Saturday, July 20, 2024
General Agriculture

Potassium Content of Soils, Forms, Sources, Relationships and Deficiency Symptoms

Soil potassium (K) directly affects crop yield since K is responsible for the maintenance of osmotic pressure and cell size, which in turn influences photosynthesis and the energy production along with stomatal opening and carbon dioxide supply.

Except nitrogen, K is a mineral nutrient plants require in largest amounts. Potassium is assimilated in relatively large quantities by the growing crop as the yield and quality are closely related to soil K (Tisdale, etal1993).

Plants require soil K for ATP production, translocation of sugars, starch production in grains, nitrogen fixation in legumes and protein synthesis.

Potassium Content of Soils

The concentration and availability of K in the soil is primarily controlled by inorganic processes. Though K does not pose the potential environmental concerns that nitrogen and phosphorus do; an understanding of K cycling and availability is important for the management of profitable long-term cropping systems.

Soil K has, however, not been given much attention by researchers as it deserves in the tropics. Possibly this neglect in K research could be due to the general assumption that most tropical soils contain adequate amounts of K to sustain crop growth.

Whilst this assumption could be true, losses of K are basically incurred through leaching in drainage waters, crop removal, continuous cropping and utilisation by living organisms.

Forms of Soil Potassium (K)

Based on availability to plants, K can be categorized into three major forms:

Relatively unavailable K; this is contained within the crystalline structure of micas, feldspars and clay minerals.

Plants cannot use the K in these insoluble forms and therefore mineral weathering must take place. Because feldspars and micas are resistant to weathering they release only small quantities of K during a single cropping season.

Slowly available (fixed) K; this form of K is trapped between the layers or plates of certain kinds of clay minerals as illite, vermiculite and chlorite.

Potassium held in this manner cannot be used much by plants during a single growing season. However, the supply of fixed K largely determines the soil’s ability to supply K over extended periods of time.

Readily available K; is that which is dissolved in soil water or held on the surface of clay particles.

Plants absorb dissolved K readily, and as soon as the concentration in the soil solution drops, more is released into the solution from the exchangeable forms.

Potassium in the soil solution, which represents a very small fraction of total soil K, is an important indicator of K availability.

Generally, 90 to 98 percent of the total K in soils is in the relatively unavailable form, 1 to 10 percent in the slowly available form and about 0.1 to 2 percent in the readily available form.

Sources of Potassium in Soils

Clay minerals are the most important sources of soil K excluding that from fertilizers. They hold the bulk of mobile K and release it when the concentration of the soil solution falls due to uptake by plants or to an increase in soil moisture.

It has also been observed that over 95 percent of the K in tropical soils is contained in primary and secondary minerals.

Potash feldspars, muscovite and biotite are generally considered the original sources of K in soils. At equal clay content, the K concentration of soil solution depends on the nature of the clay minerals.

Potassium Content of Soils

Kaolinitic clay minerals have no inter-lattice binding sites for K and a low cation exchange capacity. They do not hold non- exchangeable K and therefore behave similarly to sand and soil organic matter, as far as K dynamics are concerned.

Illitic clay minerals, vermiculite and chlorite adsorb K selectively. According to Olaitan and Lombin (1984) illites form the most important clay mineral that contains K.

Read Also : Nitrogen Content of Soils and Factors Affecting Soil Nitrogen

The selectivity of montmorillonitic clay minerals (smectite) for K is lower than that of illitic but greater than that of kaolinitic clay minerals.

Allophanes contain very small amounts of K.

Relationships between Potassium (K) Forms and other Soil Properties

It has been reported that there is greater exchangeable K availability to plants in soils of coarse texture than on fine texture. Thus, replacement of a given amount of exchangeable bases will cause release of more K ions from sandy soils than from clayey soils with equal exchangeable K content.

It has also been found that K fixation by samples of many soils of Finland increased with clay content indicating that soils with higher clay content are likely to contain more non-exchangeable

K. Sands are often made up of almost entirely of quartz and therefore contain very small amounts of K minerals.

Organic matter has no strong affinity for absorbing K. However, it has been argued that the CEC of organic matter increases with pH and that at higher pH levels organic matter may be able to serve better as a source of plant available K.

The effect of soil pH on the availability of soil K is still a debatable issue. York et al (1953) noted that the fixation of fertilizer K takes place more readily in neutral than in acid soils and liming an acid soil increases its ability to fix K.

It has also been reported that even though liming decreases K susceptibility to leaching, it might also reduce solution K to levels where plants suffer deficiencies.

Factors Affecting Availability and Fixation of Potassium Soils

1. Nature of Soil Colloids

The dominant clay species in a 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, montmorrilonite and fine grained mica (illite), are dominant readily fix K in large amounts.

The 2:1 clays have larger negative charge from isomorphous substitution of A13+ for Si4+ in their silical tetrahedral layer thereby strongly binding the K+ ions.

2. Alternate Wetting and Drying

Alternate wetting and drying and freezing and thawing has been reported to contribute to 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 leguminious crops such as soya bean 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%.

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 Deficiency and Toxicity Symptoms

On an annual basis, agricultural crops remove between 100-300kg K ha-1. The amount 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 requirement.

This is because amount available for crop uptake depends on the concentration at the root surface and its replenishment. Potassium in its form taken up by plant, that is K+, is a mobile element easily translocated to the younger parts of the plants whenever there is a short fall in the amount taken up by the crop.

Therefore, deficiency symptoms first manifest on the older plant parts. There is yellowing along the margin from leaf tips or apex of older leaves. Necrotic area along leaf margins is characteristic of K-deficiency symptom in dicotyledon plants.

There is also browning of tips of leaves down to the base. Acute shortfall in K-supply leads to stunted growth, poor root development and 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.

In summary, the concentration and availability of K in the soil is primarily controlled by inorganic processes. Except nitrogen, K is a mineral nutrient plants require in largest amounts.

Potassium is assimilated in relatively large quantities by the growing crop as the yield and quality are closely related to soil K. Clay minerals are the most important sources of soil K excluding that from fertilizers.

K is the next largest nutrient element after nitrogen needed by crops for their growth and development.

Soil potassium (K) directly affects crop yield since K is responsible for the maintenance of osmotic pressure and cell size, which in turn influences photosynthesis and the energy production along with stomatal opening and carbon dioxide supply.

The forms in which K is available in the soil are; relatively unavailable K, slowly available and readily available.

Read Also : Feed and Nutrition Management for Cattle

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Benadine Nonye is an agricultural consultant and a writer with several years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. Agric4Profits.com - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. WealthinWastes.com - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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