Adsorción y desorción de potasio en suelos de Galicia (España) desarrollados sobre distintos materiales de partida

Resumo

The potassium sorption capacity of soils is a major factor determining the dynamics of this element in the soil-plant system and the requirements for potassium fertilisation. Low contents of available potassium are often a factor limiting crop yield in Galician soils. Potassium sorption depends on the nature of clay minerals; the hydroxy-aluminium vermiculites, common in surface horizons of Galician soils, bestow on them a capacity to fix potassium. The present paper studies potassium sorption and desorption by samples representative of A and B horizons of Galician (NW Spain) soils, developed from various parent materials. The mineralogical composition of the soil clay was studied by X-ray diffraction analysis; the presence of non-crystalline materials was tested using the pH in NaF and estimated by the aluminium and iron extracted by acid oxalate. Potassium sorption was determined by shaking the soil samples with 0.01 M CaCl2 containing various K concentrations, until equilibrium is reached. In most cases the experimental data fitted best to the linear or Freundlich models, so that it is not possible to predict a limit for potassium sorption by these soils. The sorption energy is higher in soils richer in non-exchangeable potassium. Soils with high contents of non-crystalline materials show a small potassium sorption capacity. The exchange complex contributes notably to potassium sorption. Potassium desorption was determined by shaking the soil samples, after the sorption experiment, with 0.01 M CaCl2. Some of the studied soils showed a pronounced hysteresis.