The Potassium Status of West Indian Volcanic Soils

Summary The potassium status of a group of unfertilized volcanic soils was measured by the intensity, exchangeable, nitric acid‐soluble, fixation, and total potassium values. The soils were classified by the amount of weathering and predominant clay minerals as determined by field characteristics. The soils contained principally pyroxenes, soda‐lime feldspars, and variable quantities of glass and amphiboles. There was a marked absence of potassium‐bearing minerals in the fine‐sand fraction. Clay content varied from 5 to 72 per cent and total potassium averaged 0.38 per cent. Neither potassium highly soluble in nitric acid nor that of limited solubility were related to percentage clay, but both were highly correlated with total potassium. Fixation of potassium was usually low. Potassium intensity, expressed by p K‐1/2 p(Ca 4‐ Mg) values, was determined by equilibrating soil with potassium in calcium chloride solutions. Such values were shown to be similar to those from soil‐solution extracts displaced by alcohol. The potassium‐intensity status was highest in allophane soils and lowest in montmorillonite soils; and was correlated with exchangeable, fixed and highly soluble nitric acid potassium. The last was highly correlated with exchangeable potassium. Partial correlation coefficients showed that the major factors controlling potassium intensity were exchangeable potassium relative to exchangeable calcium and magnesium and the potassium‐fixing ability.