Potash for Grassland

Potash and protein production

Potash is also important to achieve good feed value as it affects the conversion of nitrate to protein. Total protein yield will be significantly reduced if potash is limiting. The chart shows the increase in both the hay yield and the % crude protein resulting from the application of potash, with the total protein yield rising by almost 45%.

Deficiency symptoms

Potash deficiency is not easily identified in grassland and is sometimes referred to as 'hidden hunger' because losses occur despite any recognisable visual symptoms. Typical deficiency symptoms in grasses are yellowing and chlorosis of the edge and tip of older leaves, with progressive senescence. Similar symptoms result from a number of other causes such as - drought, water-logging, wind, etc. Clovers become less vigorous with chlorotic speckles over the whole of the leaves which are frequently smaller than normal. After grazing, vigorous and darker regrowth in dung and urine patches is in contrast to yellower, poorer growth when potash is deficient - this can also reflect nitrogen supply however.

In practice visual diagnosis is an unreliable means of detecting potassium shortage and regular soil analysis should be used to determine potash status.

Potash Supply

Grass takes up potash from soil reserves which should be replenished by additions of manures or fertiliser equal to what is removed.

It is inefficient to allow soil reserves to fall to levels where grass growth is directly dependent upon fertiliser applications.

The quantities of potash removed in silage are large and for multi-cut systems they are often greater than for any arable system. Clearly it is necessary to replace these nutrients and logical to return manures to silage and hay fields. In practice, however, this may not occur because cutting fields are more remote or not so convenient. It is common to find fields which regularly grow maize with high applications of manure and which have excess soil P and K whilst grass silage fields do not receive manure and have deficient nutrient levels.

The situation is very different with grazing, because around 90% of the potash in grazed grass is returned directly to the sward and the need for potash application for grazing is low. The return, however, may not be even and fertility can build up where animals congregate to drink, feed or lie, which means a run-down in other areas where they only graze. Common sense sampling can identify such differences and guide the need for additional nutrients.