Changes in clay minerals and potassium fixation capacity as a result of release and fixation of potassium in long-term field experiments

We investigated to what extent soil K balances, resulting from plant off-take and K fertilizer application rates, had changed the content of illitic (10 Å) materials (as discrete illite and as illitic layers in mixed-layer minerals) in soil clay fractions. We also assessed to what extent different particle size fractions were involved in the processes. The study was performed on soil samples from five long-term agricultural experiments located on Mollisols and Inceptisols in South and Central Sweden, each having a range of K fertilizer application rates. We analysed X-ray diffraction (XRD) patterns from parallel orientated samples of the following particle size fractions: total clay (< 2 μm), fine clay (< 0.2 μm), and (at two of the sites) fine–medium silt (2–20 μm). In addition, K fixation capacity in the total fine earth (< 2 mm) was measured in order to assess changes in the number of fixation sites due to previous release or fixation of K in the field. At all sites but one, K management had measurably affected the content of the illitic components in the total clay fraction, as expressed by an ‘illite layer ratio.’ Although the content of illitic materials in phyllosilicates differed between particle size fractions, they were all altered to a similar extent by the soil K balance; i.e., the net off-take or input of K in the different fertilizer treatments. Soil K balances also affected the K fixation capacity of the soil. Overall, 8 ± 6 to 40 ± 30% of accumulated net inputs or outputs of K in the field were recovered as a change in K fixation capacity. This means that K release was reversible by K fertilization to a highly variable extent. The release of K appeared reversible in (i) two silty clays and (ii) a sandy loam developed in a parent material characterized by the occurrence of calcite. Two other soils showed evidence of irreversible loss of interlayer K from 2:1-minerals. XRD patterns and the slightly lower pH at the latter sites suggest that the occurrence of hydroxyaluminium interlayers in 2:1-minerals may have hampered K fixation in these soils. To generalize, a reversible nature of K release and fixation may be promoted if easily weatherable soil minerals are abundant.