# How to calculate cost-effective N fertiliser rates

The economic response in crop yield to nitrogen far outweighs the fertiliser cost up to the “break-even point”, which is the point at which one additional kilogramme of nutrient is just paid for by the value of the extra yield it produces.

To calculate the break-even ratio, divide the cost of N by the crop value in p/kg.

The lower the ratio the more nitrogen is economically justified, but there are environmental pressures to reduce nitrate losses to water and so inputs: Nitrate Vulnerable Zones account for a large proportion of England already, and could increase in the future to as much as 90% or 100%. There is also the Water Framework Directive.

The current Defra NVZ rules imposes a crop N requirement limit (Nmax), amongst many other things.

As the yield-determining nutrient on most farms, adequate, but not excessive, amounts of nitrogen are needed to sustain economic yields. How do you do that?

## Estimating the amount of fertiliser N required

AHDB Nutrient Management Guide (RB209) – is the basic reference. It describes two methods to calculate the fertiliser nitrogen requirement.

Both are based around an estimation of the plant-available supply of nitrogen from a soil, Soil Nitrogen Supply (SNS).

Estimating SNS is not easy because it is affected by many factors, especially winter rainfall (leading to nitrate leaching), soil type (clay soils may retain more nitrate than sandy soils) and soil organic matter from which nitrate can be released by soil microbes. The guide allows for all these.

### Field Assessment Method:

1. Determine the average annual rainfall or actual excess winter rainfall and select the appropriate SNS Index table: low, moderate or high.
2. Identify the main soil type in the field and locate that in the SNS Index table.
3. Know the previous crop to identify the SNS Index from the table.

### SMN Analysis method

1. Measure the soil mineral nitrogen (SMN) – the amount of nitrate-N plus ammonium-N in the top 90 cm of soil.
2. Add to the SMN the amount of nitrogen in the crop at the time of soil sampling, (mainly for autumn sown crops) and an estimate of the mineralisable soil nitrogen. The total, in kg N/ha, is the SNS.
3. Read off from the top of any of the SNS tables the SNS Index.

For both methods: Obtain the fertiliser nitrogen requirement from the soil type and SNS Index in the appropriate crop table.

Remember to make full allowance for nitrogen in manures. Increase the SNS index by 1 or 2 if manures have been applied regularly for several years.

## What if you need more N?

• The recommendations in the Nutrient Managment Guide are based on average responses. You might justifiably need more nitrogen.
• To justify that, if challenged, you need a good paper trail.
• Keep individual field yield records year-on-year (to show above average potential) and soil analysis data every 4-5 years (to show no risk of limiting soil fertility).

## Estimating soil N supply

• Great care is needed to get a representative soil sample for SMN analysis, especially for different soil types within one field.
• Keep the sample cool but not frozen, and get it analysed by a reputable lab as soon as possible.
• Identifying the correct SNS Index and applying the right amount of nitrogen will not succeed unless other soil factors are also optimal.

## Soil structure

• Good soil structure is essential to give roots access to air and water as well as nutrients.
• Soil structure can be damaged by compaction by vehicles or “poaching” by livestock. Compacted soils are more prone to waterlogging and the loss of nitrogen by “denitrification”, in which soil microbes convert nitrate to nitrous oxide.
• Improve your soil structure and reduce your nitrous oxide emissions by conserving soil organic matter and return as much organic material as possible.

## Balanced nutrition

Achieving optimum yields is not just a matter of applying more nitrogen.

Soil factors like pH, and P, K and Mg status also have to be right. Maintaining an appropriate soil potassium level for all crops in the rotation is essential to avoid wasting money on nitrogen fertiliser and, of course, ensuring that the phosphorus index is correct.

There is also an increasing need to apply sulphur on many UK cereal crops.

Nutrient budgets or audits that estimate the inputs and outputs of the major nutrients to and from a farm are also becoming important.

The current NVZ regulations include a whole-farm manure N loading limit, and a nutrient budget will help with this.

It will also show if nutrients are being exported off the farm, which adversely affects soil fertility, or large surpluses are being built up, which is an unnecessary cost.

## Variable rate

Direct measurement of the variation in yield across a field offers the prospect of variable-rate nitrogen applications, but yield maps are confounded by many potential causes of yield variability.

Using yield maps alone to predict nitrogen requirements without measuring other potential yield-limiting factors, such as pests and diseases, may be a waste of effort and resource.

But variable-rate nitrogen applications can result in a 60% increase in the area correctly fertilised compared to a fixed-rate application.

The best management practice for nitrogen on wheat requires good all-round agronomy:

• Choose the variety best suited to the yield and quality required, e.g. for bread-making wheat.
• Determine the nitrogen fertiliser requirement using a recommendation system such as the Nutrient Management Guide or Yaraâ€™s N Plan.
• Take full account of nitrogen in manure and other biowastes.
• Time nitrogen applications to provide nitrogen when the crop is growing quickly – “canopy management” – to develop and maintain complete ground cover as quickly as possible. Avoid unnecessary autumn and early spring applications.
• Maintain a green cover as much as is practicable to retain nitrogen. Drill autumn-sown crops early and if appropriate use a cover crop before a spring-sown crop. But this must be balanced against the risk of carry-over of pests and diseases and the need for effective weed, pest and disease control.
• Make regular soil analyses for pH, P, K and Mg and foliar analyses including trace elements.
• Use lime to maintain the appropriate pH for optimum nutrient supply (6.5 for arable crops 6.0 for grassland).
• Apply fertilisers and manures evenly and well away from watercourses using a properly calibrated spreader.
• Use appropriate controls to minimise pest, disease and weeds.

## Key points

• Fertiliser must supplement nutrients available from soil
• Asses soil status carefully – take full account of manures applied
• Ensure other nutrients and soil structure are correct
• Consider yield mapping and variable rate nitrogen applications

Keep the sample cool but not frozen, and get it analysed by a reputable lab as soon as possible.

Identifying the correct SNS Index and applying the right amount of nitrogen will not succeed unless other soil factors are also optimal.

• Apply fertilisers and manures evenly and well away from watercourses using a properly calibrated spreader.
• Use appropriate controls to minimise pest, disease and weeds.

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