With tightening farm nitrogen (N) limits, now is the time to consider steps to reduce N losses to water or air while maximising the return from all farm N sources to sustain productivity levels.
A new factsheet is now available with five simple steps to improving nitrogen use efficiency (NUE).
The following five steps form the basis of improving the utilisation of N from soil, manures and fertilisers on grassland and tillage farms and reducing N losses to either water or air while maintaining farm productivity and profitability.
1. Correct soil pH
The primary step for improving N use efficiency is soil pH, as it is a key regulator of soil N supply. On grassland mineral soils maintain a soil pH of 6.3 to 6.5, peat soils pH of 5.5 to 5.8 and tillage soils pH of 6.5 to 6.8.
Check soil test results, identify fields with a lime requirement, plan and organise lime applications over the coming weeks and months. Correcting a mineral grassland at soil pH 5.5 to soil pH 6.3 will increase nitrogen use efficiency by up to 16% providing the opportunity to tailor N rates and reduce farm costs.
2. Optimise soil P and K fertility
On intensive grassland and tillage farms aim to optimise soil P and K Indices to Index 3 to maximise the utilisation of each kilo of N applied. Develop a farm fertiliser plan and consult on a regular basis for recommended N levels on a field-by-field basis. Organic manures are a valuable source of P and K and help replenish soil levels. Target organic manures to silage fields to balance farm soil P and K levels.
Correcting soil P & K fertility from Index 1 to 3 improves NUE by 12%.
3. Account for soil nitrogen supply
Irish soils have a great capacity to supply N during the growing season ranging from 70 to 250kgN/ha, depending on soil type.
Adjust fertiliser N rates depending on soil type; for example a Moorepark-type soil will supply ~ 130kgN/ha/yr while a heavier soil type such as Johnstown Castle soil will supply up to 200kgN/ha/yr. For tillage crops adjust N applied based on previous cropping history for example after beans/peas reduce N by 30kgN/ha on the following cereal crop. Reduce fertiliser N applications on clover swards to maximise clover sward productivity. On grassland soils NUE can be increased by up to 25% while on tillage soils up to 19% depending on soil type.
4. Utilise organic fertiliser resources
Target organic manures where available to replenish soil P, K and organic matter levels.
Apply organic fertilisers in springtime with LESS techniques to maximise N recovery and reduce ammonia emissions.
Applying cattle slurry to intensively cut silage fields helps replenish soil organic matter/carbon levels with the added benefit of increasing grass yields by up to 1.5t DM/ha/yr.
Applying cattle slurry at a rate of 3,000gals/ac will reduce crop N requirements by 26%.
5. Maintain sulphur balance
A key nutrient in improving farm N use efficiency and reducing N losses is supplying recommended levels of S during the growing season.
Grass yield responses to S have been measured across all soil types with lighter soil types most responsive. Aim to apply S on a little and often basis from April to July.
Apply 15kg S/ha on grazing swards and 10 to 20kg S/ha for grass silage fields. Sulphur increases NUE between 14 and 30% on poorly to well-drained soil types.
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