ARCHIVE: Fertiliser Manual (RB209)

All Cereals – Phosphate, Potash, Magnesium and Sulphur


The amounts of phosphate and potash are appropriate to the grain yields shown. The amounts of phosphate and potash shown at the target Index, P Index 2 and K Index 2-, are needed to replace the offtake in the yield of grain shown and maintain the soil at the target Index. The upper half of the Table shows the maintenance applications for the yield shown for each cereal when straw is incorporated. The lower half of the Table shows the maintenance applications when straw is removed but not weighed, the extra amount of phosphate and potash is based on the grain yield. Where the weight of straw removed is known, use the amounts of phosphate and potash per tonne straw shown in Appendix 5 to calculate the amounts removed and apply these amounts as the maintenance dressing. The phosphate and potash recommendations at target or lower Indices can be adjusted if yields are likely to be larger or smaller than those shown in the table, by multiplying the difference in expected grain yield by the phosphate and potash content per tonne of grain yield using the appropriate value for where straw is either incorporated or removed as given in Appendix 5. For example, at P Index 1, the phosphate recommendation for wheat with an expected yield of 10 t/ha where straw is incorporated is 90 + (2 x 7.8) = 106 kg/ha.

Crops grown on soil at Index 0 and 1 would be expected to respond to the extra amounts of phosphate and potash shown in the table below. Also over a number of years, these extra amounts of fertiliser will help to raise most soils, except light sands, to Index 2. At Index 3 and above no phosphate and potash need be applied for a few years but check soil analyses regularly.

At Index 2, phosphate and potash can be applied when convenient during the year but at Index 0 and 1, they should be applied and worked into the seedbed. To avoid damage to germinating seedlings do not combine drill more than 150 kg/ha of nitrogen plus potash on sandy soils.

  P or K Index
  0 1 2 3 and higher
  kg/ha
Straw ploughed in/incorporated
Winter wheat, winter barley (8 t/ha)
Phosphate (P2O5) 120 90 60 0
Potash (K2O) 105 75 45 (2-) 20 (2+) 0
Spring wheat, spring barley, rye, triticale (6 t/ha)
Phosphate (P2O5) 105 75 45 0
Potash (K2O) 95 65 35 (2-) 0 (2+) 0
Winter and spring oats (6 t/ha)
Phosphate (P2O5) 105 75 45 0
Potash (K2O) 95 65 35 (2-) 0 (2+) 0
Straw removed
Winter wheat, winter barley (8 t/ha)
Phosphate (P2O5) 125 95 65 0
Potash (K2O) 145 115 85 (2-) 55 (2+) 0
Spring wheat, spring barley, rye, triticale (6 t/ha)
Phosphate (P2O5) 110 80 50 0
Potash (K2O) 130 100 70 (2-) 40 (2+) 0
Winter and spring oats (6 t/ha)
Phosphate (P2O5) 115 85 55 0
Potash (K2O) 165 135 105 (2-) 75 (2+) 0

Magnesium

At Mg Index 0, magnesium fertiliser should be applied every 3-4 years at 50 to 100 kg MgO/ha (see page 42).

Sulphur

Cereals are becoming more responsive to sulphur as atmospheric deposition of sulphur declines. See page 43 for more details and a map showing current deposition and areas where deficiency could occur. Where deficiency has been recognised or is expected, 25-50 kg SO3/ha as a sulphate-containing fertiliser should be applied in early spring before the start of stem extension.

Sulphur deficiency can be diagnosed by analysing whole plant shoots sampled at stem extension. A value of less than 0.2% S in dry-matter indicates deficiency. At this stage of growth there is little opportunity to correct any deficiency, but identifying deficiency allows remedial action to be taken for subsequent crops.

 

 

 

Don’t forget to:

make allowance for nutrients applied in organic manures (see Section 2)

ensure the phosphate and potash offtake is balanced by application on Index 2 soils and check that the soil is maintained at Index 2 by soil sampling every 3-5 years

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