Abstract
Potassium (K) is one of the three major elements that play important roles in plants, such as maintaining
turgor of cells, promoting activation of enzymes, and improving efficiency of photosynthesis. The types
of K in soil may affect the plant absorption of K. K in soils includes K minerals, K in layered silicates
(clay minerals), exchangeable K and K in solution. The former two can provide K very slowly through
long period and so may be seen as ineffective; the latter two are the major types for K supply to plants.
The release of the exchangeable K is basically through ionic exchange reactions. In the farm, soils with
weak acidity to neutral conditions and 90% saturation of salt base, has calcium (Ca) as the most
important cation among the exchangeable ions, followed sequentially by magnesium (Mg), K and
sodium (Na). Ca and Mg are the major competitors of K for the exchangeable sites or locations. The
effectiveness of K ions depends on the competitive absorption of Ca and Mg. However, the exchange
behaviour of Ca and Mg in soils is quite similar and so they are commonly combined as one cation to
simplify theoretical and analytical treatments of the processes. In general, quantity/intensity (Q/I)
diagram of K may serve as an index for the strength and quantity of effective K supply to plants in soils.
Some parameters such as equilibrium activity ratio (ARe), K selectivity coefficient (KG) and potential
buffering capacity (PBC) may provide important information for the K behaviour in soils. ARe indicates
the amount of K that can be provided by solution instantly in the soil. KG, a coefficient for K selectivity,
indicates higher K selectivity for the soil when its value is higher. Higher PBC shows higher K buffering
capacity of the soil. Therefore, this information may be based as references for K fertilization.
Key words: Potassium (K), quantity/Intensity (Q/I), activity ratio (AR), selectivity coefficient (KG), absorption
Abbreviation
Q/I, Quantity/intensity; ARe, equilibrium activity ratio; KG, potassium selectivity coefficient; PBC, potential
buffering capacity; AR, activity ratio; ExK, exchangeable potassium; CEC, cation exchange capacity.
