How to improve soil fertility with nitrogen

Nitrogen (N) is often referred to as the ‘engine’ of plant growth. It is a vital nutrient that plays a central role in improving soil fertility and boosting the productivity of both arable crops and grassland.

Understanding how nitrogen works in the soil, its impact on plant growth, and how to manage it effectively can significantly enhance the potential of your crop performance.

Nitrogen’s role in soil fertility

Nitrogen is a macronutrient that is essential for the formation of chlorophyll, amino acids, and nucleic acids— key components that drive photosynthesis, protein synthesis, and genetic material formation in plants. In practical terms, this means nitrogen is directly linked to the growth, development, and yield potential of arable crops and grasslands. Nitrogen’s role in soil fertility is multifaceted, influencing vegetative growth, biomass production, root development, and soil microbial activity

1. Promotes vegetative growth

Nitrogen encourages the growth of leaves and stems by promoting cell division and expansion. This results in a higher green area index (GAI), which enhances light interception and photosynthesis in both grass and arable crops. For example, wheat and maize show significant yield improvements when nitrogen levels are optimised, as more energy is available for grain filling.

2. Increases biomass production:

Nitrogen is critical for the production of plant biomass. Studies have shown that in cereals, nitrogen applications directly correlate with increased total biomass, which is a key determinant of final yield.¹ In grassland, AHDB research has highlighted that nitrogen application is vital for increasing the yield and quality of forage.² Nitrogen has also been shown to contribute to denser sward growth, which is crucial for pasture productivity and livestock nutrition.

3. Supports root development:

Adequate nitrogen supply supports robust root systems. Well-developed roots can explore a larger soil volume for water and nutrients, which is essential for the resilience and productivity of crops, especially under stress conditions. These deeper and more extensive roots, in crops like barley or ryegrass, can then contribute to better nutrient and water uptake, directly affecting yield.

4. Enhances soil microbial activity:

Nitrogen availability affects the activity of soil microorganisms, which play a crucial role in nutrient cycling. The nutrient stimulates the growth of soil bacteria that decompose organic matter, releasing nutrients back into the soil for plant uptake. This microbial activity also improves soil structure by creating stable soil aggregates, enhancing water retention, and reducing erosion.

Nitrogen cycling in arable and grassland soils

The nitrogen cycle is a complex process that regulates nitrogen availability in the soil. Understanding this cycle is essential for effective nitrogen management and optimal soil fertility.

The nitrogen cycle:

1. Nitrogen fixation:

• Atmospheric nitrogen (N₂) is converted into a plant-usable form, ammonia (NH₃), through biological nitrogen fixation (BNF).
• Leguminous crops, like peas and clover, host symbiotic bacteria (Rhizobium) in their root nodules, which fix atmospheric nitrogen into ammonium (NH₄⁺).

2. Nitrification:

• Ammonium in the soil is converted to nitrate (NO₃⁻) by nitrifying bacteria (Nitrosomonas and Nitrobacter).
• Nitrate is the primary form of nitrogen absorbed by most crops and used to fuel growth, due to its high mobility in the soil.
• However, this mobility makes nitrate prone to leaching, particularly in sandy soils or during heavy rainfall, which means responsible spreading practices are essential to help protect water quality.

3. Denitrification:

• In waterlogged or compacted soils, denitrifying bacteria convert nitrate back into nitrogen gas (N₂) or nitrous oxide (N₂O).
• Denitrification represents a significant loss of nitrogen from the soil, reducing its availability for crops and contributing to environmental challenges.
• Managing soil conditions to avoid waterlogging and compaction is key to minimising these losses – read our advice on how to improve soil structure

4. Mineralisation and immobilisation:

• Mineralisation is the process by which organic nitrogen in soil organic matter (SOM) is converted into ammonium.
• This process is influenced by soil temperature, moisture, and microbial activity.
• Immobilisation occurs when soil microbes take up ammonium or nitrate for their growth, temporarily making it unavailable to plants.
• Understanding these processes helps in timing nitrogen applications to match crop demand.

Nitrogen management: Balancing inputs and environmental impact

Effective nitrogen management is all about optimising crop performance while minimising environmental impacts. When considering steps to improve soil fertility, the following nitrogen management strategies are recommended as best practice:

1. Soil testing and monitoring:

 Regular soil testing is crucial to determine baseline soil nitrogen levels and adjust fertiliser applications accordingly. Tools like the Soil Nitrogen Supply (SNS) measure and the use of nitrification inhibitors can help tailor nitrogen applications to specific crop needs and soil conditions.

Learn how soil testing can help you make more informed decisions on nutrient management

2. Use of legumes and cover crops:

Integrating legumes into your rotation can help fix atmospheric nitrogen, while cover crops, like clover or mustard, capture residual soil nitrogen, preventing leaching and improving soil organic matter when incorporated back into the soil. Green manures are crops grown specifically to be incorporated into the soil, helping to create a balanced nutritional growing environment when used alongside nitrogen fertilisers.

Read more detail on the different roles of cover crops and green manures

Find out more about cover cropping options available within SFI for arable farmers

3. Split applications:

 Splitting applications of a nitrogen-based fertiliser, such as Nutramon, throughout the growing season can improve nitrogen use efficiency (NUE). This approach matches nitrogen supply with crop demand, reducing the risk of leaching and volatilisation losses. For instance, applying Nutramon at key growth stages, like tillering and booting in cereals, ensures the crop has sufficient nitrogen when it is most needed for grain filling.

4. Precision farming techniques:

Technologies, such as NDVI (Normalized Difference Vegetation Index) sensors and GPS-guided variable rate application, allow for more precise nitrogen management. These tools enable farmers to apply nitrogen where it is most needed, reducing waste and environmental impact. Research shows that precision application can reduce nitrogen use by 10-20% while maintaining or even improving yields.³

Learn more about the precision farming options available within SFI for arable farmers

References

1. Sylvester-Bradley, R., & Kindred, D. (2009). The Yield Response of Winter Wheat to Nitrogen Fertilizer. The Journal of Agricultural Science, 147(3), 235-246. doi:10.1017/S002185960900829X
2. AHDB (2024) – cover crops to improve soil fertility. https://ahdb.org.uk/knowledge-library/cover-crops-to-improve-soil-fertility
3. Mulla, D.J. (2013). Twenty-five years of precision agriculture research: Where do we go from here? Agronomy Journal, 105(6), 2317-2325. doi:10.2134/agronj2013.0507