Considering Soil Potassium Pools with Dissimilar Plant Availability

Abstract

Soil potassium (K) has traditionally been portrayed as residing in four functional pools: solution K, exchangeable K, interlayer (sometimes referred to as “fixed” or “nonexchangeable”) K, and structural K in primary minerals. However, this four-pool model and associated terminology have created confusion in understanding the dynamics of K supply to plants and the fate of K returned to the soil in fertilizers, residues, or waste products. This chapter presents an alternative framework to depict soil K pools. The framework distinguishes between micas and feldspars as K-bearing primary minerals, based on the presence of K in interlayer positions or three-dimensional framework structures, respectively; identifies a pool of K in neoformed secondary minerals that can include fertilizer reaction products; and replaces the “exchangeable” K pool with a pool defined as “surface-adsorbed” K, identifying where the K is located and the mechanism by which it is held rather than identification based on particular soil testing procedures. In this chapter, we discuss these K pools and their behavior in relation to plant K acquisition and soil K dynamics.

Long-term rundown of plant-available potassium in Western Australia requires a re-evaluation of potassium management for grain production: a review

“…Loam and clay soils in WA may contain large amounts of nonexchangeable K in the interlayer of clay minerals such as illite (Pal et al 2001), but the effect of illite on K supply to crops has not been demonstrated. There is evidence that feldspars, that are relatively prevalent in soils of WA, can contribute significant quantities of bioavailable K to plants, potentially from sand-sized fractions rather than clay-sized fractions (Bell et al 2021b). More research is warranted to investigate the role of non-exchangeable K and feldspar K in crop K supply and availability in order to better predict the rates of K rundown where negative K balances occur.…”

Long-term K fertilization effects on soil available K, grain yield, and plant K critical value in winter wheat

“…The contribution of non-exchangeable K to crop nutrition increases as exchangeable K decreases and can be important especially for soils with high contents of 2:1 layered clay minerals (Barré et al 2008;Damar et al 2020). The availability of non-exchangeable K for crops is also influenced by the crystallographic properties of soil mineral particles (Bell et al 2021). To measure only exchangeable K, numerous extractants are used, e.g., ammonium acetate (K-AA), ammonium acetate EDTA (K-AAE) in Switzerland, BaCl 2 -triethanolamine, NH 4 Cl, or NaHCO 3 (NFX 31-108, ISO 13536) (Colwell 1965;Mc Lean and Watson 1985;FAL 2004).…”

“…However, considering the multitude of existing methods, there is still a lack of information on the correspondence of some of the methods, for example K-AAE or K-CO 2 . Bell et al (2021) highlighted the weaknesses of this model with four pools. For instance, crop availability of K in the interlayer of various clay mineral types cannot be estimated through only one chemical extraction.…”

“…In addition, structural K can also provide substantial amounts of available K in some cases (Sadusky et al 1987). Therefore, Bell et al (2021) proposed another model to classify the K pools based on soil physico-chemical mechanisms involved in holding K rather than on sequential chemical extraction methods. For instance, a "surface-adsorbed" pool is considered instead of the exchangeable K and K pools are classified according to their crystallographic properties.…”

Mineral stabilities in soils: how minerals can feed the world and mitigate climate change

“…1, and cation exchange reactions. This gives rise to the concept of K 'pools' with differing stabilities and availability to plants within soils, thoroughly explored by Bell et al (2021), who recognise 6 different K pools. These are: (1) structural K in feldspars and feldspathoids,…”