Nitrogen is the most critical element obtained by plants from the soil and is a bottleneck in plant growth. Plants can use the nitrogen as either the ammonium cation ammonium (NH4+) or the anion nitrate (NO3–). Nitrogen is seldom missing in the soil, but is often in the form of raw organic material which cannot be used directly.
|Clover, green sweet
|Clover, mature sweet
|Humus in warm cultivated soils
|Legumes (alfalfa or clover), mature
Some micro-organisms are able to metabolise the organic matter and release ammonium in a process called mineralisation. Others take free ammonium and oxidise it to nitrate. Particular bacteria are capable of metabolising N2 into the form of nitrate in a process called nitrogen fixation. Both ammonium and nitrate can be lost from the soil by incorporation into the microbes’ living cells, where it is temporarily immobilised or sequestered. Nitrate may also be lost from the soil when bacteria metabolise it to the gases N2 and N2O. In that gaseous form, nitrogen escapes to the atmosphere in a process called denitrification. Nitrogen may also be leached from the soil if it is in the form of nitrate or lost to the atmosphere as ammonia due to a chemical reaction of ammonium with alkaline soil by way of a process called volatilisation. Ammonium may also be sequestered in clay by fixation. Nitrogen is added to soil by rainfall.