Potassium is often thought of as being the simplest macronutrient to manage as it cannot be lost as a gas or leached as quickly as nitrogen, and cannot become permanently unavailable to plants like phosphorus. Mineral potassium reserves are abundant, and it is estimated that they will last for thousands of years even if the current extraction rate were to be doubled. However, benefit can still be derived, and input costs optimised from a proper understanding of the dynamics of this nutrient.
Potassium is used by plants to fight disease and promotes vigorous growth by acting as a catalyst for enzymes during photosynthesis. Potassium is also a major factor in the blooming and quality of fruit/seed.
Potassium in the soil
The only form in which potassium exists in the soil is as a simple cation (K+), and it is in this form that it is taken up by plants. This simple chemistry also limits the type of interactions potassium can have in the soil.
Only a small percentage of the total soil potassium is in solution at any one time, and when it is taken up by plants or leached out, the soil solution is quickly replenished by other soil reservoirs. These vary in form, and readiness to release the potassium they hold so that plants can take it up. Much of the potassium, which has a positive charge, is sorbed, or held loosely, on to the various negatively charged surfaces found in the soil, mostly 2:1 clay minerals and soil organic matter. This is termed exchangeable potassium and is the most likely to enter the soil solution when the concentration of free potassium decreases, by plant uptake or leaching. The cation exchange capacity of the soil (CEC) provides a rough estimate of the potential of the soil to hold potassium in this way, although potassium is more readily released from organic matter than from clay surfaces.
Potassium does not form a part of stable soil organic matter, like carbon, nitrogen and phosphorus. The only association with organic matter is surface adsorption. However, soil organic matter is still important, especially in highly weathered soils. In these soils, most of the clay minerals are highly weathered forms which do not have a charge, and so, are not able to hold any potassium. In these cases, the soil has a low intrinsic CEC, and therefore reduced ability to prevent potassium from leaching. Most of the exchange capacity is provided by organic matter in these soils, and therefore, most of the exchangeable potassium is held by the charged surfaces of soil organic matter. The more organic matter, the more potassium can be held to be used later by plants.
Potassium is held deeper inside clay particles, because the spaces inside are negatively charged. This potassium is not as easily released compared to that on the surface of the clays. Weathering of clay particles or a strong potassium gradient (created through plant uptake or leaching) will release the potassium from the interlayer space. The potassium held in this way is slightly exchangeable, or available, and is difficult to quantify. There is often more potassium in the soil than expected because of this, and certain plants are better adapted to use this less available potassium than others. Plants which can take up potassium very quickly can create dramatic potassium gradients in the soil, which causes a release of the less available potassium held by the clay minerals. Amongst the crop plants, winter grains, such as rye and wheat, seem to be particularly good at this, being unaffected by potassium deficiency when other plants would be.
The most unavailable pool of potassium in the soil is known as the structural potassium, or potassium which is a part of the minerals which make up the soil particles themselves. This potassium only becomes exposed when the particles are weathered down into smaller pieces. It is released very slowly through weathering, but it does contribute to plant nutrition. Many plants can accelerate this process by releasing organic acids into the soil through their roots, which break down the potassium containing minerals, to release potassium for uptake. Cultivated plants which are well known for this trait include lupins, chickpea, and pigeon pea.
Managing Potassium Losses
An understanding of potassium in the environment cannot be complete without taking into account crop residue. Unlike nitrogen and phosphorus, much of the potassium taken up by the plant is not partitioned to the seed, or grain, which is removed during harvest, but rather remains in the vegetative parts of the plant. In maize, as much as 70% of the potassium taken up by the plant remains in the residue after harvest. Removal of crop residue, therefore, causes a relatively greater loss of potassium than nitrogen or phosphorus, and should preferably be avoided. In forage crops, this means that large amounts of potassium are removed from the field when hay or silage is made, with most of it being deposited elsewhere after passing through the animals. This effect is exacerbated by the fact that plants will take up more potassium than they need if it is available, this is known as luxury consumption. If the livestock are fed in a concentrated area, most of this potassium is lost. Concentrated depositing of manure and urine will generally exceed the capacity of the soil to adsorb it. One solution is to graze the pastures directly as far as possible, instead of making hay, to ensure that manure and urine, and potassium, are deposited relatively evenly on the land, instead of only where the animals are fed. This will reduce the leaching losses commonly associated with overloading the soil with a potassium source. Grazing and forage management, along with pasture crop choice is most important if pastures are to be grazed instead of cut.
Deep rooted cover crops, such as the radishes shown here, can be used to extract potassium that has leached into the subsoil, making it available for shallow-rooted crops that follow
Potassium, although less mobile than nitrogen, often ends up in the subsoil layer. Here it often remains for a long time just out of the reach of plant roots. It is logical, therefore, that plants, or even cultivars of common crop plants, with deeper root systems are less likely to show potassium deficiency. It is hypothesised that many maize cultivars were inadvertently bred for shallow root systems when the breeding work was done under ideal fertiliser and water availability regimes. These cultivars are, then, more likely to suffer from a deficiency than cultivars with a deeper root system. A compaction layer or plough pan in the soil can have a similar effect, denying root access to potassium that may be easily available, but out of reach. A sustainable method of bringing this deep potassium back to the surface, other than deep tillage, is the use of a deep-rooted cover crops. These include many of the brassicas and perennial grasses, with canola being a well-studied example that has a long tap root. When these plants die, the potassium quickly enters the topsoil before material is broken down,. Generally the potassium is leached out of the plants in a matter of weeks, where it is available to shallow rooted plants. Luxury consumption, can, in this case, be put to good use.
A more significant and permanent loss of potassium, that is unfortunately rather common, is erosion and runoff. Since potassium is adsorbed onto clay particles in the soil, it is particularly badly affected by runoff. Clay particles, which hold more potassium than other parts of the soil, such as sand and silt, are more easily washed away than the sand and silt. For this reason, relatively more potassium is lost to runoff than other nutrients. Bare fallow soil is particularly badly affected by both erosion and runoff, often causing the loss of any potassium that may have been left over from the previous season. By slowing the flow of water and trapping suspended soil particles, cover crops have shown to reduce the losses of potassium through runoff from the field by as much as 90%. A cover crop also helps to prevent the formation of a soil crust caused by raindrop impact, which can reduce the rate at which rain, and surface applied potassium fertiliser infiltrates into the soil.
The reddish colour often associated with flood water is caused by clay particles, which carries away adsorbed potassium
Little Effort, Big Rewards
Potassium is the one nutrient for which cover crops can provide the quickest, and most tangible benefits, both by reducing unnecessary losses, and increasing the plant available supply from sources that are already in the soil. Cover crops can also eventually increase the ability of the soil to hold onto excess potassium by increasing the cation exchange capacity of the soil through the addition of organic matter. Like any other nutrient that is available as synthetic fertiliser, overapplication will generally result in wastage, while there is evidence that plants can manage well with underapplication of potassium as long as the soil biology is in a healthy state.