Regular field sampling of the 0-3 in. horizon of a sandy soil disclosed an annual pattern of potassium release from the organic matter fraction and retention by the microbial population. The exchangeable + soil-solution potassium content KE+S reached a maximum in autumn (March) at a time when microbial potassium KM and non-exchangeable potassium associated with organic matter KOM were at minimum levels. In soils containing 4% or more of organic matter the KM and KOM categories were important sources of supply of potassium entering the KE+S form. Release of potassium was rapid at temperatures of 40�C and above, but was much slower at lower temperatures. Temperatures in the surface horizon of soils in parts of south-west Australia often exceeded 40�C during the summer period; at these temperatures release occurred more readily in wet soils than in dry soils. Two slight peaks in microbial retention of potassium occurred during early winter (April-May) and spring (October-November). The significance of the observed fluctuations is discussed in relation to leaching and plant uptake of potassium.
