Potash, magnesium & sodium, fertilisers for grass

The place for sodium

Sodium fertilisers will not normally give extra grass yield but they will increase the Na content of grass which will improve the palatability of herbage and may reduce the chance of grass staggers. Sodium is also associated with a greater % of live herbage, higher D values and sugar content of grass. Research from Bangor University indicates that these effects increase milk output and % butterfat and may also have a small benefit on somatic cell count. Grass palatability and milk output increase at herbage sodium levels up to 0.5% in the dry matter.

Grazing

Staggers is mainly associated with lush spring growth when magnesium and sodium % are low, potash application to grazed swards should be avoided at this time. Replacement K dressings should therefore either be made after the spring flush ie. June onwards, or applied in small more frequent applications. Potash requirements of grazed grass are small because most K is recycled back to the sward in the dung and particularly in the urine.

Staggers can occur at other times in the season when magnesium levels are inherently low, where over-dependence is placed on low dry matter grass and when animals are under other stress such as at calving. Close attention to the mineral nutrition of animals is required at these times and supplementation may be required.

Cut Grass

Silage and hay crops remove large quantities of potash from the soil. These must be replaced to maintain soil fertility and protect future yields. Restricted use of potash will seriously reduce hay and silage yields and will not necessarily reduce the incidence of magnesium disorders.

So-called "luxury uptake" will not occur if the rate of fertiliser potash applied takes acount of:-

• the nitrogen rate used (which will affect yield & therefore offtake)
• level of soil K reserves
• use of organic manure
• soil type

Potassium, magnesium & sodium in the animal

Much of the sodium consumed by cattle and sheep is used in the production of saliva which is secreted into the rumen to maintain a constant pH by neutralising acids formed by bacteria in the rumen liquor. If the sodium content of forage is too low, the animal automatically substitutes potassium for sodium as an alternative buffer in the saliva and diverts sodium to maintain blood Na level as first priority.

The resulting increase in K:Na ratio in the rumen leads to reduced resorption of Mg through the rumen wall into the blood - hence placing the animal at risk to hypomagnesaemia. However, it is only in extreme cases that a low blood level of magnesium occurs (less than 1.8mg/100ml of blood in cows) and the consequences of the condition (reduced milk yield and even death) may arise without ever detecting low blood Mg.

Fate of forage potash, magnesium & sodium

Fertilise to balance K, Mg, Na

The sodium and magnesium content of grassland can be improved by using sodium and magnesium fertilisers to balance the levels of K required for grass production. A list of fertilisers containing potassium, magnesium or sodium is given on page 4. Extra minerals may be required for high yielding dairy cows and lactating sheep. Feed analysis should be used to confirm supplements required.

Slurry

Slurry is a valuable source of minerals containing a little sodium, some magnesium and a lot of potash. Modest applications of slurry may increase K and Mg levels in herbage. With larger applications however, the high concentrations of potassium tend to depress both Na and Mg levels. The nutrient contribution of slurry should be taken into account when deciding fertiliser policy. In particular, spring potash dressings should be adjusted when slurry has been applied over winter to avoid excess levels of K being available to the grass in the spring.

Slurry effect on herbage minerals

Good Farming Practice

1 Soil analysis

Soil sample every 4-5 years to check on fertility status and trends.

2 Herbage analysis

Where mineral disorders are a problem take herbage samples when grass is growing actively (eg May) to check on K, Mg and Na status.

TARGET LEVELS