ARCHIVE: Fertiliser Manual (RB209)

Potatoes – Phosphate, Potash and Magnesium

The amounts of phosphate and potash shown at Index 2 are those recommended to achieve a total yield of 50 t/ha. The phosphate recommendations are intended to achieve optimum yield and should not be adjusted even if larger or smaller yields than 50 t/ha are expected. The potash recommendation at target or lower Indices can be adjusted when yield is likely to be larger or smaller than 50 t/ha by multiplying the difference in expected yield by the potash content per tonne yield given in Appendix 5. For example, at K Index 1, the potash recommendation for an expected yield of 70 t/ha is 330 + (20 x 5.8) = 446 kg K2O/ha.

Crops grown on soil at Index 0 and 1 would be expected to respond to the extra amounts of phosphate, potash and magnesium shown in the table below.

  P, K or Mg Index
  0 1 2 3 4 and higher
Phosphate (P2O5) 250 210 170 100 0
Potash (K2O) 360 330 300 150 0
Magnesium (MgO) 120 80 40 0 0


The amount of phosphate recommended for soils at P Index 2 or 3 is more than sufficient to replace the phosphate removed by a 50 t/ha crop (about 50 kg P2O5). The surplus phosphate will help to maintain the soil at a target P Index 2 for an arable crop rotation and should be allowed for when assessing the need for phosphate of one or more following crops. On soils at P Index 0 and 1 the surplus phosphate will help increase the soil P Index and no allowance should be made when deciding the phosphate requirement of a subsequent crop. On soils at P Index 2 or below a large proportion of the phosphate should be water-soluble.

The amount of potash recommended at K Index 2 will only replace the amount removed by a 50 t/ha crop and potash should be applied for the next crop in the rotation to maintain the soil at K Index 2. The extra amounts of potash shown for K Index 0 and 1 soils will slowly increase the soil K Index.

Timing of application

All the phosphate should be applied in the spring and either worked into the seedbed or placed at planting.

Where more than 300 kg K2O/ha is required, apply half in late autumn/winter and half in spring. On light sandy soils, all the potash fertiliser should be applied after ploughing and no sooner than late winter. Large amounts of potash can sometimes reduce tuber dry matter content. Where this occurs the decrease may be smaller when muriate of potash (MOP, potassium chloride) is replaced by sulphate of potash (SOP, potassium sulphate).

These recommendations should be used for both bed and ridge and furrow systems. Where fertiliser is placed, a small reduction in the recommended rate of phosphate and potash could be considered.

Don’t forget to:

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

ensure the potash offtake is balanced by application of potash fertiliser on Index 2 soils, and check that the soil is maintained at Index 2 for both phosphate and potash by soil sampling every 3 – 5 years

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