Release of non-exchangeable potassium from some British soils cropped in the glasshouse

1. Continuous cropping with perennial ryegrass in the glasshouse was used to measure the release of non-exchangeable potassium from twenty soils representative of a wide variety of soil series.2. Releases of non-exchangeable potassium ranged from almost none to > 2000 lb. K/acre.3. The main reason for the variations in the potassium-releasing powers of the soils was traced to differences in the amount and potassium content of the finer clay fractions. Small potassiumcontents in the fine clay and small contents of fine clay were associated with small releases of nonexchangeable potassium.4. Accumulated potash fertilizer residues or the presence of such minerals as glauconite can be responsible for the good potassium-releasing powers of some soils.5. Some of the limitations of glasshouse work for assessing the potash status of soils are discussed.6. Evidence was obtained for the existence in many soils of at least two categories of useful nonexchangeable potassium.