Forms and Functions of Potassium in Plants
After nitrogen and phosphorus, potassium is the third most important fertilizer nutrient, especially in the more humid climatic areas of southern Nigeria where leaching of soluble potassium is most prevalent.
The three elements (nitrogen, phosphorus, and potassium) called primary nutrients, are generally added to crops as N:P:K compound fertilizer to correct deficiencies.
Forms and Functions of Potassium in Plants
Amount in Plant Tissue
Potassium accounts for 1 and 3% of plant tissue (dry weight) on average but could reach up to 12% in young tissues.
Potassium occurs in plants only as a mobile soluble ion, K+, but does not form an integral part of any specific compound in plant cells.
Due to its extreme mobility, potassium is easily translocated across plant membranes and its high concentration helps in ionic balance and regulation in plant cells.
Functions of Potassium in Plant
Among the multi of various functions of potassium in plants are cell division, formation and translocation of carbohydrates (sugar and starch), an activator of several enzymatic systems (over 60 enzymes are known to require potassium for activation), regulation of osmosis or control of water in the plant, cell permeability, the resistance of some plants to certain diseases and insects attack, increased strength of stem to prevent lodgings such as maize, conversion of sugar to organic acids in roots, the regulation of N-uptake by roots thus preventing excessive elongation and many other functions.
Potassium is known to increase grain formation in cereal crops and also for tuber development in roots and tuber crops which respond highly to generous application of potassium fertilizers.
Most plants have luxurious consumption of potassium. That is, if excessive quantities of potassium fertilizers are applied to soil, the plants will absorb potassium in excess of that required for optimum yields (critical level).
In most cases, the uptake curve is linear implying a direct relationship between soil quantities of potassium and plant tissue potassium.
Potassium does not enter into the organic structure in plant tissue as do calcium and magnesium in calcium pectate of middle lamella and chlorophyll, respectively.
Read Also: The Effect of pH of Soils on Micro-organisms
Potassium Deficiency and Toxicity Symptoms
On an annual basis, agricultural crops remove between 100-300kg K ha-1. The amount is taken up annually by a good cereal crop yielding 5 to 10t ha-1 grain is between 200-300kg K ha-1 while a good crop of potato could also be up to 300kg K ha-1.
Potassium uptake by grass could be very much higher than the figures quoted for common arable crops.
Although the total amount of potassium in soil may be several times larger than uptake, the potassium may not be present in the soil in the available form to meet crop requirements. This is because the amount available for crop uptake depends on the concentration at the root surface and its replenishment.
Potassium in its form taken up by the plant, that is K+, is a mobile element easily translocated to the younger parts of the plants whenever there is a shortfall in the amount taken up by the crop. Therefore, deficiency symptoms first manifest in the older plant parts.
There is yellowing along the margin from leaf tips or apex of older leaves. Necrotic areas along leaf margins are characteristic of K- deficiency symptoms in dicotyledon plants. There is also browning of the tips of leaves down to the base.
The acute shortfall in K-supply leads to stunted growth, poor root development, and a reduction in the production of fruits and grains.
Fertilizer K is normally added to correct K deficiencies. Deficiencies occur in soils that are low in micas, soils that are low in clay (few exchange sites), and acid soils of pH 4.0 – 6.0 due to leaching by high rainfall.
Excess potassium has been found to induce the deficiency of magnesium (Mg) and cobalt (Co). Excess application of K-fertilizer generally leads to deficiency of other cations such as Mg- deficiency in oil palm referred to as orange frond. This condition is called ion antagonism in plants.
At the end of the growing season, some K is passed back to the soil through the roots. Potassium moves up the plant as salt by passive means in water solution through the xylem vessels and moves down as organic K through the phloem.
Soil Potassium
Although potassium is the third most important and most used element in fertilizers, it is one of the elements whose usual chemical compounds are most soluble, yet its soil mineral forms (micas and orthoclase feldspar (KA1si3O8) are only very slowly soluble and occur in relatively unavailable forms.
Soil total potassium is between 1-3%, while 90-98% of soil total K is found in primary minerals –K- feldspar (Orthoclase and micas, muscovite, and Biotite). These minerals are resistant to weathering and supply relatively very small quantities of K during a given breakdown of these primary minerals due to the presence of acid clay and the action of other solvents such as carbonated water.
A large portion of plant K+ comes from exchangeable and solution K+, especially in neutral and basic soils. In acid soils, exchangeable K+ is the major source. Potassium accumulates as the mica and feldspars weather and as K+ in plant residues is released into the soil solution.
Potassium can be lost in leaching waters and or entrapped between layers of hydrous mica (illite), smectite, and vermiculite. The interlayers of these 2:1 expanding clay mineral accommodates K+ and NH4+ ions to become “fixed” into non-exchangeable potassium and ammonium ions.
Read Also: Factors Affecting Solubility and Fixation of Phosphorus in Soils
