Physical chemistry of soil potassium

Potassium is a major component of the earth’s crust, soils, and plants. In the earth’s crust, K is the seventh most abundant element. The lithosphere contains an average of about 25.9 g K kg−1 (Hurlbut and Klein, 1977). In the soil, the common range of K content is 0.4 to 30 g K kg−1 (Jackson, 1964; Helmke, 2000). Of the major and secondary nutrient elements, K is generally the most abundant in the soil (Rich, 1968a; Sparks and Huang, 1985). Relative abundance and some chemical characteristics of K and certain elements common in the earth’s crust are given in Table 4–1. Among the mineral cations essential for plants, K is the largest in size. Therefore, the number of oxygen ions coordinating K in mineral structures is high. Consequently, the strength of each K–O bond is relatively weak. Potassium has a lower polarizability than NH4, Rb+, Cs+, and Ba2+. By contrast, K, relative to Ca2+, Mg2+, Li+, and Na+ ions, has a higher polarizability. Ions with higher polarizability would be preferentially selected in ion exchange reactions. Compared with Li+, Na+, Mg2+, and Ca2+, K+ has a lower hydration energy (Helfferich, 1962) and thus would cause little swelling in the interlayer space. The role of K in soils is prodigious; of the many plant nutrient–soil mineral relationships, those involving K are of major significance (Sparks, 2000b). Potassium plays a very important role in enzyme catalysis, photosynthesis and respiration, assimilation and transport, protein and oil metabolism, legume dinitrogen fixation, disease reduction, and interactions with other nutrients and with crop varieties or hybrids (Munson, 1985). Therefore, K is vital in sustaining plant growth, yield formation and crop quality. Furthermore, K is essential in human and animal nutrition and is closely related to certain medical and health aspects. The availability of soil K to plants is related to the nature of soil K reserves, the chemistry of the structural configurations and surface properties of soil components involved, and the dynamics and equilibria of K in soil environments. The objective of this chapter is to integrate the current knowledge and discuss future prospects on this subject.

Chemistry of Potassium in Soils

Chemical thermodynamics augmented by the Gay-Lussac-Ostwald step rule provides a theoretical approach to weathering phenomena in soils that requires few ad hoc constitutive assumptions. The efficacy of this approach is illustrated with several examples of natural and anthropogenically-induced weathering reactions involving kaolinite. In soils, kaolinite exhibits a continuum of structural disorder that is reflected by a corresponding spectrum of solubility product constants. The composition of the soil solution vis-a-vis this spectrum can be interpreted with the help of the step rule. The concept of a soil kaolinite continuum appears necessary to the evaluation of activity-ratio diagrams and other thermodynamic constructs.

Chemical Models of Weathering in Soils

The concentrations (per cent) of secondary clay minerals (mica, vermiculite, montmorillonite, chlorite and kaolinite) in the A-horizon of 99 soil sample sites were entered into a cluster program to determine the common clay mineral assemblages found in U.S. soils. Five major assemblages were identified with the most common assemblage consisting of mica, montmorillonite and kaolinite. A suite of climatic variables was determined for each of the 99 sites, including the water balance variables and Arkley's leaching index. A discriminant analysis was then used to predict the occurrence of a clay assemblage from the climate data. Sixty per cent of the sites could be correctly predicted, although not all assemblages could be predicted equally well; two assemblages could be predicted with 88 and 82 per cent accuracy. The precipitation of the wettest month and Arkley's leaching index accounted for most of the predictive power. Thus this study supports theoretical models which suggest that seasonal moisture supply is an important control of clay genesis and persistence. A residuals analysis indicated that many of the palaeosols in the data set and many of the soils from glacial parent materials possessed clay mineral assemblages not expected on the basis of modern climate.

Climatic control of the assemblages of secondary clay minerals in the A-horizon of United States soils

Influence of rotation cropping and sugarcane production on the clay mineral assemblage

A steady state reaction of apparent equilibrium of K mica + Ca/sup 2 +/ reversible Ca vermiculite + K/sup +/ was indicated by prolonged dissolution extractions from Blount soil clay (from northern Indiana) abundant in dioctahedral mica and vermiculite, with log K/sub eq/ = 2.92 for the reaction when extrapolated to infinite time. From this and published free energies of formation of mica and kaolinite, a mineral phase stability diagram depicting the phase joins of Ca vermiculite, muscovite, and kaolinite was constructed with the solute activity functions pH-pK/sup +/, 2pH-pCa/sup 2 +/, and pSi(OH)/sub 4/. These solute functions for 14-day reactions of calcareous (and dolomitic), poorly drained Harps soil (from central Iowa) fell near the calcite-dolomite-CO/sub 2/-H/sub 2/O phase join, suggesting equilibrium. These functions for Harps soil and the control minerals muscovite, biotite, and (or) vermiculite plus calcite were plotted on the mica-vermiculite stability diagram for various CO/sub 2/ partial pressures. The points fell on the vermiculite-stable side of the mica-vermiculite plane at CO/sub 2/ partial pressures of 0.15 and 0.20 atm (similar to soil air that would exist under frozen soil during winter and early spring; 2pH-pCa/sup 2 +/ approx. = 10.3). They fell on the muscovite-stable side ofmore » the muscovite-vermiculite plane at CO/sub 2/ partial pressures of 0.0001 and 0.001 atm and therefore K/sup +/ would be expected to be fixed. The 2pH-pMg/sup 2 +/ values determined for Harps soil at the various CO/sub 2/ partial pressures plotted either in the Mg montmorillonite stability field or on the Mg-montmorillonite-kaolinite phase join, in concordance with the abundance of montmorillonite and some kaolinite in the medium and fine clay fractions. The solute values for the nearby Clarion soil (upland, noncalcareous) plotted on the montmorillonite-kaolinite join, or with higher CO/sub 2/ partial pressure, in the kaolinite stability area.« less

H. E. Doner

Papers

Cation and silica relationships of mica weathering to vermiculite in calcareous harps soil