Potash for Grassland

Timing

Phosphate

All fertiliser phosphate amounts should be applied early in the season for the first and second cuts especially if soil P reserves are low. For long term improvement less soluble forms of phosphate may be applied over winter.

Timing of phosphate contributed by manures will largely depend upon practicalities of manure management and effects on the efficiency of use of other nutrients, particularly nitrogen. Care must be taken to avoid manure run-off; this risk should be assessed before applications in wet conditions or when heavy rainfall is possible.

Potash

• No more than 90 kg K₂O/ha should be applied for any one cut during the growing season. Apply the balance over the previous autumn/winter as muriate of potash (60% K₂O). If this application is missed, apply the balance to aftermath grazings at 30 kg/ha per grazing or during the next autumn/winter.
• Experiments carried out by ADAS at 23 sites in England and Wales showed no differences in first cut yields between K applied in November-December compared with mid-February to mid-March, although K recovery in the grass was slightly lower with the earlier application time.
• Make allowance for potash in FYM/slurry applied by reducing the fertiliser application nearest to the time of application of the manures. For example if manure is applied in mid season, reduce the fertiliser potash rate to second cut silage or mid season grazing.
• In multi-cut systems potash applications should be made for each cut.
• Do not apply potash for grazing during spring except in high N low PK fertilisers providing less than 15 kg K₂O/ha per application.

Economics of potash use on grass

Silage yield responses to potash for index 0 and 1 soils

Around 50% of all grassland soils are at index 0 or 1 and for these the justification for using potash has been shown in many trials as illustrated. On the most deficient soils there may be a greater return from expenditure on potash than on nitrogen for the reasons detailed on page 4 especially as £1 spent on nitrogen will buy maybe twice as much potash!

Index 0 soils

Trials have shown grass yield responses on K index 0 soils of between 37 and 65%, depending on the rate of potash applied (see figure). With muriate of potash at £140/t for example, the cost of the potash fertiliser applied per tonne of extra grass dry matter produced is only £6 with low input of fertiliser, increasing to £17 at the highest application rate. This is much below the value of extra grass dry matter which varies according to production system from £50 to £100/t.
This is a return of £3-17 per £1 spent on potash

Index 1 soils

The Figure shows increases in grass yield of between 14 and 28% on K index 1 soils. The cost of potash fertiliser per tonne of extra grass dry matter produced in this case is £10 at the low input increasing to £24 at the highest rate.
This is a return of £2-10 per £1 spent on potash

Financial risks of cost saving

With livestock profits under severe pressure there is a great need to look for cost savings, but these need to be carefully targeted so that yield reductions do not outweigh any costs saved, for example by not applying potash. Nationally around 40% of grass soils are at K index 2 and are unlikely to show direct response to applied potash (fertilisers or manures) and for those at the higher range of this index band (index 2+) there are useful opportunities for potash savings. However, if instead of a normal replacement policy at index 2-, no potash is used in order to save costs, soil reserves will fall at a rate related to the soil type and amount of potash removed each year. Medium and light textured soils under a two cut silage policy could be expected to fall from index 2- to index 1 in a single season if no fertiliser or manure is applied. Fertiliser use would then need to be raised to restore soil reserves and yields in the short term might still not be as satisfactory as if soil K had been maintained.

If the same extreme cost saving approach is continued at index 1, yield penalties are severe.

Yield penalties if nil potash used on soil K index 1
Year 1	Year 2	Year 3	Year 4
13%	25%	49%	53%
yield loss	yield loss	yield loss	yield loss