Nutrient budgets provide a framework which can be applied systematically across a range of systems and scales from single fields to whole farm systems.
Why are nutrient budgets used?
- As a tool to allow farmers and growers to make optimum use of available nutrients.
- To design and evaluate the viability and sustainability of arable and horticultural crop rotations by organic advisers.
- To assess an arable or horticultural rotation or whole farm system against organic production standards by an inspector.
- To indicate likely surpluses of N in the farm or horticultural systems and therefore risk of losses by leaching to ground and surface water, especially in Environmentally Sensitive Areas, or Nitrate Vulnerable Zones.
How do I calculate nutrient budgets?
- Calculate inputs and outputs for each nutrient which will allow any surpluses or deficits to be dealt with.
- Although there is variation, published values can be used for the nutrient content of harvested crops and organic manures. There are also values for the amount of N fixed by leguminous crops.
- There is a need to interpret results with caution as these published figures are based on averages and should therefore not be used to give exact recommendations.
- Ideally use the budgets in conjunction with soil analysis to take account of the levels of available nutrients (apart from Nitrogen) in the soil.
The balance of nutrients
This shows the flow of nutrients through the farm system.
These come mainly through N fixing bacteria in legumes, purchased inputs, rainfall and deposition from the atmosphere.
Nitrogen fixing of legumes
Insert pdf table
Take note – the actual amount of Nitrogen that is fixed is difficult to assess and the figures given are the average values. The amount of N fixed will depend on the fertility of the soil as well as the amount of N fixing by the bacteria in the legume.
Inputs in rain and dry deposition vary across the UK. Atmospheric inputs may be increased in close proximity to intensive animal production units due to increased deposition of ammonia.
Inputs of nutrients in seed
Table – inputs of nutrients in seeds
Inputs of nutrients in manures and fertilisers
Table – N, P and K in fresh manures and in fertilisers
The nutrient content of manures are very variable, depending on animal type, diet, production level, bedding, housing and manure handling.
It is good practice to analyse the nutrient content of organic manures, green waste and compost routinely.
Inputs of nutrients in bought – in feeds
Table – N, P and K in bought – in feeds
Where possible, use actual feed analysis values rather than average values.
These occur mainly through sales of crops and animal products and losses of ammonia and nitrogen through leaching and volatilisation.
Nutrient output for different crop products
Table – nutrient offtake for a crop product
Remember that actual yields will vary widely depending on the season and soil type.
The area of all crops should be recorded along with the yields of crop sold off farm. For whole farm budgets, straw should only be included if it is sold off-farm.
Nutrient outputs for animal products
Table – nutrient outputs per kg livestock product
Minimise losses of ammonia and Nitrogen from organic manure application by incorporating them into the soil within 6 hours for slurry and within 24hrs for FYM.
Calculating a nutrient budget
Example farm gate annual nutrient budget – mixed cattle and arable
- Nitrogen is often the nutrient limiting crop growth and is usually the first nutrient assessed when planning rotations.
- If P or K are low in the soil, the use of budgets should allow the planning of additional fertiliser materials.
- Ca and Mg are less important, unless potential deficiencies make them significant for example in fruit growing, glasshouse tomatoes or with livestock.
Make sure that you are consistent in using the same units (e.g. kg/ha) throughout the budget. Wherever possible use actual values rather than average values.
How to interpret the results
- IMPORTANT – soil analysis is an essential part of the process. Tests should be carried out at least once in every rotation cycle, at the same stage in the rotation and at the same time of year to help interpret results. A good time is at the beginning of the fertility building phase when nutrient levels are likely to be at their lowest.
- Interpreting must take account of the soil type including nutrient reserves, susceptibility to leaching and suitability to the planned rotation and stocking.
- Changing farm management in response to the results of nutrient budget should only be undertaken with careful advice.
- Aim for a surplus of around 30kgN/ha/yr where this is expressed for a complete rotation or whole farm system.
- N surpluses much higher than 30kg/ha may indicate a pollution risk depending on the climate and soil type.
Phosphorus and potassium
- Should show a surplus close to 0.
- Many soils in the UK show high reserves of P and K in the soil, and where this is the case it may be acceptable for a farm system to show a deficit. This should be carefully monitored.
- Combine nutrient budgeting with soil analysis to help with P and K interpretation – if there are high P and K soil indices then you can let the levels come down.
Source: Institute of Organic Training and Advice Technical Leaflet 6. A Guide To Nutrient Budgeting on Organic Farms, Dr Christine Watson, Dr Kairsty Topp and Dr Liz Stockdale.
Interested in how to build fertility in your soils and use legumes more effectively? Check out our section on legume management. Or for more information on how to minimise losses from manure applications and storage have a look at the diffuse water pollution, ammonia and greenhouse gas emissions manual for more management options.