How Does Redox Status Influence Exchangeable Potassium In Soil?.

Potassium in soils may be held in readily exchangeable, slowly exchangeable (temporarily fixed), and nonexchangeable (fixed) forms. In soils where 2:1 layer silicates dominate the clay fraction, potassium exchangeability (and availability to plants) is regulated in complex ways by the seasonal variability in the redox status of the soil. In this context, some soil characteristics that affect potassium exchangeability / fixation are more or less permanent factors, and some are affected by soil management practices.

      Permanent factors include the abundance of vermiculite and smectite in the soil. High-charge vermiculitic layers provide opportunities for long-term fixation of applied potassium. Moreover, the negative layer charge associated with Fe-rich smectites is a function of redox potential, and when it is low enough for structural Fe to be reduced, the capacity for short-term fixation of potassium increases. In many poorly drained soils, the abundance of Fe oxides is relatively low because of dissolution and leaching over many redox cycles. It may be hypothesized that low concentrations of Fe oxides make temporary fixation by smectite more likely since the redox buffering capacity of the soil is low. On the other hand, organic matter often occurs in higher concentrations in poorly drained soils, providing negative charge sites where potassium ions can be readily exchanged as well as blocking access to possible fixation sites in vermiculitic layers or smectite quasicrystals. Thus redox-related, temporary fixation of potassium may be driven in one direction by the abundance of Fe-rich smectites but in another direction by the chemical and physical interactions of Fe oxides and organic matter.

     Soil redox status also affects potassium fixation potential indirectly by its impact on pH. Over long periods, low soil pH (< 5.5) promotes the formation of hydroxy aluminum polymers in the interlayer regions of vermiculite and smectite. Pillaring of clay layers with hydroxy aluminum interlayers tends to increase the likelihood that potassium will be trapped in long-term fixation sites. The concentration of sites where potassium might be readily exchangeable also declines when clay interlayers are filled with hydroxy aluminum polymers. Potassium fixation in pillared clay, however, is more likely to occur in well drained soils where the redox potential is high, organic matter is low, and pH values decline when the soil dries. There is evidence that liming acid soils promotes dissolution of these pillars and, presumably, decreases the potential for long-term fixation of potassium. In opposition to the impact of low redox status on potassium fixation by layer silicates, reducing conditions during long periods of water saturation lead to an increase in pH, mitigating the risk of hydroxy interlayer aluminum and increasing the abundance of pH-dependent, readily exchangeable sites on soil organic matter. Maintaining soil pH values above 6 reduces the potential for potassium fixation by interlayer hydroxy aluminum polymers.

     Besides pH, another management factor that influences the exchangeability of potassium in the context of redox status is nitrogen fertilization. While usually present in lower concentrations than exchangeable potassium, ammonium cations compete with potassium for the same retention sites where they can be readily exchangeable, slowly exchangeable, or nonexchangeable. Common nitrogen fertilizers include ammonium or are transformed to ammonium quickly in the soil. Low redox conditions in organic matter-rich soils can lead to conversion of nitrate to ammonium via dissimilatory nitrate reduction. Under saturated conditions, at low redox potential, ammonium cations are thermodynamically stable. Thus during wet spring periods when annual crop roots are not present to actively take up ammonium, there is opportunity for ammonium – potassium competition in which some potassium ions can be displaced from readily exchangeable and (perhaps) slowly exchangeable sites. In this context, low redox conditions promote exchangeability of potassium.

     Clearly, the complexity of potassium forms and their dependence on factors both intrinsic and extrinsic to the soil present challenges to routine soil testing. It is unlikely that potassium which would be “plant-available” over an entire growing season could be predicted by a single extraction technique (such as 1 M NH₄OAc or the Mehlich 3 extractant) applicable to all soils. Perhaps by taking into account the impacts of redox status on clay mineralogy, Fe oxides, organic matter, pH, and recent nitrogen fertilization, models of potassium availability as well as routine soil assessment of potassium availability might be improved.