ARCHIVE: Fertiliser Manual (RB209)

The measurement method

This method is particularly appropriate where the SNS is likely to be large and uncertain (see page 29 and Appendix 2). This includes fields with a history of organic manure application and vegetable rotations where the timing of residue incorporation can strongly affect Soil Mineral Nitrogen (SMN) for the following crop. Nitrogen residues also can be large following outdoor pigs. The SNS Index can be identified using the results of direct measurement of (SMN) to 90 cm depth or maximum rooting depth in shallow soils over rock. The crop nitrogen content (at the time of soil sampling) and an estimate of net mineralisable nitrogen must be added to the SMN result when calculating the SNS. The Measurement Method is not recommended for peat soils where net mineralisation can be very large and uncertain and the measured SMN may be a relatively small component of SNS. For these soils, the Field Assessment Method or local experience will be better guides to SNS.

Do not confuse SNS (Soil Nitrogen Supply) and SMN (Soil Mineral Nitrogen).

SMN is the measured amount of mineral nitrogen (nitrate-N plus ammonium-N) in the soil profile.

SNS = SMN (0-90 cm or maximum rooting depth in shallow soils over rock) + crop N (at time of sampling for SMN) + estimate of available N from mineralisation of organic matter.

The Measurement Method does NOT take account of the available nitrogen supplied from organic manures applied after the date of soil sampling for SMN. The available nitrogen from manures applied after sampling should be calculated separately using the information in Section 2, and deducted from the nitrogen rate shown in the appropriate recommendation table. The nitrogen contribution from manures applied before sampling for SMN will be largely taken account of in the measured value and should not be calculated separately.

When using the Measurement Method there are four steps to follow:

Step 1. Measure SMN
Step 2. Estimate nitrogen already in the crop.
Step 3. Make an allowance for net mineralisable nitrogen
Step 4. Identify SNS Index

In detail these four steps are:

Step 1. Measure SMN

Sampling the soil to 90cm depth is difficult to do manually, especially as a minimum of 15-20 soil cores per field (based on 10 ha) will be needed to obtain a representative sample. Information on sampling and analysis for SMN is in Appendix 2. Note samples should not be frozen but cooled and maintained at less than 5°C until analysed.

Where SMN is measured to only 30 or 45 cm soil depths for shallow rooted crops (for example lettuce, onions and other salad crops), use of the analysis results will underestimate the SNS Index which is based on SMN in the 0-90 cm soil depth. A decision support system such as WELL_N can be used to help interpretation of these soil nitrogen measurements. Alternatively, estimate SMN to 90cm depth by assuming a uniform concentration of mineral nitrogen for all soil layers. Further information is in Section 5 Vegetables and bulbs.

Analysis for SMN is the best method for measuring nitrogen residues following grassland but is not recommended during the first year after ploughing out. See page 29 for more details and Appendix 2 for sampling guidelines.

Step 2. Estimate nitrogen already in the crop

Where a crop is present when SMN is measured, the amount of nitrogen already taken up must be estimated. For cereals, this is often a small though important component of the SNS but for oilseed rape, it can be as large as SMN.

The crop nitrogen content in cereals can be assessed according to the number of shoots present (main shoots and tillers), as follows:

Shoot number/m2 Crop nitrogen content (kg N/ha)
500 5-15
1000 15-30
1500 25-50

Use the smaller crop nitrogen content of the range shown when assessing crops in late autumn and the larger crop nitrogen content for crops in early spring.

In oilseed rape, the crop contains around 50 kg N/ha for every unit of Green Area Index (GAI). Alternatively, the nitrogen content of an average density crop can be assessed by measuring the average crop height.

Crop height (cm) Crop nitrogen content (kg N/ha)
10 35-45
15 55-65
20 75-85

Add the estimate of nitrogen in the crop to the measured SMN.

Step 3 Make an adjustment for net mineralisable nitrogen

Nitrogen mineralised from soil organic matter and crop debris after soil sampling is a potentially important source of nitrogen for crop uptake. However, in mineral soils of low to average organic matter content (less than about 10%), the amount of net mineralisable nitrogen will be small and for practical purposes, no adjustment is needed when using the recommendations in this Manual.

For field vegetable crops allowances are made in the recommendation tables for future mineralisation depending on planting and harvesting dates, see the field vegetables section.

An adjustment may be needed where soil organic matter content is above average or where there has been a history of regular manure applications. Adjustments can be made on the basis of a measurement of the topsoil organic matter content, or data from a laboratory anaerobic incubation or from agronomic factors using a computer model. As a guide where measurement is not done, a soil that has a topsoil organic matter content of 10% may release 60-90 kg/ha more potentially available nitrogen than an equivalent soil with 3%organic matter content. However, some soils with an organic matter content of around 10% may release little nitrogen and local knowledge must be used in estimating mineralisable nitrogen.

Add any adjustment for net mineralisable nitrogen to the total of SMN and nitrogen in the crop to give SNS.

Step 4. Identify the SNS Index

SNS = SMN + N in crop + net mineralisable N (kg N/ha) SNS Index
Less than 60


81 - 80


81 - 100


101 - 120


121 - 160


161 - 240


More than 240


Example 1

Spring barley (feed) is to be grown on a light sand soil following sugar beet. Annual rainfall is 650 mm. There have been no organic manures applied or grass grown in the last 5 years.

Select Table B (SNS Indices for moderate rainfall areas). On a light sand soil following sugar beet, the SNS Index is 0. Refer to the spring barley recommendation table on page 111 which gives a recommendation of 110 kg N/ha.


Example 2

Sugar beet is grown on a medium soil after winter wheat. 30 m3/ha of pig slurry (4% DM) was applied in February and incorporated into the soil within 6 hours. Although the average annual rainfall is 650 mm, in an unusually dry winter the excess winter rainfall was found to be 100 mm.

Since the winter was dry, select Table A (SNS Indices for low rainfall areas). On a medium soil after winter wheat, the SNS Index is 1. Refer to the sugar beet recommendation table on page 125 which gives a recommendation of 120 kg N/ha.

Since the pig slurry was applied after harvest of the last crop, its nitrogen contribution must be calculated separately. This manure application provides 65 kg/ha of available nitrogen that is equivalent to inorganic nitrogen fertiliser (see Section 2).

120 – 65 = 55 kg N/ha as fertiliser should be applied.


Example 3

Winter wheat is grown on a medium textured, low organic matter soil after potatoes which received some FYM. Annual rainfall is 750 mm. The soil is sampled in early February and analysed for SMN.

The analysis report shows that the SMN (0-90cm) is 115 kg N/ha and the crop nitrogen content is estimated to be 25 kg N/ha (see Appendix 2). Because the soil contains little organic matter, no extra allowance is made for net mineralisable nitrogen. The SNS is therefore 140 kg N/ha. Refer to any of the SNS Index tables which show that the SNS Index is 4. Refer to the winter wheat recommendation table on page 105, which gives a recommendation of 120 kg N/ha for a medium soil.


Example 4

Winter barley is to be sown following a 3 year pure grass ley which has been managed in the last 2 years using 280 kg/ha/year total nitrogen. An average application of slurry has been applied in early spring each year before taking one cut of silage followed by grazing. The soil is a medium soil in a moderate rainfall area.

The previous grass management is classed as ‘High N’. Using Table D2 for medium soils (see page 94), select the category ‘3-5 year leys, High N, grazed’. The SNS Index appropriate for the winter barley crop is Index 3. The SNS Indices for the next two crops following the winter barley are Index 3 and Index 2 respectively.


Example 5

Winter wheat is to be sown following spring barley that followed a 2 year grazed ley which has been managed using 300 kg/ha/year total nitrogen. The soil is a deep clay in a high rainfall area.

Using Table C, the SNS Index would be 1. Using Table D4, the previous grass management is classed as ‘High N’ and grazed. The SNS Index from this Table is 2. The higher of these two Indices from Tables C and D4 is 2 and this should be used for the recommendation tables.

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