Potash for Grassland

Recent surveys indicate that around 50% of grassland is on soils with K index of either 0 or 1. With the average application of fertiliser K to grassland having fallen by 30% in the last ten years, we are heading towards further reductions in soil potash levels. Increased focus on K is required to improve efficiency of grassland production and reduce risks to the environment.

Deficiency of K will not only reduce grass yield, but also reduce the efficiency of use of N, leading to increased losses of N compounds to water and the atmosphere. Changes in farm support and low milk prices mean that farmers must seek to reduce production costs and make the best possible use of grassland and forage. This means increased attention to potash.

Experiments throughout the country showed that grass yields on soils with K status of index 1 are reduced by 25% if no K is applied - the loss of 3 tonnes of silage dry matter (DM) per hectare. The table shows that large economic benefit results from potash application when the extra silage is valued in relation to alternative feeds. If there is a shortage of forage on the farm and extra grass production is translated directly into extra milk production, the value of the extra 4000 litres of milk produced on soil Index 1 soils is some £680. It is realised though that there would normally be other costs in order to achieve this extra milk production, but this illustrates the large penalties that can result from under-production on the grassland area.

The financial benefits from correcting deficiency are even greater with soils of K index 0:

The requirements for potash are most marked on silage areas, because about 24 kg K₂O are removed for every tonne of silage DM produced (6 kg per tonne of silage with 25% DM). In high yielding sites the removal of K in two silage cuts is about 230 kg K₂O/ha. Nutrient applications should at least replace the quantity of K removed. Major contributions to the requirements can be made from timely slurry and manure applications. Slurry can be applied at up to 50 m³ per ha; this will supply 160 kg available K₂O per ha - some 70% of the annual requirement, and a useful potential saving on bought-in fertiliser.

Key points for effective and efficient potash manuring are:

• Carry out soil analysis at least every third year.
• Apply K in manures and fertilisers at least to replace the quantity of K removed in silage or hay. If soil index is 0 or 1, applications should be higher in order to build up the index.
• Work out a plan for use of the manure produced on the farm and use mineral fertiliser to increase K supply to satisfy the requirement.
• Slurry is best applied in late winter-early spring and applications should not exceed
  50 m³ per ha.
• Do not apply slurry (or fertiliser) if heavy rain is probable within the next 3 to 4 days (particularly in early winter).
• There is little difference in response to fertiliser K between applications in November-December compared with mid-February to mid-March.
• In multi-cut systems apply K before the growing season and after each cut.

Requirements for potash are much lower in grazing systems, because most of the K in the grass is returned directly to the sward in urine. Nevertheless, many grazed swards still have low K status, reflecting low previous applications of K. Fertiliser K recommendations for swards that are grazed only are 60, 40 and 20 kg K₂O per ha for soils with K index of 0, 1 and 2- respectively. Fertiliser K application in spring should be restricted to less than 15 kg K₂O per ha in high N low PK fertilisers.

See PDA Leaflet 14, Potash for Grassland for Silage and Grazing

For further information please contact:
PDA info@pda.org.uk
Potash Development Association

update: November 2005