Potassium (K), often referred to as potash, is a plant essential macronutrient important for stress resistance and water regulation. A lack of potassium may lead to lodging, reduced protein formation and the onset of disease.
Macronutrient contributing to 2-5% of a a plant's nutritional makeup
Involved in water regulation, photosythesis and enzyme formation
Deficiency symptoms include scorching of leaf edges, lodging and onset of other diseases
Small amount of potassium can be lost via run-off or leaching if applied in adverse conditions
Rates and timings depend on the potassium soil index and expected crop offtakes
Potassium containing products include Muriate of potash (50%) and Sulphate of potash (42%)
Of all the essential nutrients, potassium is taken up by the plant in the greatest quantities. For example, an average cereal crop will take up over 300kg/ha potassium during peak spring growth, compared to around 225kg/ha nitrogen.
Potassium is required for the activation of over 60 plant enzymes. It is responsible for improving water use from roots to shoots through the regulation of photosynthesis and the transportation of sugars.
It improves the uptake and use of several nutrients including nitrogen, which makes it useful in boosting protein production.
Potassium provides an insurance against adverse growing conditions by improving the plants ability to withstand disease, pest and drought stress. In good growing conditions the response to potash may be modest, especially for crops like cereals, but in adverse years its contribution to optimum yields will be substantial.
It has a major role in many aspects of crop quality such as grain size and appearance, tuber size, oil content, dry matter and starch content.
Potassium is the third most abundant nutrient within an animal’s body, with only phosphorous and calcium present in greater quantities.
As with plants, potassium is involved water regulation and in the activation of enzymes within animal cells. It also aids in the metabolisation of carbohydrates and proteins and helps maintain a regular heartbeat.
Potassium is essential for rumen function therefore ruminants typically have a greater demand for potassium than non-ruminants. Insufficient supply of potassium to ruminating animals could result in reduced feed intake.
Excessive levels of potassium in an animals diet can lead to an nutrient imbalance. When potassium intake is high, animals substitute sodium for potassium which leads to resorption of magnesium in the blood, leaving animals vulnerable to magnesium deficiency (grassland staggers). To help prevent this, the potassium:sodium and potassium:magnesium ratios of the forage should be kept below 20:1 and ideally less than 10:1.
Animals tend to consume more potassium than they require, and so excess potassium is excreted in the animal’s urine and contributes to soil potassium reserves.
Potassium for grazing at stocking rate 170kg/ha Org N¹
Soil K index
K Advice (kg/ha)²
For stocking rates above 170kg/ha Org N increase rates by 5 kg/ha for each increase in stocking rate of 40 kg/ha. For each decrease in stocking rate of 40 kg/ha below 170kg/ha Org N decrease rates by 5kg/ha.
Chemical K application rates can be calculated by deducting the quantity of K applied in organic fertilisers.
*Fertiliser K can be applied during closed periods in autumn and winter.
Typical K levels in organic fertilisers¹
Organic fertiliser type
Total K contained in 1 tonne² (kg/t)
Dungstead manure (cattle)
Slurry (layers 30% dry matter)
Layers (55%) dry matter
Dry matter and nutrient contents can vary widely between farms
1 tonne of slurry=1m³. 1000 gallons=4.5tonnes. 1000gallons/acre=11tonnes/ha
Potassium exists in soil solution as the potassium ion, K+, the form that is taken up by plants. However only very small amounts of potassium are maintained in the soil solution at any one time.
Most soil potassium is bound to clay particles and organic matter within the soil profile. The speed at which this potassium can be made available for plant uptake depends on the strength of the bond – E.g. potassium which is only loosely held by clay particles is available more quickly.
Some potassium exists within soil minerals which release potassium very slowly as they weather over time. The supply of potassium from soil minerals is not sufficient to meet crop demand in most cases.
[infographic of the '4 pools']
Plants deficient in potassium present with burnt or scorched leaf edges which gradually moves to the centre of the leaf over time. Since potassium is mobile within the plant, the older leaves are the first to show signs of deficiency.
Plants are also more vulnerable to lodging and the onset of pest and disease damage.
Where is the risk of deficiency highest?
Light soils with low organic matter content
High rainfall areas
Where high levels of soil magnesium impede potassium uptake
To help prevent potassium deficiency, soil testing should be completed at least every 3-5 years.
Maximum cereal and grass yields are typically reached in soils which contain 100-150mg/litre potassium – otherwise known as index 3. Where soil potassium levels are below this threshold, farmers should work to build soil reserves towards the target index, which can take several years.
While leaf analysis can be used to give an indication of plant potassium levels at a point in time, it is not useful in predicting the nutritional requirement of a crop over an entire season and so should be used in conjunction with soil analysis rather than in isolation.
For most soils – those which have a clay content of 5% or above - where recommended rates of potash are applied, potassium which is not taken up by the crop will remain within the soil profile.
However, losses can occur when potassium is applied under adverse conditions – for example when soils are waterlogged or compacted. Most of this loss is by surface run-off and can be avoided by following the code of good agricultural practice.
Soils with less than 5% clay (sands or loamy sands) have a much lower retentive capacity for potassium. Such soils, especially if shallow and subjected to heavy rainfall, have a greater risk of potassium loss.
The quantity of potassium within a Goulding Soil Nutrition blend can be adjusted to meet the requirement of individual crops and soil types.
The timing of potassium applications is partly dependent on the potassium index. Demand for potassium is greatest when crops are actively growing in the spring.
Applying large quantities of potassium to waterlogged, frozen or compacted soils should be avoided to avoid nutrient loss.
Where the potassium index is at target, enough potassium should be applied to meet the demands of the growing crop – otherwise known as maintenance. Where the potassium index is below target, maintenance levels of potassium should be applied, plus additional potassium to help build soil reserves closer to the optimum level.
Farmers should also take into account the crop offtake - the amount of potassium which is expected to be removed in crop material (E.g. straw, grain, silage) - when considering potassium requirements.
Muriate of Potash
Naturally mined granular potash
Sulphate of Potash
0-0-42%K + 16%S
Naturally mined granular potash
19.2%S + 11.6%K + 3.6%Mg + 12.2%Ca
High sulphur multi-nutrient. Mined from natural rock deposits. Organic certification. Low chloride.