Potassium is an essential nutrient for all plants. Enzyme processes, water movement, and transport of the products of photosynthesis are all reliant on potassium. It is essential that crops have a good supply of potassium to ensure high yields and consistent quality.

Role

Potassium activates many enzymes by changing their shape and ‘unblocking’ their active site, as well as keeping the pH constant and neutral at around 7 so reactions can take place. It is necessary for the formation of ATP, which is largely involved in both the light dependent and light independent reactions of photosynthesis. With less potassium available, fewer reactions can take place and therefore the plant is less likely to have the products needed for growth and reproduction.

Potassium regulates the opening and closing of stomata, and this means it can control how much carbon dioxide and oxygen is taken up by the plant, and how much water is lost through evapotranspiration. Stomatal pores open when potassium enters guard cells surrounding the stomata. Water moves into the guard cells via osmosis, causing them to expand. Gases can then move in or out. If there is insufficient potassium, gas exchange may not be as efficient.

Deficiency Symptoms

Weak and less vigorous swards, and white spots on clover leaves. Yellow tinge on edges of leaves (scorch on grass leaf margins). Leaves may appear wilted, as well as the sward having increased susceptibility to disease.

Sources

• Soil
• Animal recycling
• Manures
• Fertilisers

Grazing

80% potassium ingested by grazing animals is excreted, mostly in urine, so it is chemically available but not well distributed. However, livestock have less aversion to grazing on urine than dung pats.

Potassium from stored manures

The composition of manure is variable and is dependent on: the species of animals and their diet, as well as the housing system (slurry vs FYM) and bedding material (for example, straw is high in potassium). The system should also be taken into account, for example parlour washings and rain can have a dilution effect.

Potash Fertilisers 

• Manufactured fertilisers are in a readily soluble form:
• Muriate of potash  (K2O) 60% potash
• Sulphate of potash (K2SO4) 48% potash

Luxury Uptake

Plants can uptake more potassium than their requirement in a process known as ‘luxury uptake’. If a sward is repeatedly cut, soil potassium reserves become deleted. They need to be replenished via fertiliser or manure. It is applied as an potassium oxide, often referred to as potash (K2O). Peaty and sand or gravel will have least potassium, whilst silty soils, clays and loams hold and release more.

Potassium content of herbage can become artificially high (3-4%) and this will result in luxury uptake. However, high potassium content  can be dangerous for grazing animals as grass magnesium content and absorption are both reduced. If grass magnesium levels fall below 0.2% in the DM, animals are likely to be at risk from hypomagnesaemia (grass staggers)

When Should Potassium Fertiliser be Applied?

For silage making, fertiliser potassium should be applied for each cut, splitting the total application. For grazing, less is needed but not in spring (most risky as soil K is at its peak).

To increase soil reserves, potassium should be added when the grass has stopped taking it up. This increases the chance of potassium becoming attached to the soil cation exchange complex. Potassium supplies should be replenished and this is difficult because grass tend takes up potassium whether or not it is needed, so timing is critical