Potash, magnesium & sodium, fertilisers for grass

Potash use is linked to fears that so-called "luxury" uptake may cause staggers (hypomagnesaemia) and other mineral disorders. Metabolic problems can be costly but so is below-optimum use of potash. This leaflet reviews the facts.

Natural mineral content of grass

The natural pattern of uptake of minerals such as Potassium (K), Magnesium (Mg), Sodium (Na) and Calcium (Ca) varies during the course of the growing season. The dry matter of young rapidly growing grass, especially in the spring, has a high K content which declines as the season progresses, whilst the concentration of other minerals increases through the summer.

Seasonal pattern of mineral uptake in grass swards

Fertilisers and mineral uptake

The application of a correct balance of nitrogen, phosphate and potash is essential to obtain the best yields of low-cost on-farm forage. In addition to their effect upon yield, N and K fertilisers influence mineral content of grass, especially Mg, Na, and Ca. Adequate nitrogen will tend to stimulate the uptake of all minerals but the ease with which plant roots absorb different nutrients varies:

EASIER		UPTAKE		LESS EASY
N as NO3 : P : K : Na : Mg : Ca

This is in line with the needs of grass which for optimum growth requires large amounts of potassium and lesser amounts of magnesium and sodium. For animals however, the balance of requirements is for more magnesium and sodium and only modest potassium. An adequate supply and balance of these minerals is important to minimise the risk of metabolic disorders such as hypomagnesaemia (staggers or grass tetany) and milk fever and to ensure fertility.