Cation - noun, chemistry - a positively charged ion
By Gurbir Dhillon Ph.D.
This month Soil Savvy gives you some facts to consider about potassium in your soils. It is segment two in the soil potassium series. See segment one here
The parent geologic minerals store a major portion of the potassium contained in the soils. It undergoes transformations as it cycles through soil-plant system to become available to crops through weathering of minerals.
Alberta's parent geologic material is considerably rich in potassium-containing minerals. These mineral reserves form 90-95% of the total potassium in soils. However, this form of potassium is not available to the crops for uptake.
The minerals slowly weather at the surface and edges to release potassium locked in the parent geologic mineral structure into the soils. The potassium in soils forms up to 5 to 10% of the total potassium reserves. The plant-available potassium is loosely attached to the exchange sites in soils or dissolved in soil solution where it is available for plant uptake.
The plant-available forms generally account for up to 1-2% of the total potassium. In most Alberta soils, the available and exchangeable levels of potassium range between 300-1000 kg/ha, which is sufficient for adequate crop growth.
The positively charged potassium ions attached on to the negatively charged surfaces of clay and humus particles tends to be fairly immobile. Despite its relative immobility, potassium is more susceptible to leaching than phosphorus.
Sandy soils with low cation exchange capacity generally have low capacity to maintain potassium levels. Late-season potassium deficiency may result in such soils as the dissolved potassium in soil solution is removed through intensive crop production.
The fine-textured soils with higher cation exchange capacity are generally able to maintain a consistent supply of potassium throughout the growing season.