ARCHIVE: Fertiliser Manual (RB209)

Assessing the Soil Nitrogen Supply (SNS)

For the purposes of using this Manual, the Soil Nitrogen Supply is defined as follows:

“The Soil Nitrogen Supply (SNS) is the amount of nitrogen (kg N/ha) in the soil (apart from that applied for the crop in manufactured fertilisers and manures) that is available for uptake by the crop throughout its entire life, taking account of nitrogen losses.”

The SNS is different to, but includes Soil Mineral Nitrogen (SMN). The calculation of SNS must include three separate components of nitrogen supply as follows.

Soil Nitrogen Supply (SNS) = SMN + estimate of nitrogen already in the crop + estimate of mineralisable soil nitrogen

Where:

  • Soil Mineral Nitrogen (kg N/ha is the nitrate-N plus ammonium-N content of the soil within the potential rooting depth of the crop, allowing for nitrogen losses.
  • Nitrogen already in the crop (kg N/ha) is the total content of nitrogen in the crop when the soil is sampled for SMN.
  • Mineralisable soil nitrogen (kg N/ha) is the estimated amount of nitrogen which becomes available for crop uptake from mineralisation of soil organic matter and crop debris during the growing season after sampling for SMN.

The SNS depends on a range of factors which commonly vary from field to field and from season to season. It is therefore important to assess the SNS for each field each year. The key factors influencing SNS are:

  • Nitrogen residues left in the soil from fertiliser applied for the previous crop

  • Nitrogen residues from any organic manure applied for the previous crop and in previous seasons.

  • Soil type and soil organic matter content.

  • Losses of nitrogen by leaching and other processes (the amount of winter rainfall is important).

  • Nitrogen made available for crop uptake from mineralisation of soil organic matter and crop debris during the growing season.

Mineral nitrogen residues after harvest 

The management and performance of a crop can have a significant effect on the amount of residual mineral nitrogen (nitrate-N and ammonium-N) in the soil at harvest. Residues are likely to be small if the amount of nitrogen applied matched crop demand in high yielding years or where the amount of nitrogen applied was less than that required by the crop. The residues may be larger than average when yields are unusually small due to disease or drought. Residues following cereals are generally lower than those following break crops.

Well-established cover crops, such as mustard, forage rape or Phacelia, sown after harvest can take up significant amounts of soil mineral nitrogen and reduce the risk of nitrate leaching over winter. Generally, the earlier the cover crop can be established, the more mineral nitrogen will be taken up. Following destruction of the cover crop, this nitrogen will be gradually mineralised over many years. However, the amount becoming available for uptake by the next crop is relatively small and difficult to predict. Where cover crops have been used regularly, soil analysis can be a useful technique to help estimate the overall supply of soil mineral nitrogen.

 

 

Effect of excess winter rainfall

The amount of nitrate leached will depend on the quantity in the soil when the water content reaches field capacity and through-drainage starts, the soil type and the amount of water draining through the soil (the excess winter rainfall).

The excess winter rainfall is the actual rainfall between the time when the soil profile becomes fully wetted in the autumn (field capacity) and the end of drainage in the spring, less evapotranspiration during this period (i.e. water lost through the growing crop). In England and Wales, typical total evapo-transpiration between October and February inclusive is around 50mm with a further 28mm in March. The Met Office can provide estimates of excess winter rainfall in different locations and for different soil/cropping situations.  

Excess winter rainfall (mm) = Rainfall between the time a soil reaches field capacity and the end of drainage – evapo-transpiration

 

Light sand soils and some shallow soils can be described as ‘leaky’. Nitrate in these soils following harvest is fully leached in an average winter even where substantial residues are present in the autumn. The SNS Index is nearly always 0 or 1 and is independent of previous cropping except in low rainfall areas or after dry winters.

Deep clay and silt soils can be described as ‘retentive’. The leaching process is much slower and more of the nitrate residues in autumn will be available for crop uptake in the following spring. Differences in excess winter rainfall will have a large effect on SNS in these soils. Low levels of SNS (Index 0 and 1) are less frequent than on sandy soils. Other mineral soil types are intermediate between these two extremes.

Because of both regional and seasonal differences, separate SNS Index tables are given for three different rainfall situations (see Section 3).

  1. Up to 600 mm annual rainfall (up to150 mm excess winter rainfall)
  2. 600-700 mm annual rainfall (150-250 mm excess winter rainfall)
  3. Over 700 mm annual rainfall (over 250 mm excess winter rainfall)
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