How to improve nitrogen use efficiency
Nitrogen (N) is a crucial nutrient for crop growth, yet it’s also one of the most challenging to manage effectively. This macronutrient exists in multiple forms in the soil and environment, each with varying degrees of mobility and susceptibility to loss. Unlike other nutrients such as phosphorus or potassium, nitrogen is highly dynamic, undergoing several transformations (e.g., mineralisation, nitrification, volatilisation, and leaching) that can result in nutrient losses, inefficiencies for growers, and environmental consequences.
Taking steps to improve nitrogen use efficiency (NUE) can mitigate these issues, optimising crop yields, improving livestock nutrition, and reducing environmental impacts. This guide sets out numerous methods for improving NUE on UK farms, providing recommendations based on AHDB guidelines.
Understanding nitrogen use efficiency (NUE)
NUE is a critical metric in agronomy that measures how effectively a farming system utilises the nitrogen inputs applied through fertilisers, manure, and other sources. Essentially, NUE is the ratio of the amount of nitrogen taken up and used by crops (yield) to the total nitrogen applied. It provides an indication of how much of the applied nitrogen contributes to productive growth versus how much is lost to the environment.
Calculating NUE on your farm involves using the formula:
When interviewed in Farmers Weekly, independent crop adviser, Allison Grundy recommended that farmers should aim for 70-80% NUE. If NUE values are higher than this, it brings an increased risk of soil-mining, depleting the soil’s supply of mineral nitrogen. However, an NUE value below this threshold – especially if it drops below 50% – indicates a nitrogen surplus. This raises the risk of nitrogen leaching and surface run-off, posing significant issues to water quality and the loss of valuable nutrients. By calculating and monitoring NUE using the following methods, farmers can identify inefficiencies and make informed decisions about nitrogen management.
1. Soil testing and monitoring
Soil testing is the cornerstone of precision nutrient management. Understanding soil characteristics, such as pH, organic matter content, and nutrient levels, allows farmers to tailor nitrogen applications to crop requirements.
Testing frequency: It is recommended to conduct regular soil testing every 3-5 years to keep track of soil nutrient status and make adjustments to fertiliser applications. For accurate nitrogen management, soil organic matter testing is particularly important as it determines the soil’s ability to supply nitrogen through mineralisation.
Soil pH management: Maintaining optimal soil pH (6.5-7.0) enhances nitrogen availability by facilitating microbial activity and root nutrient uptake. If soil tests indicate a low pH, liming can improve NUE by creating favourable conditions for both nitrogen mineralisation and crop growth.
2. Optimal fertiliser application timing
Timing is crucial to optimise nitrogen management. Nitrogen should be applied when crops have the highest demand to minimise losses and ensure maximum uptake. Splitting nitrogen applications and matching them with critical growth stages is a proven strategy for improving NUE.
Split applications: AHDB’s Nutrient Management Guide (RB209) endorses the use of split fertiliser applications during key growth stages (e.g., tillering and stem elongation for cereals) to help match nitrogen supply with crop demand. This can also help reduce the risk of nutrient loss and environmental damage through leaching or volatilisation.
Controlled release fertilisers: Using controlled-release fertilisers can provide a steady nitrogen supply to the crop, reducing the likelihood of losses to the environment. According to research conducted by Map of Ag and Harper Adams University, slow-release fertilisers can increase NUE by ensuring that nitrogen remains available to the plant over an extended period, particularly in wet or sandy soils where leaching is more prevalent.
3. Precision farming techniques
Precision farming techniques, including soil mapping, remote sensing, and variable rate technology (VRT), allow farmers to apply nitrogen more accurately based on real-time field conditions. This helps to improve the NUE ratio.
Variable rate technology (VRT): VRT enables the application of nitrogen fertilisers at variable rates across a field, accounting for differences in soil texture, organic matter, and historical management practices – which can all affect nutrient use efficiency. By using soil maps, sensors, and application maps, VRT can potentially increase NUE by up to 20%, AHDB research shows.
NDVI and soil sensing: Remote sensing technologies like NDVI (Normalized Difference Vegetation Index) imagery and soil sensors provide data on crop health and soil nitrogen status. These tools enable farmers to apply nitrogen more precisely, reducing the risk of over-application and nitrogen loss.
4. Incorporating legumes and cover crops
Legumes and cover crops are effective tools for enhancing soil nitrogen content and improving NUE. Legumes, such as clover and beans, fix atmospheric nitrogen through symbiotic bacteria in their root nodules, adding organic nitrogen to the soil.
Cover crops: Planting cover crops such as mustard, vetch, or rye during fallow periods can capture residual nitrogen in the soil, reducing the risk of leaching. Research carried out by Harper Adams University and Map of Ag highlights that cover crops can decrease nitrate leaching by up to 30-50% while improving soil structure and organic matter content, contributing to long-term soil health and resilience.
5. Efficient manure management
Manure is a valuable source of organic nitrogen, and efficient application is key to maximising NUE.
Incorporation methods: Farmyard manure and slurry can be more effective for improving crop nutrient uptake when applied using techniques like slurry injection or band spreading. These methods reduce ammonia volatilisation and enhance nitrogen availability to crops. For best results, manure application should be timed with crop growth stages to maximise nitrogen uptake.
Regular testing: It is recommended to test manure and slurry for their nutrient content to tailor application rates accurately. By knowing the nitrogen content in manure, farmers can avoid over-application, reducing the risk of nitrogen losses through runoff and leaching.
6. Improving grassland management
Livestock farms can also benefit from improved nitrogen management on grassland. For example, strategic grazing practices, reseeding with nitrogen-efficient grass varieties, and optimising fertiliser applications can significantly enhance pasture NUE.
Rotational grazing: Implementing rotational grazing systems promotes uniform grass utilisation and nitrogen uptake from the soil. This helps to combat overgrazing, which can lead to uneven nitrogen distribution and runoff, and undergrazing, which results in wasted nitrogen. A controlled rotational system ensures pastures have enough recovery time, enhancing nitrogen capture by the plants.
High-sugar grasses: Reseeding pastures with high-sugar grass varieties can improve nitrogen use efficiency in livestock systems. These grasses enhance protein utilisation in the rumen of cattle and sheep, resulting in lower nitrogen excretion and reduced environmental impact.
Further advice on making the most of your fertiliser applications
For further guidance on how to maximise nutrient uptake in your crops, we recommend reading the following blogs on our website: