MINERAL CHANGES IN A DANISH ALFISOL CAUSED BY 30 YEARS OF POTASSIUM-DEPLETION IN THE FIELD

The changes caused by 30 years of potassium (K) depletion in a Danish, neutral Alfisol formed on glacial till were studied by comparing K po ols and mineralogy of samples from the Ap horizon of a fertilized plot (100 kg ha(-1) year(-1)) and a non-fertilized plot. The two soils hav e almost identical particle size distributions and contained similar a mounts of total K, but the sample from the fertilized plot is richer i n water-soluble K, exchangeable K and HNO3-extractable K. The differen ce between the sums of these pools could, however, only account for 30 % of the K added to the fertilized plot. Despite the long period of K depletion, the soil still seems able to release substantial amounts of K to the plants. According to X-ray diffraction analysis, the clay an d silt fractions contained the same layer silicates including kaolinit e, illite or mica, chlorite, vermiculite and a component considered to be a vermiculite-illite intergrade. The main effect induced by K depl etion as shown by the diffractograms of these fractions was a decrease in the intensity of a peak at 1.34 nm and a corresponding increase in a peak at 1.43 nm. This change, which does not seem to have been repo rted before, is interpreted as a transformation of a vermiculite-illit e intergrade (peak at 1.34 nm) into vermiculite (peak at 1.43 nm) by K depletion.