Eight-year long potassium fertilization effects on quantity/intensity relationship of soil potassium under double rice cropping

Potassium-Fixing Clay Minerals as Parameters that Define K Availability of K-Deficient Soils Assessed with a Modified Mitscherlich Equation Model

“…It is well established in the recent literature (Das et al 2019;Islam et al 2017;Jalali 2007;Khan et al 2014;Moody and Bell 2006) that K fertilization management based on NH 4 OAc soil testing procedure often fails in predicting the exact amounts of K that have to be added, in order to replenish exhausted soil K pools and to increase crop yields to desired levels. The latter becomes even more important for soils that contain significant amounts of 2:1 fixing clay minerals like illite or vermiculite which is a common soil clay assemblage alongside with smectites in temperate soils like Greece (Velde, 2001), or for soils under intensive cropping.…”

“…The above consideration is also reported in literature especially through quantity-intensity studies which support that there is a minimum K status in Kdeficient soils in which K ions are retained by the soil and do not exchange with Ca 2+ , despite their extraction by NH 4 OAc (Schneider 1997a, b). This portion of soil K may represent the amount of K ions fixed on some clay interlayer sites (Das et al 2019;Islam et al 2017), and thus, it could be said that it corresponds to a fraction of exchangeable K which is largely unavailable to plants.…”

“…Thus, initial soil K availability and therefore fertilization adjustment in order to achieve desired crop yields have to be addressed with taking into account factors that control K fixation and release from soil reserves, as they constitute key factors of soil K balance formation. Schneider (1997a, b) described one such of these aforementioned factors as the minimum soil K status under which K ions are retained by the soil and therefore are not in equilibrium with Ca 2+ of the soil solution, despite the fact that they are extracted with conventional soil tests of available K. This minimum exchangeable K could thus be considered as a fraction of exchangeable K which, although is extracted with respective soil tests of available K, is almost unavailable to plants (Islam et al 2017).…”

“…Therefore, the quantification of available K contents of soils with respect to the contribution of both exchangeable and non-exchangeable K, in conjunction with the study of possible relationships between them and the respective modification of clay minerals, could become an essential K management tool (Li et al 2017). The above becomes even more important for soils under intensive cropping, which are expected to have higher fixation capacities and therefore, conventional extraction methods like ammonium acetate (NH 4 OAc) are proven inadequate in explaining K availability (Das 2019;Islam et al 2017;Jalali 2007). In soils like the above, the contribution of non-exchangeable K on K availability due to low levels of exchangeable K (Hinsinger 2002), as well as a possible vermiculitization effect which could result in high K-fixing properties (Hinsinger and Jaillard 1993), should be taken under consideration.…”

Potassium Supplying Capacity of Diverse Soils and K-Use Efficiency of Maize in South Asia

“…It is well known that the K availability to the plants does not only depend on the size of the available K pool in the soil but also K release patterns, and its transport from soil solution to the root zone for its uptake by plants [45,46]. It is hypothesized that soils K availability to plants differ in terms of mineralogy, soil K reserves, K-supplying capacity and its allowable drawdown factors [47,48]. Thus, some soils would require more while others would require less K to grow profitable maize crops [40].…”