in soil size fractions of arid soils

Amounts of potassium (K) fixed in soil fractions of 10 calcareous soils of southern Iran were measured to evaluate the contributions of different soil size fractions to K fixation. Soil particles were fractionated after dispersion of the soils with an ultrasonic probe. Potassium fixation analysis was done by addition of 1000 mg K/kg samples. Mineralogy of the size fractions was determined by X-ray diffraction. The clay fractions were dominated by smectite, chlorite, mica, and palygorskite. Potassium fixation capacities ranged from 104 to 148 mg/kg for clay, from 102 to 155 mg/kg for silt, and from 96 to 187 mg/kg for sand fractions. A positive and significant relationship (P < 0.05) was obtained between K fixation capacity and smectite content for the clay fractions. High amounts of K fixed in the sand fraction may be explained by a larger diffusion path of fixed K out of the frayed edges of micaceous and smectitic minerals into the extracting solution, low cation exchange capacity (CEC) of the coarse fraction and thereby the high concentration gradient along solution and interlayers, physical entrapment of K ions in coarse aggregates cemented by carbonates, and by the presence of clay particles in coarse fractions due to incomplete dispersion of coarse aggregates.