ARCHIVE: Fertiliser Manual (RB209)
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Finding the Phosphate, Potash and Magnesium Recommendations


For phosphate, potash and magnesium, the results of a recent soil analysis will be needed showing the soil Index. The use of soil analysis as a basis for making fertiliser decisions is described in Section 1, and the procedure for taking soil samples in Appendix 3. Soil samples should be taken from every field every 3-5 years.

Recommendations are given in the tables as phosphate (P2O5), potash (K2O) and magnesium oxide (MgO). Conversion tables are given in Appendix 8.

  • Recommendations at target Indices (2 for P and 2- for K) are maintenance dressings intended to maintain soil reserves and prevent depletion of soil fertility rather than give a yield response. Recommendations at Indices lower than target include an allowance for building-up soil reserves over several years as well as meeting immediate crop requirement.
  • All recommendations are given for the mid-point of each Index (mid-point of 2- for potash).
  • Phosphate and potash recommendations are given for crops producing average yields. The recommendation may be increased or decreased where yields are substantially more or less than this yield level. These adjustments can be calculated using the values for phosphate and potash in grass (see Appendix 5).
Example:

Soil analysis shows P Index 3 and K Index 2-. A pure grass sward is to be closed up for 2 cuts of silage. No organic manures will be applied.

The table on page 211 recommends 20 kg P2O5/ha and 80 kg K2O/ha for the first cut, and nil phosphate and 90 kg K2O/ha immediately after the first cut for second cut growth. A further 80 kg K2O/ha should be applied after the second cut – see notes beneath the table.

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