ARCHIVE: Fertiliser Manual (RB209)

Finding the phosphate, potash and magnesium recommendations

Phosphate, potash and magnesium recommendations are based on achieving and maintaining target soil indices for each nutrient in the soil throughout the crop rotation. Soil analysis should be done every 3-5 years. The use of soil analysis as a basis for making fertiliser decisions is described on page 29, and the procedure for taking soil samples in Appendix 3.

The phosphate and potash recommendations shown at Index 2 and 2- respectively are those required to replace the offtake in the yield shown (apart from potatoes where the phosphate recommendation at Index 2 is greater than offtake). The recommendation should be increased or decreased where yields are substantially more or less than this. The amount to apply can be calculated using the expected yield and values for the offtake of phosphate and potash per tonne of yield given in Appendix 5. The larger recommended applications for soils at Index 0
and 1 will also bring the soil to Index 2 over a number of years.

Recommendations are appropriate where the phosphate or potash balance for preceding crops have been close to neutral. Adjustments can be made where the balance for the preceding crop was significantly positive or negative. For example, potatoes can leave a positive phosphate balance so that less than the normally recommended amount might be needed by the following crop. On the other hand, a phosphate or potash ‘holiday’ can result in a need for greater than normally recommended amounts for following crops.

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

Example 1

Soil analysis shows P Index 2 and K Index 1. The next crop to be grown is spring barley, the expected grain yield is 6t/ha and the straw will be baled and removed from the field.

The table on page 114 recommends 50 kg P2O5/ha and 100 kg K2O/ha.

Other important points to consider when using the recommendation tables are:

  • All recommendations are given for the mid-point of each Index. For some crops, there are different recommendations depending on whether the soil is in the lower half (2-) or upper half (2+) of K Index 2.
  • Where a soil analysis value (as given by the laboratory) is close to the range of an adjacent Index, the recommendation may be reduced or increased slightly taking account of the recommendation given for the adjacent Index. Small adjustments of less than 10 kg/ha are generally not justified.

Example 2

Soil K analysis is 65 mg/litre which is at the low end of K Index 1 (range 61 and 120 mg/litre K – see Appendix 4). Winter wheat is to be grown, expected grain yield is 8t/ha and straw will be removed.

The table on page 114 shows a recommendation of 115 kg K2O/ha for winter wheat (straw removed) at soil K Index 1. This recommendation is for a soil K analysis value of 90 mg/litre, the mid-point of K Index 1. Because the soil is at the bottom of Index 1, it would be more appropriate to apply 130 kg K2O/ha, a value between that for K Index 0 and 1.

  • Where more or less phosphate and potash are applied than suggested in the tables adjustments can be made later in the rotation.

Example 3

Soil analysis shows P Index 2 and K Index 2, and main crop potatoes are to be grown and expected yield is 65 t/ha tubers.

The Table on page 123 recommends 170 kg P2O5/ha and 300 kg K2O/ha for a crop yielding 50 t/ha. Using Appendix 5, a crop yielding 65 t/ha will remove:

Phosphate:       65 x 1.0 = 65 kg P2O5/ha
Potash:            65 x 5.8 = 377 kg K2O/ha

For phosphate, the recommendation is much larger than the offtake because potatoes are likely to respond to extra phosphate at P Index 2. The surplus phosphate, 105 kg P2O5/ha (170 – 65), should be allowed for when deciding on the phosphate application for the next crop(s) grown in the rotation.

For potash, the recommendation is less than the offtake and the deficit (377 – 300) should be made good later in the crop rotation.

  • Where organic manure is applied it is important to calculate the quantity of each plant available nutrient added in the manure (see Section 2) and adjust the amount of fertiliser by this amount. Where organic manures are applied frequently it is essential to calculate the immediate and residual plant-available amount of each nutrient and adjust the fertiliser applied appropriately. Allowing for the nutrients in manure reduces the need for fertiliser, improves farm profits and reduces the risk of nutrient pollution of water.
  • Construct a nutrient balance sheet for each field and ensure that the phosphate and potash offtake is balanced by an equivalent application of phosphate and potash on Index 2 soils and check that the soil is maintained at Index 2 by soil analysis every 3-5 years.
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