How to prevent nitrogen leaching over winter
Nitrogen is a critical nutrient for crop production, but applications need to be carefully managed to help protect the environment, particularly during the winter months.
Nitrogen leaching – the process by which nitrogen, particularly in the form of nitrates, is washed out of the soil into groundwater – reaches its peak risk during the wet, cold winter season.
This not only leads to nutrient loss, negatively affecting crop yields, but also contributes to water pollution, impacting ecosystems and water quality.To help arable and livestock farmers manage nitrogen effectively during winter, it is crucial to understand how leaching occurs and implement strategies that minimise nutrient losses while protecting the environment.
Understanding the risk of nitrogen leaching in winter
Nitrogen leaching occurs when heavy rainfall infiltrates the soil, carrying soluble nitrates beyond the root zone and into deeper soil layers or groundwater. This process is exacerbated in winter when frequent rain saturates the soil, reducing its capacity to retain water and nutrients. Nitrates, highly mobile macronutrients, are easily lost in these conditions – particularly in areas with poor soil structure or inadequate drainage.
During the winter, grasslands and autumn-sown arable crops, such as winter wheat, continue to take up nitrogen, but at a much lower rate compared to the peak growing season. This lower uptake is a result of reduced metabolic activity and slower root growth as temperatures drop. In some cases, the uptake can be as low as 20-30% of what would be absorbed during warmer months. This limited nitrogen absorption, coupled with waterlogged soils, creates an ideal scenario for nitrate leaching, as the soil’s ability to hold onto nutrients is compromised.
The risk of nitrogen leaching is particularly high when:
- Nitrogen fertilisers are applied too late in the growing season or just before heavy rainfall
- Soils are compacted or poorly structured, leading to faster water movement through the soil profile
- There is a lack of cover crops or vegetation during the winter months to absorb residual nitrogen
Key strategies to manage nitrogen leaching on-farm
To mitigate nitrogen leaching over winter, there are several practices that can improve nitrogen use efficiency and protect both soil health and water quality.
1. Use cover crops to capture residual nitrogen
Cover crops are one of the most effective ways to reduce nitrogen leaching. When sown after the main crop, cover crops absorb residual nitrogen from the soil, preventing it from being washed away by winter rain. Research carried out by AHDB has shown that non-leguminous cover crops, such as brassicas, cereals and phacelia (if sown early) work best at reducing nitrate leaching.
Findings show deep-rooting crops, like fodder radish, plantain and stubble turnips, help reduce nitrogen leaching into watercourses by up to 40%.
These crops not only capture nitrogen but also improve soil structure, reduce erosion, and increase organic matter, providing long-term benefits to soil health.
Read more advice on the benefits of cover cropping and practical recommendations.
2. Consider when to apply nitrogen fertiliser
Applying nitrogen at the right time is crucial to minimising leaching. Nitrogen-based fertilisers such as Nutramon should be applied when crops are actively growing and are able to take up nitrogen, rather than during the late autumn or early winter, when uptake is low. AHDB’s Nutrient Management Guide (RB209) advocates that avoiding nitrogen applications in high-risk periods – particularly late autumn – can significantly reduce leaching losses.
Before spreading, routine soil testing is essential to determine baseline nitrogen levels. Using a tool such as the Soil Nitrogen Supply (SNS) index, farmers can tailor fertiliser applications to the specific needs of crops and prevailing soil conditions. It is also recommended to split nitrogen applications, applying a portion in early spring and the remainder later in the peak growing season when crops have higher nutrient demands.
3. Improve soil structure and drainage
Soil structure plays a key role in nitrogen retention. Poorly structured or compacted soils allow water to move quickly through the soil, increasing the risk of nitrogen leaching. In contrast, well-structured soils, with greater porosity and more stable aggregates, allow water to infiltrate and percolate more evenly. This prevents waterlogging in heavy rain and reduces the speed at which water passes through the soil, limiting the loss of soluble nutrients like nitrates. Learn how to assess your soil structure.
Adopting reduced tillage or no-till practices is proven to help improve soil structure, benefitting nutrient retention and reducing erosion. Min-till practices allow plant residues and cover crops to decompose on the soil surface, breaking down organic matter. This contributes to the formation of stable aggregates, which in turn improves the soil’s water-holding capacity and nutrient retention.
4. Manage manure and organic fertiliser applications
For farmers applying organic fertilisers to crops during the colder seasons, careful manure management is critical to reducing nitrogen leaching. Applying manure during the winter months, when soils are saturated and plants are dormant, can lead to significant nutrient losses.
So, manure and organic fertilisers should be spread during periods of low rainfall and incorporated into the soil as soon as possible to minimise surface runoff. Farmers should also be mindful of the nitrogen content in manure. Testing the nutrient levels of manure before application ensures it is applied at the right rate, avoiding excess nitrogen that could be lost to leaching.
5. How precision nutrient management tools can protect the environment
Precision farming techniques can help farmers manage nitrogen more efficiently and reduce leaching. Technologies such as variable rate application (VRA), nitrogen modelling tools, and drone imaging allow farmers to apply nitrogen where and when it is needed most with more precision than ever before. Using tools like the Nitrogen Use Efficiency (NUE) calculator can help farmers assess their current nitrogen practices and identify areas for improvement. By tailoring nitrogen applications to the specific needs of each field, farmers can reduce waste and improve nitrogen retention.
Check out more advice on how to improve nitrogen use efficiency to benefit crop performance.