ARCHIVE: Fertiliser Manual (RB209)

Oilseed rape and Linseed – Phosphate, Potash, Magnesium and Sulphur


The amounts of phosphate and potash are appropriate to the seed yields shown. For each crop in the Table the amounts of phosphate and potash shown at the target Index, P Index 2 and K Index 2-, are required to replace the offtake in the yield shown and maintain the soil at the target Index. The recommendation at target or lower Indices can be adjusted if the yields are likely to be larger or smaller than those shown by multiplying the difference in expected yield by the phosphate and potash content per tonne of seed given in Appendix 5. For example, at P Index 1, the phosphate recommendation for winter oilseed rape with an expected yield of 4.5 t/ha is 80 + (1 x 14) = 94 kg P2O5/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 period of years, this additional amount of fertiliser will help raise the soil to Index 2.

Apply phosphate and potash when convenient during the year except on Index 0 and 1 soils when it should be applied and worked into the seedbed.

  P or K Index
  0 1 2 3 and higher
  kg/ha
Winter oilseed rape (3.5 t/ha)
Phosphate (P2O5) 110 80 50 40
Potash (K2O) 100 70 40 (2-) 20 (2+) 0
Spring oilseed rape (2 t/ha) or Linseed (1.5 t/ha)
Phosphate (P2O5) 90 60 30 0
Potash (K2O) 80 50 20 (2-) 0 (2+) 0

Magnesium

At Mg Index 0 and 1, magnesium at 50 to 100 kg MgO/ha should be applied every three or four years (see page 42 for details).

Sulphur

Oilseed rape will respond to an application of sulphur on all mineral soils. Spring crops may be less susceptible to sulphur deficiency than winter crops. 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 suspected, 50-75 kg SO3/ha as a sulphate containing fertiliser should be applied in early spring.

Sulphur deficiency can be diagnosed by analysing young fully expanded leaves at early flowering stage. Critical values of less than 0.4% S in dry-matter or an N: S ratio of more than 17: 1 indicate 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. The leaf sulphate: malate ratio test can predict potential deficiency at an earlier stage of growth. Sulphur deficiency symptoms include stunting, interveinal yellowing of middle and upper leaves and pale flower petals.

Don’t forget to

make allowance for 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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