The effect of soil mineral composition on K availability to plants

Potassium (K) is an essential macronutrient that takes part in a wide variety of processes in the plant,
from enzymatic activity to cell development and osmotic balance. Excess K has a minor effect on the plant,
mostly due to imbalanced nutrition. Accurately determining K requirements for optimal growth and high
fertilizer utilization efficiency is therefore challenging, and often results in excess fertilizer application. It
is customary to refer to four soil K reservoirs which vary greatly in their size and availability to the plant:
(1) soil solution is the smallest and the one from which plants uptake K, (2) exchangeable - K adsorbed
onto clay mineral surfaces, oxides and organic matter (1-2%), (3) interlayer - K fixed between clay mineral
sheets (up to 10%), and (4) structural K – most commonly found in K-feldspar and usually considered
unavailable to the plant (90-98%). The interlayered - K is a dynamic and important pool that may supply
K for the plant in case of K deficiency, or act as a sink by fixing excess K.
Fertilization recommendations often relay on soil tests that estimate the exchangeable-K pool. The
common paradigm is that exchangeable K represents the capacity of the soil to supply plant-available-K.
In alkaline soils, however, there is a growing number of studies from both short- and long-term experiments
(several growth seasons to a few decades), reporting little to no response to K fertilization, indicating that
the soils supply greater amounts of K than indicated the exchangeable K tests. These findings evoke
questions regarding the intrinsic soil K reservoirs and the extent to which the natural supply of K to the
soils from weathering or dust deposition, readily supply K to plants.
In this study, we examine the affect of different K-bearing mineral phases (soil K pools) on the soil K
cycle and K availability to the plant. Our study focusses on intensively used agricultural soils in Israel that
vary in their clay compositions- illite and illite-smectite and K-feldspar. We think that illite and illite-
smectite are instrumental in buffering the excess K fertilizer that we observe in short- and long-term
experimental plots. Over time, we suggest, that agriculturally driven enhanced weathering of the illite and
K-feldspar releases significant amounts of K, that had been previously considered unavailable. Taking into
consideration the abundance of these mineral phases in the regional dust, we further suggest that dust
deposition and location along the regional dust gradient play a significant role in replenishing and
maintaining soil K levels.