Potassium is an essential plant nutrient vital to a range of plant functions including seedling development and plant growth, tolerance to stress and disease resistance.
Historical management to meet water-limited yield potential has led to a depletion of soil potassium in some soils of the south-western agricultural region of Western Australia. Potassium deficiency initially appeared on deep sands on new land, with emerging deficiency on duplex soil in the 1990s – and more recently on loam soil types with higher natural potassium reserves. Decades of negative potassium balances where plant demand is greater than the supply of potassium, have resulted in this deficiency becoming more prevalent – and exacerbated where export of grain and hay is high.
The capacity of a soil to provide adequate potassium for growth is closely related to the soil’s properties, such as clay content and clay mineralogy. Crop residues can also contain significant quantities of potassium which is readily soluble and released by rainfall. Potassium is highly mobile in soil and readily leached.
Why is potassium important for soil and plants?
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Photosynthesis
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Enzyme activity
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Fruiting
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Water regulation

Role of potassium in plant tolerance to stress
Professor Richard Bell (Murdoch University) explains how potassium helps plants buffer against stress like disease and drought. From: Soil Quality: 10 Plant Nutrition (Scanlan et al. 2023). Video: Lomax Media.
Potassium cycle
The capacity of a soil to supply adequate potassium for maximum plant growth is closely related to clay content. As clay content increases, the level of native potassium in soil increases, as does the capacity to reduce leaching through adsorption onto negatively charged surfaces.
Potassium supply to crops and pastures is mostly from the exchangeable pool in soil. This plant-available pool is adsorbed to negatively charged surfaces on clay particles and organic matter, and exists in equilibrium with potassium that is in the soil solution and non-exchangeable potassium.
When plants take up potassium from the soil solution, the potassium concentration of the soil solution drops, and some potassium is desorbed from the exchangeable pool until a new equilibrium is reached. In sands, potassium supply depends primarily on the organic matter content of soil. In loam and clay soil, the amount and type of clay minerals have a major influence on potassium supply.

Potassium in the soil solution consists of monovalent cations (K+) dissolved in the aqueous phase of the soil. In the absence of mineral fertiliser or manure added to the soil, the concentration of soluble potassium is often relatively low (100–1000 μM). The soil’s cation exchange capacity can influence soil solution potassium concentration. As it increases, the concentration of potassium in the soil solution decreases.
Plant roots take up potassium ions from the soil solution. Factors that affect the rate of plant uptake include:
- rate of potassium diffusion in the solution from surrounding soil
- how much desorbable potassium is present on the exchange complex
- other cations in the soil, such as calcium (Ca), magnesium (Mg) and sodium (Na)
- rooting density
- cation exchange capacity of the soil.
When potassium levels in the soil are sufficient, plant roots take up potassium using a low affinity transport system. In this system, the membranes of root cells contain channels that potassium ions can pass through. If the soil environment contains limited or inadequate potassium levels, plants will take up potassium using a high affinity transport system. This uptake process is more complex and involves several proteins that mediate the movement of potassium ions into the plant roots.
Exchangeable potassium is the potassium associated with negatively charged sites in soil, and is readily available for release into the soil solution in response to depletion by plant uptake. If the concentration of potassium ions is low in the soil solution potassium pool, then these adsorbed potassium ions can be released from the surfaces until equilibrium is re-established.
Conversely, if the plant root is preferentially taking up other ions, such as Ca2+ , then the build-up of potassium ions in solution will move to the exchange or adsorbed phase until equilibrium is achieved. Thus, exchangeable potassium is the most commonly used indicator of soil potassium supply for crops and pastures. The sodium bicarbonate-extractable soil potassium test (Colwell K) has been extensively used by soil testing laboratories for soil exchangeable potassium in Australia.
Non-exchangeable potassium includes interlayer potassium in micas interlayer potassium in secondary layer silicates. Interlayer potassium is potassium that is bound up in layers of minerals called micas. These negatively charged layers are made up of two tetrahedral sheets bound on either side of an octahedral sheet, all bound together by positively charged potassium ions. As a result of weathering, the positive charge of the micas decreases and potassium ions are released from the structure.
Secondary layer silicates are produced from the weathering of primary minerals. Vermiculite and smectite are examples of secondary layer silicates that influence potassium dynamics in soil. These both contain water in the interlayer, which mediates the movement of cations in and out. Measuring potassium using a soil test that relies on cation exchange will measure potassium from this interlayer.
The non-exchangeable potassium was thought to be poorly or not bioavailable. However, there is growing evidence that the potassium in these pools is bioavailable to some extents, varying with soil type and mineralogy. Nonexchangeable potassium has been commonly estimated by the difference between the 1 M nitric acid-extractable potassium and exchangeable potassium, as the strong acid could amplify the intensity of the hydrolysis reaction to dissolve potassium-bearing silicates.
Structural potassium is the portion of potassium incorporated into the crystal structure of primary minerals in the soil, particularly feldspars and micas. It has also been referred to as unweathered potassium, native potassium or matrix potassium. This type of potassium constitutes a significant proportion, often reaching up to 94% or more, of the total potassium content in soil. This form of potassium is tightly bound and not easily extracted by conventional methods. Its availability is dependent on a number of factors, including the levels of potassium in other forms (such as solution potassium, exchangeable potassium and non-exchangeable potassium) as well as the extent of weathering of the feldspars and micas comprising the structural potassium fraction. Generally, structural potassium necessitates the process of mineral weathering and gradual breakdown over time to release potassium ions that can be taken up by plants. It is considered a long-term source of potassium for plant nutrition.
Potassium is leachable in soil and the leaching losses can vary depending on factors such as soil type, drainage conditions and agricultural practices. Leaching will likely occur when inputs of potassium are greater than the soil’s potassium holding capacity and plant uptake of potassium. Leaching is especially problematic on sandy soil. Such losses can undermine efficient potassium use in agriculture. If potassium-rich fertilisers are applied to sandy soil, heavy rain or irrigation can wash away the extra potassium. In the higher rainfall parts of the south-western agricultural region of Western Australia, significant amounts of soil potassium may leach from the root zones of pasture paddocks. Leaching becomes more likely when the input of potassium surpasses the soil’s capacity to hold it and the plants’ ability to use it. Leaching losses are therefore influenced by a combination of:
- rate of potassium fertiliser applied
- timing of fertiliser and/or manure application
- soil type
- crop type
- soil drainage pathways.
Potassium can leach from soil through large pores left by plant roots, soil fauna such as worms, or from other geological- or human-causes. Potassium can also leach from soil due to rainfall amount and intensity as potassium dissolved in water moves through the soil profile. In general, potassium leaching is greatest in sandy soil, then loamy soil and least in clay soil. This is due to soil particle surface area, where clay soil particles are the smallest size, and so have a larger surface area. A bigger surface area means there are more available sites that can bind potassium from the soil solution.
Crop residues hold a significant portion of absorbed potassium in cereals, with more than 70% found in the straw of wheat and barley. Soil fauna and enzymes from microorganisms break down these plant cells that have been added to the soil as residues. This break down causes the fluid of the cell and the potassium ions it contains to enter the soil solution. By retaining stubble and practicing no-till farming, the potassium within these residues is predominantly recycled into the topsoil, potentially benefiting subsequent crops. The manner in which potassium is released from crop residues is influenced by factors like plant species, rainfall, time after desiccation, and is amplified by physical treatments like humidification and particle size reduction . In field conditions, particularly in instances where heavy rains occur and there is no immediate crop demand for potassium, such as during the summer in Western Australia, there is a possibility that potassium might be washed out of residues.
The spread of harvest residues (organic matter) can influence spatial variation in soil properties such as soil pH and potassium concentration. From: Soil Quality: 10 Plant Nutrition (Scanlan et al. 2023). Image: from Adobe Stock 476692614. Editing: Science with Style.
Potassium removal increases where export of grain is high; and is higher still in areas where hay or baled plant residues are cut and removed from the paddock. Product harvest constitutes a significant component of potassium output from the farming system and is more likely to result in a negative potassium balance and greater likelihood of potassium deficiency. As an example, three tonnes of wheat residues contain about 45 kilograms of potassium per hectare.
Excreted potassium from livestock comes mostly from urine. The potassium in manure is in a soluble form and so enters the soil solution potassium pool.
Most inorganic potassium fertiliser granules are water soluble and if soil moisture content is adequate then potassium ions are rapidly released from solid granule form into the soil solution.
Potassium fertilisers are available in nitrate, chloride and sulfate forms. Potassium nitrate and chloride are the more expensive forms per unit of potassium supplied. Too much potassium chloride can cause increases in salinity and be harmful to bacteria in the soil. An excess of potassium can also affect the uptake of magnesium, ammonium and calcium and sodium. There is some evidence that potassium uptake by plants may be affected by the large amounts of sodium present in saline and sodic soils – sodium partly substituting plant uptake of potassium.
Annual potassium balance in wheat

Potassium and soil mineralogy
Exchangeable potassium is the form in which most plant-available potassium is stored in soil. The level of exchangeable potassium depends on the organic matter content of soil, especially in sands. In loam and clay soil, the amount and type of clay minerals and their negative charge have a major influence on exchangeable potassium. It is higher in mica-, vermiculite- and smectite-rich soil than in kaolinitic soil, because of the increased negative charge of the clays. However, the amount of exchangeable potassium is a relatively small proportion of the total soil potassium reserves, the majority of which resides in potassium-bearing minerals.
When exchangeable potassium is depleted, it is replenished at various rates depending on the amount and characteristics of non-exchangeable potassium held by illite clays and by potassium-bearing minerals in soil. The potassium-bearing primary minerals include feldspars, biotite, and muscovite mica. The amount of potassium in primary minerals depends on the type of parent materials and the age of soil. The release of potassium from those minerals is quite slow but differs according to their resistance to weathering. Feldspar and mica are very resistant to weathering. By contrast, potassium is less strongly held in soil dominated by kaolin clay (highly weathered soil), sand or organic matter than in soil dominated by illite–vermiculite clay minerals. The order of release of plant available potassium is: biotite > muscovite mica > potassium feldspars (orthoclase and microcline).

What are the symptoms and problems caused by potassium deficiency?
Potassium deficiency symptoms include yellowing of older leaves, death of leaf tips and scorching of leaf margins in grasses, or ‘spotting’ in legumes.
Farmers may also observe grasses replacing clover in mixed pasture, because legumes are generally more susceptible to potassium deficiency than grasses.
Assessing potassium in the paddock
Plant testing
The youngest mature leaf is often the first to show potassium deficiency symptoms, and tissue tests can be used to determine the deficiency status at the time of sampling but cannot be used to predict future requirements for potassium.
Soil testing
Available potassium is measured by the Colwell or Skene methods, or can be estimated by multiplying the exchangeable potassium test result by a factor of 391. With the exception of alkaline or recently limed soils, the values from either the Colwell or Skene method are similar. Available potassium depends on soil type, with clay soils needing higher levels of available potassium than sandy soils due to a high buffering capacity.
Crop requirements for potassium can be estimated by taking into account the amount of potassium removed from the paddock in yield, soil type, current soil potassium status and stubble retention (up to 50% of potassium may be lost in crop residues when lost during burning or stubble baling).
PAGE REFERENCES AND ACKNOWLEDGEMENTS
Material on this page adapted from:
- Hoyle FC (2007). Soil Health Knowledge Bank.
- Scanlan C, Weaver D, Bell R, Borrett R and Cheng M (2023). Soil Quality: 10 Plant Nutrition. SoilsWest, Perth, Western Australia. [Access]
Last updated July 2024.