ARCHIVE: Fertiliser Manual (RB209)

Allowing for the nutrient content of livestock manures

Nitrogen

Estimates of the percentage of the total nitrogen content that is available for uptake by the next crop from different manure applications are given in the tables on page 63 (FYM), page 64 (poultry manures), page 66 (cattle slurries and dirty water) and page 68 (pig slurries). The tables take account of differences in readily available-N and dry matter content, the effects of application method and timing, soil type and autumn/winter rainfall following application. The footnotes should be used to adjust the values where appropriate.

Where more detailed field specific guidance on the fertiliser nitrogen value of manures is required, use of the MANNER-NPK (MANure Nutrient Evaluation Routine) or PLANET decision support systems is recommended. MANNER-NPK/PLANET will predict the fertiliser nitrogen value of field applied manures, taking into account the manure type, manure analysis data (total N, ammonium-N, nitrate-N and uric acid-N), soil type, application timing and technique, ammonia-N, nitrate-N and denitrification losses, and the mineralisation of organic-N.

Where there is uncertainty about the level of residual nitrogen present in the soil, for example, where manures have been applied regularly or at unknown application rates, soil sampling to measure soil mineral nitrogen (SMN) is recommended (see page 26).

Phosphate, potash, magnesium and sulphur

Manures are valuable sources of other nutrients as well as nitrogen, although not all of the total nutrient content is available for the next crop. Typical values for the total and available phosphate and potash contents of farm manures are given in the tables between pages 63 and 69. Nutrients which are not immediately available will mostly become available over a period of years and will usually be accounted for when soil analysis is carried out. The availability of manure phosphate to the next crop grown (50-60%) is lower than from water-soluble phosphate fertilisers. However, around 90% of manure potash is readily available for crop uptake.

Where crop responses to phosphate or potash are expected (e.g. soil Indices 0 or 1 for combinable crops and grassland) or where responsive crops are grown (e.g. potatoes or vegetables), the available phosphate and potash content of the manure should be used when calculating the nutrient contribution. Soils at Index 0 will particularly benefit from manure applications. Where the soil is at target Index or above (usually Index 2 or above, see page 37) for phosphate or potash, the total phosphate and potash content of the manure should be used in nutrient balance sheet calculations. For most arable crops, typical manure application rates can supply the phosphate and potash requirement. At soil P Index 3 or above, take care to ensure that total phosphate inputs do not exceed the amounts removed in crops during the rotation. This will avoid the soil P Index reaching an unnecessarily high level. It is important to manage manure applications to supply phosphate and potash for the crop rotation.

Manures also supply useful quantities of sulphur and magnesium, but there is only limited data on availability for the next crop grown. Sulphur and magnesium inputs from manures should largely be regarded as contributing to the maintenance of soil reserves.

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