Nitrogen Management

Nitrogen is one of the most important essential elements for plants and is required in comparatively large amounts. Successful nitrogen management can optimize crop yields and increase profitability while minimizing nitrogen losses to the environment. However, nitrogen management is unique and might be a complex task.

Nitrogen disorders in plants

Nitrogen deficiencies might result in stunted growth, chlorotic leaves and significantly reduced yield.

Excess of nitrogen might result in poor root system, soft tissue, delay in harvestable products, low quality yield and higher susceptibility to disease and pests.

Nitrogen is mobile within the plant and, therefore, deficiency symptoms are expressed on older leaves.

Nitrogen deficiency symptoms

Nitrogen sources and available forms

Nitrogen behavior is complex and it is determined by many physical, chemical and biological processes. These processes are very much affected by environmental factors.

Natural nitrogen is present mainly in air and soil.

Atmospheric nitrogen - The atmospheric nitrogen is a major reservoir of nitrogen, but it is unavailable to most plants. Only legume plants can use atmospheric nitrogen in biological processes that involve bacteria. Small amounts of nitrogen are deposited by rain.

Soil nitrogen – most of the nitrogen in soil is contained in organic matter. The organic matter is relatively stable and it is not directly available to plants.

Plants can absorb nitrogen only in its inorganic forms, NO3 (nitrate) and NH4 (ammonium). Only about 2-3% of the nitrogen in the organic matter becomes available to plants per year, in a process called "mineralization".

This process involves bacteria that convert organic nitrogen to mineral nitrogen, which is available to plants.  The mineralization process is influenced by environmental factors, such as temperature, moisture, aeration, and soil pH.  

For example, excess moisture limits the availability of nitrogen and slows down the mineralization. Mineralization is optimal at 30C and at neutral to slightly acidic pH.

Nitrogen Losses

Nitrogen might be lost from the soil and, therefore, become unavailable for plants, in several ways:

1. Leaching – nitrate (NO3) easily moves downward along with water, as it is not held by soil. As a result it might be washed out below the root zone, with the flow of water.

2. Volatilization – nitrogen is lost as an ammonia (NH3) gas. This might happen when fertilizers containing urea are surface-applied.

3. Denitrification – nitrate-nitrogen (N-NO3) is converted back, by bacteria, into nitrogen gas, that is lost into the air. This process occurs when the soil is saturated or very wet.

Nitrogen Management

Successful nitrogen management can optimize crop yields and increase profitability while minimizing nitrogen losses to the environment.

Timing - One of the main challenges in deciding on a nitrogen fertility program is the timing of the application. In fertigation systems, the best practice would be to apply frequent small applications, at rates that meet the crop requirements.

In less intensive crops, like cereals and grains, where only a few fertilizer applications are made, timing of nitrogen application is critical.

Applying nitrogen too early holds the risk of losing it through leaching, before the crop takes it up, especially if rains are to come. The common approach, in such cases is to split the nitrogen application, where most of the nitrogen fertilizer just before the crop's maximum demand for nitrogen.

However, there is a risk of applying the nitrogen fertilizer "too late", if logistic or weather conditions do not allow applying it when planned.  

Determining nitrogen application rates - Nitrogen goes through of the quick and constant changes between its different forms and it is highly mobile in the soil. As a result, testing soil nitrogen gives a reading that is valid only the same moment of testing, and might lead to erroneous recommendations for nitrogen application.  

Therefore, the common approach is to give nitrogen recommendations based on yield goal and the nitrogen uptake of the crop.

Nitrogen credits, due to organic matter in the soil and residues of previous crops, should be also taken into account when making nitrogen fertilizer rate recommendations.

New methodologies and approaches for testing soil nitrogen are currently being developed and evaluated.
 

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