Factors Affecting Availability and Fixation of Potassium Soils
The following are the factors affecting the availability and fixation of potassium soils;
1. Nature of Soil Colloids
The dominant clay species in soil determines the extent to which added fertilizer K could be fixed. Soils in which 1:1-type clays, such as Kaolinite, are dominant fix very little K.
On the other hand, soils in which 2:1-type clays, such as vermiculite, montmorillonite, and fine-grained mica (illite), are dominant readily fix K in large amounts.
The 2:1 clays have a larger negative charge from the isomorphous substitution of A13+ for Si4+ in their silica tetrahedral layer thereby strongly binding the K+ ions.
2. Alternate Wetting and Drying
Alternate wetting and drying and freezing and thawing have been reported to contribute to the fixation of K into non-exchangeable form as well as its ultimate release to the soil solution.
During wetting, the 2:1 expanding clay minerals increase their interlayer spaces and K+ ions could easily move into the spaces.
On drying, the expanded layers collapse to entrap the K+ ions between the interlayer spaces, thereby preventing the release of the potassium. The same mechanism is believed to occur during freezing and thawing.
3. Influence of Lime
Application of lime usually results in an increase in K-fixation and thus conserved against leaching losses.
Nevertheless, in soils where the negative charge is pH-dependent, liming can greatly reduce the level of K in the soil solution. High calcium levels in the soil solution also reduce potassium uptake by the plant.
4. Frequency of Application
Frequent light applications of K are found to be superior to heavier ones. Frequent light applications are recommended to avoid luxury consumption, leaching losses, and fixation of excess potassium.
5. Crop Removal
Crop removal of K is higher than all other nutrient elements except nitrogen. Annual losses by crop removal could be as high as 200kg ha-1 of K, especially in leguminous crops such as soybean and cowpea.
The percent recovery of K from fertilizer – K by crops on most soils is about 70% but if the clay content is up to 27%, especially illite clay, recovery is only about 30%.
Read Also: Factors Affecting Solubility and Fixation of Phosphorus in Soils
6. Presence of other Nutrient Elements
Potassium is supplied as cation K+ and it is readily available to crops. However, there is competition between NH+4 and K+ uptake and between Ca2+ and K+ as in calcareous soils where uptake of K may be suppressed.
Potassium Fertilizers
The principal K-fertilizer is KCI (potassium chloride) called muriate of potash which is 60% K2O.
Second in importance is sulphate of potash (potassium sulphate, K2SO4) which is about 50% K2O. These two K-fertilizers are very soluble and are sold in crystal forms.
Potassium Nitrate (KNO3) which is 40% K2O potassium- magnesium sulphate (20% K2O) another K-fertilizer material.
Frequent light applications of potassium have advantages over heavy applications in preventing luxury consumption by certain plants, reducing leaching losses, and minimizing fixation into unavailable forms.
Potassium fertilizer use is yet to reach the optimum in developing agriculture compared to developed agriculture of the world.
Soil incorporation of residues of leguminous crops such as the haulm of groundnut and soya bean is an important practice for conserving soil potassium, especially in savanna areas.
In conclusion, the primary (Feldspars and micas) and secondary (clay) minerals are the main sources of soil potassium. Despite its abundance in soils, potassium mostly occurs in an unavailable form for plant uptake. Hence potassium could easily become deficient where crop demands outweigh available amounts in soil solution.
Losses of potassium are mainly through leaching especially in regions of heavy rainfall, through crop removal where the whole plant is harvested and removed away from the farmland, and by fixation into non-exchangeable forms which, however, are often released to the exchangeable form.
Management of soil potassium should emphasize small split applications so as to avoid luxury consumption, leaching losses, and chloride toxicity.
Soil pH should be maintained at optimum (pH 6-6.5) while crop residue and manures should be returned by incorporation during cultivation.
Read Also: Forms and Functions of Potassium in Plants
