The Nitrogen Economy of the Soil - Technical Version

Your Soil May Already Be Releasing £140/ha of Nitrogen Each Year

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Most soils already contain several thousand kilograms of nitrogen per hectare, yet modern farming still relies heavily on fertiliser nitrogen produced through energy-intensive industrial processes and global supply chains. Nitrogen fertiliser markets have started moving again, driven by energy prices, geopolitics and tightening global supply. That raises an important question for farmers: How dependent should crop production be on imported nitrogen? As fertiliser prices move again, conversations about nitrogen are becoming more common across the farming community. Some of the most useful discussions are happening not in advertisements or marketing material, but between farmers comparing results, sharing observations and asking simple questions about how crops actually access nitrogen in the soil. When input prices become volatile, practical agronomy tends to move back into focus. This article examines where crop nitrogen ultimately comes from and why biological processes in the soil are becoming increasingly important.

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Version 1 – The Nitrogen Economy of the Soil (Technical)

Why fertiliser prices rise, and why soil nitrogen deserves more attention

Recent fertiliser market reports suggest ammonium nitrate (34.5% N) has moved back towards the £450–£500 per tonne range in the UK, with urea not far behind. The drivers are familiar: rising energy prices, geopolitical instability affecting shipping routes and continuing export restrictions from major producing countries.

Modern nitrogen fertiliser is produced through the Haber–Bosch process, which converts atmospheric nitrogen into ammonia using natural gas and high pressure. This process remains one of the most important industrial technologies ever developed, but it also means fertiliser production is tightly linked to global energy markets.

When gas prices rise, fertiliser prices tend to follow.

For farmers this creates a recurring challenge. Nitrogen fertiliser has become one of the most expensive and volatile inputs on the farm, yet crop nutrition strategies are still often built around the assumption that fertiliser is the primary source of nitrogen for the crop.

In reality, agricultural soils already contain large reserves of nitrogen, most of which are stored within soil organic matter. Understanding the scale of this nitrogen reserve helps put fertiliser inputs into perspective.

To illustrate this, it is useful to convert fertiliser prices into the value of nitrogen itself.

Using a mid-range fertiliser price of £480 per tonne, ammonium nitrate contains 34.5 percent nitrogen. Each tonne therefore supplies approximately 345 kilograms of nitrogen, giving a nitrogen price of roughly £1.39 per kilogram of nitrogen.

This figure provides a useful benchmark for comparing fertiliser nitrogen with the nitrogen already stored within soils.

Most soil organic matter contains approximately five percent nitrogen.

A hectare of soil to a depth of around twenty centimetres typically weighs in the region of 2.6 million kilograms. If that soil contains four percent organic matter, the organic matter pool equates to roughly 104,000 kilograms per hectare.

With organic matter containing around five percent nitrogen, the nitrogen reserve within that organic fraction is approximately:

5,200 kilograms of nitrogen per hectare.

This nitrogen is not immediately available to crops because it is bound within organic compounds that must be broken down by microbial activity. However, these reserves represent a very large store of fertility sitting within the soil profile.

Each year a small proportion of this organic nitrogen becomes available to crops through microbial decomposition, a process known as mineralisation.

Under typical temperate conditions between one and three percent of organic nitrogen may mineralise annually, depending on soil temperature, moisture, cultivation practices and biological activity.

If we assume a mid-range mineralisation rate of two percent, the nitrogen released each year from that organic matter pool would be approximately:

104 kilograms per hectare.

In fertiliser terms, using the previously calculated nitrogen price of £1.39 per kilogram, this represents roughly:

£144 per hectare of nitrogen supplied directly by the soil before any fertiliser is applied.

This calculation highlights an important point.

A significant proportion of crop nitrogen demand is already being supplied by the soil itself, with fertiliser often used to supplement this biological system rather than replace it entirely.

In many fields the soil may already be supplying as much nitrogen to the crop each year as the fertiliser spreader does.

Yet fertiliser nitrogen also suffers from an efficiency problem. In cereal crops, nitrogen use efficiency typically ranges between 50 and 70 percent. Loss pathways such as nitrate leaching, ammonia volatilisation, denitrification and microbial immobilisation mean that not all applied nitrogen is captured by the crop.

Improving the efficiency of nitrogen use therefore becomes just as important as the total quantity applied.

This is where soil biology begins to play an increasingly important role.

Certain soil microbes are capable of fixing atmospheric nitrogen using enzymes such as nitrogenase. In non-legume crops such as cereals this form of biological nitrogen fixation is usually modest, often estimated between 5 and 40 kilograms of nitrogen per hectare, depending on soil conditions and microbial activity.

While this contribution alone cannot replace fertiliser inputs, it represents an additional nitrogen flow entering the crop system.

More importantly, microbial populations drive the processes responsible for organic matter decomposition and nutrient cycling.

The most active biological zone within the soil is the rhizosphere, the narrow region surrounding plant roots. Roots release sugars, amino acids and organic compounds that stimulate microbial populations, often creating microbial densities ten to one hundred times greater than those found in bulk soil.

These microbial communities play a central role in breaking down organic matter and releasing nutrients in forms that plant roots can absorb.

One criticism sometimes raised about microbial systems is the relatively small quantity of microbial material applied per hectare.

However, microbes do not behave like fertiliser inputs.

Microbial populations expand through reproduction. Under favourable conditions bacterial cells divide rapidly through binary fission, allowing populations to increase exponentially.

If a relatively small inoculation of one million bacterial cells establishes around the root system, ten cycles of cell division would produce more than one billion cells.

The objective of microbial inoculants is therefore not to dominate the entire soil mass, which would be impossible, but to establish functional populations in the rhizosphere where plant roots and microbial activity interact most intensely.

For this reason the role of biological systems is often better understood in terms of nitrogen efficiency rather than nitrogen replacement.

Fertiliser nitrogen will remain an essential component of modern agriculture. However, the economics of nitrogen increasingly depend on how efficiently crops utilise the nitrogen already present within the soil system.

Most agricultural soils contain thousands of kilograms of nitrogen per hectare locked within organic matter. The real challenge is not whether nitrogen exists in the soil, but how efficiently it is released and captured by the crop during the growing season.

Increasingly, farmers are beginning to view nitrogen not simply as a bagged input, but as part of a wider system involving soil organic matter, microbial activity and plant roots.

Bio-N has now been used on thousands of hectares across the UK to support nitrogen efficiency in both arable and grassland systems. By strengthening the biological processes that release and recycle nitrogen in the rhizosphere, it helps crops make better use of the nitrogen already present within the soil.

Many growers are now using Bio-N as part of their nitrogen strategy to reduce fertiliser dependence without compromising crop performance.

In an era of volatile fertiliser prices, improving the efficiency of the nitrogen already present in the soil may prove just as valuable as the nitrogen delivered through the fertiliser spreader.

Steve Holloway

Technical Manager