Farm level nitrogen use estimates methodology

More detailed methodology and quality information about data collected in the FBS and its headline measure of income, namely Farm Business Income (FBI), is available in Scottish farm business income: annual estimates: methodology.

The ‘Scottish agriculture greenhouse gas emissions and nitrogen use’ publication reports on the following headline metrics of nitrogen use:

• Nitrogen balance (kg N surplus/ha)
• Nitrogen use efficiency (NUE, %)

Nitrogen balance is the difference between total nitrogen input and ouput. A higher balance indicates less efficient use of nitrogen. It provides an estimate of the size of the nitrogen surplus not being captured in agricultural products that is potentially available for losses.

NUE is the ratio of nitrogen outputs to inputs. It indicates the proportion of nitrogen used in the farm system. This measure allows for better comparison across farms. NUE values should always be interpreted in relation to nitrogen surpluses and nitrogen outputs.

A higher NUE typically indicates a more efficient use of nitrogen but very high values may indicate unsustainable “soil mining”. The best range of NUE values depends on the type of farming activity as well as environmental conditions, livestock types and feed types.

Estimate methodology follows guidance from the EU Nitrogen Expert Panel for assessing nitrogen at farm level.

This method assumes standard quantities for the nitrogen content of inputs and outputs for each year. The amount of nitrogen is estimated for all farm inputs and outputs up to farm gate, where possible.

Total nitrogen input is estimated as the nitrogen in operating resources and feed ( in kg N per hectare per year). It includes estimates for nitrogen in:

• fertiliser
• imported feed and fodder
• biological fixation from peas and beans
• atmospheric deposition
• seed and planting material
• bedding material
• imported manure
• irrigation water
• animal manure, compost and sewage sludge

Limited information is available about the use of clover for biological fixation of nitrogen. As such, organic farms are excluded from estimates. Organic farms make up around 5% of the Farm Business Survey sample.

Limited information is available about farm grown feed and fodder and no estimate is included.

Seed quantity for the purpose of estimating a nitrogen input is calculated by assuming sowing rates follow best practice for crops.

Limited information is available about type of fertiliser, farm stocks of slurry, farmyard manure and compost, or application methods. This limits the quality of the nitrogen input estimates and understanding of the potential for nitrogen leaching. Manure estimates are based on the average number of animals on farm over the year and no attempt is made to account for changes in manure stocks.

Total nitrogen output is estimated as the nitrogen in produce exported from farm (kg N / farmed area (ha)) (kg N/ha/yr). It includes estimates for nitrogen in:

• crop products
• livestock sold
• livestock products

The method does not take into account the changing status of nitrogen in soils over time.

Crops products, including fodder, are included where these are for sale. There are occasionally large sales of fodder which result in high NUE estimates for some farms.