Understanding Nitrogen

Author: Rory Geldard

17th June 2021

NRM

Agriculture

Sources and soil processes

Nitrogen is the key yield building nutrient for all crops, but excess levels can be detrimental to the natural environment. Understanding how nitrogen reacts in the soil and is used by crops will help farmers and growers increase profitability by using nitrogen more effectively in crop and forage production.

Sources of nitrogen

1. Mineralisation of nitrogen from soil organic matter

If adequate oxygen is available, when soils are moist and the temperature is above 5°C microbes in the soil will break down organic matter, which contains nitrogen. The AHDB Nutrient Management Guide shows the likely nitrogen availability from soil organic matter, and for more accurate information NRM analyses samples for levels of nitrate and ammonium in the soil profile. Understanding the soil nitrogen supply is essential to manage the total nitrogen required by crops.

2. Cover crops and crop residues

These can range from high nitrogen material typified by brassica leaves to highly lignified straw from cereal crops.

3. Organic manures

Depending on the type, organic manures can have a significant effect on nitrogen supply to crops. The Nutrient Management Guide provides average data but, to utilise manures most effectively, analysis of specific batches with NRM’s manure and slurry packages will provide more accurate results.

4. Fertilisers

Fertilisers help to balance crop nitrogen requirements after all other nitrogen supplies have been accounted for.

Nitrogen loss from soils

There are three key risk areas that determine how and why nitrogen is lost from soils.

1. Leaching

Nitrate (NO₃) is highly mobile in soils and at risk of leaching (being lost in excess water draining away) when supply exceeds crop uptake requirements.

2. Volatilisation

This occurs when ammonium ions revert to ammonia (NH₃) and are lost to the atmosphere. In addition, in waterlogged conditions, microbes searching for oxygen break down nitrate, releasing nitrous oxide (N₂O) into the atmosphere.

3. Run Off

When it rains after manures are applied to steep or wet land, the organic nitrogen can run into water courses. The UK’s Nitrate Vulnerable Zones (NVZ) manure management rules are designed to minimise this risk.

Nitrogen fertilisers

There are two main types of nitrogen fertiliser: ammonium nitrate and urea.

When ammonium nitrate is applied, it dissolves with very little moisture, with the nitrate readily available for crop uptake.

Urea, which is highly soluble, must be converted into ammonium, which can then be held in the soil. In warm conditions, this conversion process can be very rapid. To slow the process, urease inhibitors are used, reducing ammonia losses to the atmosphere.

Soil nitrogen supply

The most accurate way to determine soil nitrogen supply is to analyse soils to assess the level of mineralised nitrogen. This, combined with the nitrogen content of the growing crop and an estimate of excess nitrogen from the soil, provides a more accurate nitrogen supply figure for crop nutrition and nitrogen planning for each field.

The function of nitrogen in plants and crop requirements

Functions of nitrogen in plants

Nitrogen is one of the key components in chlorophyll formation in the leaf and stem. Chlorophyll is essential to facilitate photosynthesis as it captures carbon dioxide and produces the necessary carbohydrates for successful crop growth. However, excess nitrogen drives further plant growth, leading to large canopies and more chance of lodging, which prevents the plant from utilising radiant energy from the sun as efficiently.

Nitrogen is also a key component of amino acids, the building blocks of proteins. The total amount of nitrogen required by the plant will depend on the required protein content of the final produce.

Crop requirements

Roughly two-thirds of the total nitrogen taken up by cereals is removed in the grain. For feed wheat, the optimal grain nitrogen content is approximately 1.9% of the dry matter, while milling wheat requires a nitrogen content of 2.2-2.3%. Where nitrogen is being utilised efficiently, this will be satisfied by applying an extra 50 kilograms of nitrogen, but inefficient utilisation could mean as much as an extra 70 kilograms per hectare is needed for milling wheat.

Example

For the amount of nitrogen required by a feed wheat crop with a yield potential of 10 t/ha (tonnes per hectare):

10 tonne yield at 15% moisture = 8.5 tonnes dry matter x 1.9% nitrogen = 160 kilograms of nitrogen removed per hectare.

This therefore requires 240 kilograms per hectare of nitrogen uptake by the crop.

The total available nitrogen needed from all sources depends on nitrogen use efficiency.

Nitrogen uptake can be monitored throughout the season with the use of plant tissue analysis. As crops bulk up, total nitrogen concentrations decline, but total nitrogen content continues to rise until crop maturity.

Guideline nitrogen contents of the latest expanded leaf by crop:

Winter barley for feed              3.5%

Winter wheat for feed              4%

Winter wheat for milling          4-4.5%

Winter oilseed rape                              5%

For many other crops, such as potatoes and sugar beet, boosting leaf nitrogen content above optimum is undesirable as excess nitrogen leads to large canopies which are inefficient in utilising sunlight. Detailed guidance for each crop is provided in the Nutrient Management Guide.

Managing crop canopies

Canopy size is calculated by determining the square metre of leaf in each square metre of soil to give the leaf or green area index (GAI).

In wheat crops at full flag leaf, the target GAI is around 6. Throughout May, the key growing month due to the warmer weather, there should be one GAI unit increase every nine days. Fertiliser programs should be designed to support this rapid growth.

In winter oilseed rape crops, a target GAI of 3.5 is needed by the start of flowering. Final yield is dependent on the upper leaves photosynthesising effectively. Without an adequate nitrogen supply, yield will be more dependent on the less efficient pods and stems. The AHDB growth guides for individual crops, found in their online Knowledge Library, provide benchmarks by growth stages.

Planned fertiliser applications should be fine-tuned during the growing season to optimise canopy development, achieve optimum GAI targets, and improve nitrogen use efficiency. Sending samples to NRM Laboratories for nitrogen analysis is the most accurate way to monitor soil nitrogen levels and crop nitrogen accumulation while simultaneously monitoring all other crop nutrients. Contact your agronomist or NRM directly to arrange for soil or plant samples to be analysed.