Botany Plant Physiology Nitrogen Absorption in Plants Nitrates Nitrogen

Nitrogen is an integral part of the plant world apart from hydrogen, oxygen and carbon.  It is found freely in the atmosphere which surprisingly isn’t absorbed or imbibed by the plant.  However, plants receive their nitrogen proportions from the soil through ammonia rich compounds, uric acid and all forms or organic and inorganic matter, rich in nitrogen.

While in the plants, nitrogen combines with many organic compounds to form proteins, vitamins, chlorophyll and amino acids.  These are but some of the important nitrogen containing compounds.  The many enzymes derived from proteins have a rich supply of nitrogen and aid in numerous plant functions.  Nitrogen that is absorbed in the plants and converted to proteins also form a vital part of the plant cell protoplasm.  Seeds, which are a form of stored food, also contain nitrogen in its insoluble form.

As mentioned earlier, plants obtain their nitrogen not from the air, but from the inorganic and organic nitrogenous compounds present in the soil.  Humus, the organic constituent of the soil is rich in proteins from decaying matter.  These proteins cannot be absorbed by the plants directly but are first broken down into amino acids, amines and urea before they are absorbed.  Urea is further hydrolysed to ammonia and carbon dioxide in many cases. 

Nitrate, nitrites and ammonia compounds are the inorganic, nitrogen-rich compounds in the soil.  Nitrate nitrogen is what the plant seeks out most and it is one of the most important forms of nitrogen for the overall growth of the plant.  Ammonia is a major source of nitrates too, however, ammonia should be supplied in sparing quantities.  Large quantities of ammonia can prove toxic to the plant.  Ammonia nitrogen can work well in large quantities as long as the soil is rich in lime. 

How ammonia (in lime soil) is broken down and absorbed:

Explanation 1:  Ammonia oxidises to form nitric acid in the soil by the soil bacteria.  Lime in the soil, neutralizes the nitric acid to form calcium nitrate which can be absorbed by the plant. 

Explanation 2:  Ammonia salts consist of two radicles; basic NH4 and an acid.  The basic NH4 radicle is readily absorbed by the plant as compared to the acid radicle.  This results in an increase in the acid content in the soil which, if left by itself is dangerous to plant roots.  However, with the lime present in the soil, this remaining acid gets neutralized. 

Nitrites are another form of nitrogen present in the soil.  They do not serve much purpose to the plant except act as reserve source of nitrogen in the soil for the plant.  These nitrites are first oxidised to form nitrates before they can be absorbed by the plant roots. 

In conclusion, it can be stated that nitrates in the form of ammonium compounds, nitrates or obtained from nitrites, are the main source of nitrogen for the plants absorbed from the soil through the plant roots.  The nitrates that enter the transpiration system of the plant enter the xylem and are carried to the leaves.  The leaves are the organs of the plant responsible for the further breakdown of nitrogen especially in the formation of chlorophyll, the energy source for the plant.