How to assess your Soil Nitrogen Supply (SNS) for spring starts with understanding how much nitrogen your soils and crop can provide before any bagged or liquid fertiliser is applied. Getting SNS roughly right is what underpins sensible first-pass nitrogen rates and helps avoid both underfeeding and expensive over-application.
What SNS actually means
Soil Nitrogen Supply (SNS) is an estimate of how many kilograms of nitrogen per hectare the soil–crop system will deliver to the crop over the season without extra fertiliser. In RB209 this is converted into an index (typically 0–6) which you then plug into crop-specific tables to get a recommended total N rate.
SNS has three main components: nitrogen already in the soil as nitrate and ammonium (soil mineral N), nitrogen already taken up by the crop, and additional nitrogen that will be released through mineralisation as the season progresses. Together these give both a kg N/ha figure and an SNS index that drive your fertiliser plan.
Mineralisation and organic N
Mineralisation is the process through which soil microbes convert organic nitrogen in manures, residues, and soil organic matter into plant-available nitrate and ammonium. Warmer, moist conditions and well-structured soils with good organic matter typically increase mineralisation, lifting SNS and reducing the amount of applied N needed.
Fields with long-term manure use, regular composts or fertile grass leys often show higher SNS because of larger organic N pools that mineralise through spring and summer. Conversely, low organic matter soils or those repeatedly cropped without organic inputs tend to mineralise less N, placing them in lower SNS indices and needing more fertiliser.
Field history and previous cropping
Field history is central to the RB209 field-assessment route for SNS, which uses book values based on soil type, cropping and manuring history, and winter rainfall. Previous high N-input crops, regular slurry or FYM, or long-term grass build background fertility and push SNS upwards.
By contrast, cereals after cereals with minimal organic returns usually sit in lower SNS categories, while crops such as pulses, forage legumes or grass/clover can significantly raise SNS for the following crop. Recording recent fertiliser rates, manure applications and rotational changes every 3–5 years helps keep SNS assumptions realistic.
Soil type and rainfall effects
Soil type determines both how much nitrogen can be stored and how much is at risk of loss over winter. Light sands with low organic matter hold less mineral N and are prone to leaching, while deeper medium or heavier soils with more clay and organic matter generally retain and supply more N to the crop.
Winter rainfall then modifies this picture: high rainfall on shallow or sandy soils typically leaches nitrate down the profile, lowering SNS, while lower rainfall on deeper or heavier soils allows more over-winter N to remain available into spring. For this reason RB209 emphasises pairing soil type with local winter rainfall data when using the field assessment method.
Measurement vs book values
There are two main ways to assess SNS in spring: a measurement method based on soil mineral N (SMN) sampling to depth, combined with crop N and estimated mineralisable N, and a field assessment method using soil type, cropping and rainfall look-up tables. SMN sampling close to first N timing can give more accurate SNS values, particularly on leachable, high-rainfall sites, but costs more and needs careful sampling to depth.
For many farms, using RB209 book values for SNS index, adjusted with local knowledge on manures, residues and previous crop, is a practical compromise that still sharpens N rates compared with blanket assumptions. Whichever route is used, it must be done before any spring nitrogen is applied, otherwise fertiliser masks the true soil contribution.
Turning SNS into first-pass N rates
Once you have an SNS index, RB209 crop tables convert it to a recommended total nitrogen rate for the season. Higher SNS indices (for example on manured, heavier soils) pull total recommended N down, while low indices on light, low-organic-matter soils push total N up to meet yield potential.
First-pass rates then become a proportion of this total, timed to match crop demand and local conditions, with later splits adjusted based on crop biomass, weather and grain price. Using protected, high-analysis liquid N and N+S grades, such as Nitrasol products with AdvaNShield, helps that planned rate translate into effective crop uptake by reducing volatilisation and leaching losses from each early pass.
Are you considering the move to liquid fertiliser? Get in contact with us today to get started.
