ARCHIVE: Fertiliser Manual (RB209)

Grass Silage – Phosphate, Potash, Magnesium and Sulphur

The amounts of phosphate and potash are appropriate to the fresh weight yields shown. The yields are based on wilted silage at 25% dry matter content as carted into a silage clamp. Where yields are likely to be greater or smaller, phosphate and potash applications should be adjusted accordingly. Appendix 5 gives typical values of the phosphate and potash content in crop material per tonne of yield.

Different potash recommendations are given for the lower half (2-) and upper half (2+) of K Index 2. When considering the maintenance recommendations (marked ‘M’), full account must be taken of note b) beneath the table.


P or K Index

  0 1 2 3 4 and over
1st cut (23 t/ha)
Phosphate (P2O5)a 100 70 40M 20 0
Potash (K2O)b – previous autumn
                        – spring
80M (2-)
60 (2+)
2nd cut (15 t/ha)
Phosphate (P2O5)a 25 25 25M 0 0
Potash (K2O)b 120 100 90M (2-)
60 (2+)
40 0
3rd cut (9 t/ha)
Phosphate (P2O5)a 15 15 15M 0 0
Potash (K2O)b 80 80 80M (2-)
40 (2+)
20 0
4th cut (7 t/ha)
Phosphate (P2O5)a 10 10 10M 0 0
Potash (K2O)b 70 70 70M (2-)
40 (2+)
20 0
  1. At soil P Index 2 or over, the whole of the total phosphate requirement may be applied in the spring. At P Index 0 and 1, the phosphate recommendation for the 3rd and 4th cuts may be added to the 2nd cut recommendation and applied in one dressing.
  2. To minimise luxury uptake of potash, no more than 80-90 kg K2O/ha should be applied in the spring for the 1st cut. The balance of the recommended rate should be applied in the previous autumn.

At soil K Indices 2+ or below, extra potash is needed after cutting as follows:

  • In one or two cut systems apply an extra 60 kg K2O/ha following the last cut or by the autumn. Where grazing follows cutting, this may be applied as an extra 30 kg K2O/ha per grazing for up to two grazings.
  • In three cut systems, apply an extra 30 kg K2O/ha after cutting.
  • In four cut systems, no extra potash is needed.

For magnesium recommendations, see page 183.

Soil and herbage analysis to assess potash use

Since large quantities of potash can be removed in intensive silage systems, soil analysis at least every 4 years is of particular importance to monitor changes in soil K Indices and adjust potash fertiliser use accordingly. Because soil K levels can fluctuate widely during the growing season, it is important that soil sampling is carried out during the winter or early spring period, leaving the longest possible time interval between sampling and the last potash fertiliser or manure application.

Herbage analysis can also be useful to assess the adequacy of recent potash applications and as a basis for adjusting potash use for future cuts. Uncontaminated samples of herbage should be taken just before cutting. Potash deficiency is indicated if the herbage potassium concentration is below 2% K (in dry matter) or the N:K ratio of the herbage is above 1 to 1.3.


Sulphur deficiency is common in grassland, especially at second and later cuts in multi-cut silage systems using high rates of nitrogen. Deficiency is possible on all mineral soil types though is likely to be particularly severe on sandy and shallow soils in areas of low atmospheric deposition. Deficiency at first cut is less common but can occur on light sand and shallow soils. The map on page 43 shows areas likely to be deficient in sulphur. Grass on organic or peaty soils is not likely to show sulphur deficiency.

The symptoms of sulphur deficiency are a general paling of growth similar to nitrogen deficiency. Analysis of uncontaminated herbage sampled just before cutting is a useful indicator of deficiency. The information can be used to assess the need for sulphur for future cuts. The critical level is 0.25% total sulphur or an N:S ratio greater than 13:1.

Where sulphur deficiency is indicated, apply 40 kg SO3/ha as a sulphate containing fertiliser applied at the start of growth before each cut.

Newly sown swards

In the first season after autumn or spring sowing, deduct the amounts of phosphate and potash applied to the seedbed.

Don’t forget to deduct nutrients applied as organic manures – see Section 2

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