Nitrogen in the soil
Nitrogen is present in the soil in various forms and is subject to a number of transformation processes. These determine whether the nutrient is absorbed directly by the plants and used for their growth, whether it becomes available only at a later point or whether it is lost by leaching or in gaseous form.
- Plant uptake: The greatest part of nitrogen uptake in plants occurs with nitrate ions (NO3-) dissolved in groundwater. Ammonium ions (NH4+) are also available for uptake by plants but are less mobile than nitrate and participate in exchangeable bonding with particles in the soil (primarily clay minerals and humus).
- Mineralisation: A large part of the nitrogen in the soil is bound to organic substance (humus), as protein or its degradation products, for example. The nitrogen contained therein is broken down by microorganisms as part of the mineralisation process into plant-available ammonium and nitrate forms.
- Leaching: Nitrate ions in groundwater are very mobile and can be leached out very easily. Particularly in winter, when high precipitation occurs and there is little evapotranspiration, nitrogen losses occur by extraction into deeper soil layers.
- Nitrification: During nitrification, Nitrosomonas and Nitrobacter bacteria oxidise ammonium first to nitrite and then to nitrate.
- Ammonia losses: Gaseous nitrogen losses can occur during fertilisation with organic materials like liquid manure or fermentation substrate as well as cases of improper urea application. Depending on conditions (method of spreading, temperature and humidity), a part of the nitrogen applied is in the form of ammonia (NH3). This is particularly the case with a high soil pH.
- Denitrification: In slightly acid soils, denitrification occurs in cases of waterlogging. Bacteria scavenge oxygen from the nitrate ions to ensure their supply of it. Molecular nitrogen (N2) and various gaseous compounds result. The consequence of this is nitrogen losses to the air and, for example in the case of degradation to nitrous oxide (N2O), a release of trace gases relevant to the climate.
- Immobilisation: In groundwater, available nitrate and ammonium are taken up by microorganisms and eliminated as part of their corporeal proteins. This occurs frequently when crop residues with a high carbon to nitrogen ratio, such as straw from grains, are worked into the soil. Only after renewed mineralisation is this nitrogen available to plants once again.
- Nitrogen fixation: Certain microorganisms enable the use of molecular nitrogen (N2) from air by reducing it to form ammonium ions and incorporating it into their corporeal proteins. Here the symbiosis of rhizobia on the roots of legumes such as peas, clover or alfalfa play a primary role.
Using nutrients as efficiently as possible is an order of the day for agriculture around the world. Currently, an average of only 40 % of the quantity applied is actually utilised by plants. Nutrient losses of this magnitude not only pollute the environment but also reduce the gains of cultivation. Add to this statutory requirement such as the new German Fertiliser Ordinance (DüV), which further increases the pressure to use nitrogen and other nutrients efficiently. Balanced application of fertilisers further contributes to this optimisation. Potassium and magnesium in particular ensure good uptake of nitrogen and its optimal utilisation in plants.
Nitrogen in plants
Plants require comparatively high quantities of nitrogen for their growth and development.
The role of nitrogen in plants
- Nitrogen is a component of amino acids, which form proteins. Thus nitrogen fertilisation at the level needed ensures a high protein content of the harvest.
- Nitrogen is a component of chlorophyll and is therefore important in photosynthesis.
- Nitrogen is a component of enzymes, which perform important tasks in plant metabolism.
- Nitrogen is also contained in nucleic acids (DNA and RNA).
Symptoms of nitrogen deficiency
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 mobilised in the plant and transported to the centers of growth. This is a key distinguishing feature from sulphur 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.
Nitrogen deficiency in wheat, corn and rapeseed (left to right)