ARCHIVE: Fertiliser Manual (RB209)

Forage Maize

The amounts of phosphate and potash shown at target Index 2 are needed to replace the offtakes in a fresh yield of 40 t/ha, and to maintain the soil at the target Index. The phosphate and potash recommendations at target or lower Indices can be adjusted if yields are likely to be larger or smaller than 40 t/ha by multiplying the difference in expected yield by the phosphate and potash content per tonne of yield given in Appendix 5. For example, at P Index 1, the recommendation for an expected yield of 50 t/ha is 85 + (10 x 1.4) = 99 kg P2O5/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. Also over a number of years, these extra amounts of fertiliser will help to raise the Index level of most soils, except light sands.


SNS, P or K Index






4 and higher

Nitrogen (N) All mineral soils 150 100 50 20 0
Phosphate (P2O5) 115 85 55 20 0
Potash (K2O) 235 205 175 (2-)
145 (2+)
110 0

Using organic manures in rotations that include maize

Large quantities of organic manures are commonly applied before maize is grown. The recommendations in the table apply to crops where organic manures are not applied. The nutrients supplied from recent and past applications of organic manures must be allowed for when deciding on fertiliser application rates (see Section 2).

In an NVZ, applications of manufactured nitrogen fertiliser and organic manures must comply with all of the relevant NVZ Action Programme measures. On land that is not in an NVZ, organic manure applications should follow the recommendations in Protecting Protecting Our Water, Soil and Air: A Code of Good Agricultural Practice (see Section 9), which includes a limit of 250 kg N/ha of organic manure total nitrogen every 12 months.

Where maize is grown continuously and organic manures are also regularly used, excessively large amounts of nitrogen and phosphate can build up in the soil. This can greatly increase the risk of nitrate and phosphate transfer to watercourses. Protecting Our Water, Soil and Air: A Code of Good Agricultural Practice states that, where the soil P Index is 3 and organic manures are used, no more than maintenance rates of phosphate should be applied during the rotation.

To minimise the risk of building up a large excess of nutrient in soil, maize should be grown in rotation with other crops so that an acceptable nutrient balance is reached for the rotation. In maize rotations, extra care must be given to avoiding excessive nutrient inputs as organic manures or fertilisers.

Placement of nitrogen and phosphate

To encourage rapid early growth, all of the phosphate requirement and up to 10-15 kg/ha of the nitrogen requirement may be placed below the seed at drilling. The remainder of the nitrogen requirement should be top-dressed as soon as the crop has emerged.


Potash should be applied before seedbed preparation and thoroughly worked in.


Where sugar beet or potatoes do not feature in the rotation, magnesium fertiliser is only justified at soil Index 0 when 50 to 100 kg MgO/ha should be applied every three or four years (see page 42).

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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