Form of Soil K:
Soils contains large amounts of K but only a small parts usually less than 1% of the total K is in exchangeable form and much smaller amounts are in soil solution. Most of the K in the soil is present in the non exchangeable forms.
The slowly and readily available forms of K may comprise a substantial portion of the K that is available for plant uptake during the growing season.
1. Mineral (Structural)
2. Non exchangeable (Fixed or difficult available)
Plants take K in the form of K+ from soil solution and its level ranges from 1 to 10 ppm. Like other exchangeable cations K+ is held around negatively changed soil colloids usually less than 1% of total K is in exchangeable form and much smaller amounts are in soil solution K present in soil other than solution K and exchangeable K is referred as non exchangeable and mineral K. although non –exchangeable K reserves are not always immediately available can contribute significantly to maintenance of labial pool in soil.
Principle of Extraction by Ammonium Acetate Method:
The term available K incorporates both exchangeable and water soluble forms of the nutrient in soil. The readily exchangeable plus water soluble K is determined in neutral normal ammonium acetate Extractant of soil. The NH4 ion provides a sharp rapid separation of K from exchange complex.
Principle of K determination by Flame Photometer:
The extract is atomized in the flame where the atoms of the element are excited , emitting radiations of characteristics wavelength. The radiation emitted by the K atoms is passed through the filter which falls on photocell emitting electrons. i.e electric current which is measured on galvanometer of flame photometer. The electric current generated is proportional to the concentration of K into the extract.