Plant

Nitrogen is an essential nutrient for plant growth, development and reproduction. Providing other factors are non-limiting, nitrogen usually produces the greatest yield response of all nutrients, as well as the highest return on investment.

It promotes rapid vegetative growth and gives plants a healthy green colour largely due to its presence within chlorophyll – the compound by which plants photosynthesise.

Nitrogen is also a significant component of DNA and RNA – the genetic material which allows cells to grow and reproduce.

It contributes to the formation of plants proteins. Where crop quality is based on protein content (E.g. milling wheat, livestock forage), ensuring adequate rates of nitrogen are applied is of particular importance.

The ability of a plant to use nitrogen effectively is dependent on the presence of several other nutrients. For example, sulphur can significantly improve nitrogen use efficiency and can also help to build plant proteins, therefore growers should aim to apply 1kg sulphur per 12kg of nitrogen (or 1kg SO₃ per 5kg N) to maintain an optimum N:S ratio.

Soil

There are two forms of nitrogen within soil; organic nitrogen and mineral nitrogen.

Organic nitrogen: This form of nitrogen is unavailable for plant uptake and accounts for around 99% of total soil nitrogen. Bacteria within the soil can convert organic nitrogen into plant available forms in a process called mineralisation. This process requires aerobic conditions and only occurs when soil temperatures reach around 4ºC. The higher the soil temperature, the faster the rate of mineralisation.

Mineral nitrogen: Forms of nitrogen within the soil which are available for plant uptake including ammonium (NH₄⁺) and nitrate (NO₃^-). Because of its negative charge, nitrate is unable to bind to clay particles within the soil and so is most at risk of leaching, however it is also most available for crop uptake. Ammonium is slightly less readily available for crop uptake in comparison to nitrate, but is less prone to leaching and denitrification and is therefore considered the most stable from of mineral nitrogen. 

Plants deficient in nitrogen appear weak and their growth is often stunted. Older leaves tend to turn light green or yellow.

As nitrogen is mobile in the plant and moves to where it is most needed, older and lower leaves are always first to show deficiency symptoms.

Nitrogen deficiency should not be confused with sulphur deficiency, which is seen first in the younger leaves.

[deficiency pics to appear down RHS]

Where is the risk of deficiency highest?

• Light, sandy soils
• Areas of high rainfall
• Soils with low pH

Testing

Soil Mineral Nitrogen (SMN) tests can help determine the quantity of soil nitrate and ammonium which is likely to be available for uptake and can be used tailor fertiliser plans to prevent deficiencies within the season. 

Tissue testing is also useful in determining the nitrogen concentration of plants mid-season in order to compare against optimum levels. Although laboratory results may be available too late to correct the deficiency in the current crop, they can be useful for decisions on nitrogen use for future crops.

Testing the nitrogen concentration within the grain or seed at harvest gives an indication of whether the nitrogen policy was successful or whether any adjustments need to be made for the following season. 

On applying nitrogen to the soil surface, typically 50-60% is taken up by the growing crop within the season of application. So what happens to the rest of the nitrogen which has been applied?

Some nitrogen remains within the soil profile, but in forms which are unavailable for plant uptake. However, a proportion of the nitrogen applied is at risk of being lost to the environment via the following methods…

Nitrate leaching

Nitrate is soluble in the soil solution and, unlike ammonium, is not held on soil particles. Once the soil is fully wetted, nitrate may leach into field drains or subsurface aquifers as drainage water moves through the soil. The amount of winter rainfall has an important influence on the amount of nitrate leached.

Under normal conditions, ammonium-N in the soil is rapidly converted to nitrate by bacteria in a process called nitrification. Therefore, sources of ammonium-N will have a similar risk of leaching as sources containing nitrate when used in excess of the requirement of a crop.

Denitrification

In anaerobic soils (poorly aerated soils lacking oxygen), nitrate can be denitrified and lost to the atmosphere as the gases nitrous oxide, a greenhouse gas, and nitrogen (N₂).

Denitrification is a biological process and is most significant in wet and warm soils where there is a supply of nitrate after harvest or where there has been a recent nitrogen application and there is enough organic matter for the microbes to feed on. Some nitrous oxide is formed during nitrification of ammonium-N to nitrate-N and some of this also can be lost to the atmosphere.

Ammonia Volatilisation

Nitrogen may be lost to the atmosphere as ammonia gas. Significant losses commonly occur from livestock housing, livestock grazing and where organic manures are applied to fields and are not immediately incorporated by cultivation.

There can also be significantly larger losses of ammonia when urea is applied to a growing crop compared to losses when other forms of nitrogen fertiliser, such as ammonium nitrate, or inhibitor treated urea, are used.

Right Product

Where soil pH is high (7 or above) or where nitrogen is likely to be applied in warm, dry conditions the use of unprotected urea is likely to lead to higher nitrogen losses and so products such as protected urea (OEN or Sustain) or nitrate-based products should be considered.

Alternatively, those in high rainfall areas or on sandy soils should be cautious when applying ammonium nitrate due to the associated leaching risk. A dual action inhibitor such as OEN can help prevent both nitrate leaching and ammonia volatilisation losses.

Right Time

In general, nitrogen applications should be limited to the spring/summer when crops are actively growing and when the leaching risk is lower. However, there are exceptions for some crops such as oilseed rape (OSR) which have an autumn nitrogen requirement.

Right Place

Even application of nitrogen is essential for uniform crop growth, to maximise use efficiency and minimise the risk of environmental losses. Well maintained and regularly calibrated fertilisers spreaders, together with high quality fertilisers, will help ensure even nutrient distribution.

Right Rate

Apply nitrogen little and often to maximise crop performance, particularly on light, sandy soils. Consult with your local nutrition agronomist or RB209 to ensure applications do not exceed crop requirement or NVZ regulations, but that nitrogen applications are based on the economic optimum.