Nitrogen

Nitrogen is required by plants in comparatively high quantities for their growth and development. It is present in the soil in various forms and is subject to a number of transformation processes. Nitrogen plays a key role as a component of proteins.

Nitrogen is contained in the soil in various forms and is exposed to various transformation processes. These determine whether the nutrient can be taken up directly by the plants and used for their growth, only becomes available at a later stage or is lost via leaching or gaseous losses.

• Plant uptake: The majority of nitrogen uptake by plants occurs with nitrate ions (NO3^-) dissolved in soil water. Ammonium (NH4⁺) is also available for uptake by plants, but is less mobile than nitrate and forms exchangeable bonds with soil particles (predominantly clay minerals and humus).
• Mineralization: Much of the nitrogen in the soil is bound to organic matter (humus), for example in the form of proteins or their decomposition products. The nitrogen contained therein is degraded by microorganisms during mineralization into the plant-available forms ammonium and nitrate.
• Leaching: The nitrate ions in soil water are very mobile and can be easily washed out. Particularly during the winter months, when high precipitation occurs and evapotranspiration is absent, nitrate losses occur due to translocation to deeper soil layers.
• Nitrification: Nitrosomas and Nitrobacter bacteria oxidize ammonium in nitrification first to nitrite and then to nitrate.
• Ammonia losses: Gaseous nitrogen losses can occur during fertilization with organic substances such as liquid manure or fermentation substrate, but also during improper application of urea. Depending on the conditions (application method, temperature, and humidity), part of the applied nitrogen escapes in the form of ammonia (NH₃). This is especially true when the soil pH is high.
• Denitrification: On slightly acidic soils, denitrification sets in when waterlogging occurs. In this process, bacteria take up oxygen from the nitrate ions to ensure their oxygen supply. Molecular nitrogen (N₂) and various gaseous compounds are formed. This results in nitrogen losses to the air and, for example, in the degradation to nitrous oxide (N₂O), also in a climate-relevant release of trace gases.
• Immobilization: Nitrate and ammonium available in soil water are taken up by microorganisms and incorporated as components of their physical proteins. This often occurs during the incorporation of crop residues with a wide C:N ratio, such as cereal straw. Only after remineralization, this nitrogen becomes available to the plants again.
• Nitrogen fixation: Certain microorganisms enable the utilization of molecular nitrogen from the air (N₂) by reducing it to ammonium ions and incorporating it into endogenous protein. Primarily, the symbiosis of nodule bacteria (rhizobia) on the roots of legumes such as peas, clover or alfalfa plays a role here.

Aiming for a high nitrogen efficiency

Using nutrients as efficiently as possible is crucial for agriculture worldwide. At present, only about 40 percent of the nitrogen applied is actually absorbed by the plants. Nutrient losses on this scale not only harm the environment, but also reduce the efficiency of cultivation. Legal requirements such as the new Fertilizer Ordinance further increase the pressure to use nitrogen and other nutrients efficiently.

Balanced fertilization makes an important contribution to optimization. Potassium and magnesium in particular ensure good nitrogen uptake and optimum nitrogen utilization in the plant.

Plants require comparatively high quantities of nitrogen for their growth and development.

Functions of nitrogen in the plant: 

• Nitrogen is a component of amino acids, from which proteins are formed. Therefore, nitrogen fertilization according to requirements provides for a high protein content of the harvested crop.
• Nitrogen is a component of chlorophyll and is therefore important for photosynthesis.
• Nitrogen is a component of enzymes performing important tasks in the plant's metabolism.
• Nitrogen is also contained in nucleic acids (DNA, RNA).

Nitrogen deficiency occurs primarily in soils which are sandy, low in humus or acidic, or after heavy winter rains.

• Insufficient nitrogen retards plant growth.
• Crops exhibit a light green to yellow-green coloring.
• Chlorosis occurs, first visible in older leaves, as nitrogen is mobilized in the plant and transported to the centers of growth. This is a key distinguishing feature from sulfur deficiency, in which chlorosis appears first in younger leaves.
• The chlorosis begins at the leaf tips, often along the leaf veins.
• Plants which are deficient in nitrogen typically exhibit a rigid appearance in stalks and leaves. They are upright and close to the stalk. Phosphorous deficiency, in contrast, is characterised by slightly bent leaf tips.