The Role of Fertilizers: the nitrogen cycle

In the middle of the 1800's, Justis von Liebig (1803 - 1873) analyzed plant material for its chemical components. He found that while there were many different substances present, phosphorus, potassium, and in particular nitrogen were mainly responsible for the growth of plants. These three  N, P and K  are the numbers you see when you read the label on a bag of fertilizer, and indicate the percentage by mass of the element in the fertilizer. For example, 33-0-0 is ammonium nitrate, which means that it contains 33% nitrogen, and no phosphorus or potassium.



NH 4 NO 3 2 (N) = 2(14.01)

4 (H) = 4(1.008)

3 (O) = 3(16.00) Total (g/mol) 28.02

4.03

48.00

======

80.05 28.02/80.05x100 = 35%

All plant and animal material contains nitrogen, for it is a part of chlorophyll, amino acids, proteins, enzymes and nucleic acids (RNA and DNA). While there is a great amount of nitrogen in the earth's atmosphere, virtually no plants are able to use it directly. Instead, they absorb nitrates (NO 3 -) from the soil throughout their roots. Plants that grow on nitrogen deficient soil will be yellow, since nitrogen is needed for development of chlorophyll.

In the natural nitrogen cycle, small quantities of nitrates are produced directly by lightning storms (at the high temperature of a lightning strike, N 2 and O 2 react to form nitrogen oxides (NO x ) which react with water to form nitric acid). However, most nitrates are produced by nitrifying bacteria which live in nodules on the roots of legumes such as peas, beans, lentils, clovers and vetches. Animal excrement, and the dead bodies of plants and animals are recycled by decomposing bacteria to form ammonium (NH 4 +). Other bacteria convert ammonium into nitrates.

In addition to the nitrates which are taken up by plants to promote growth, nitrates are removed from the cycle by leaching as water moves through the soil, which puts them below root level; runoff into streams, lakes and other surface waters; and by denitrifying bacteria which reduce the nitrates to nitrites (NO 2 -) and elemental nitrogen (N 2 ).

Because it is critical for plant  and ultimately animal  growth, nitrogen is a limiting reagent and usually a scarce commodity in a natural environment. However, man has introduced very large quantities of nitrates into the environment in the form of nitrates or anhydrous ammonia used as fertilizer. Another human source of nitrate is acid rain. At high temperatures  in internal combustion engines for example  the same reaction as occurs in lightning takes place: N 2 + x O 2 + energy 2NO x which then turns into nitric and nitrous acids on reaction with water.

There is absolutely no doubt that without artificial nitrogen fertilizers, almost all of which are produced from the Haber-Bosch process, the majority of the world's population would starve. This graph shows the increase in fertilizer use, and and in food production over the last 30 years. During the same time period the world's population has almost doubled.

This has not been without environmental cost. The enormous amounts of fertilizer being used cause changes in soil salinity, and composition. Runoff of nitrates into water systems promotes the growth of algal blooms, which destroy water quality. It seems that the Haber-Bosch process has led us into an interesting dilemma:

the use of nitrogen based fertilizers allows us to grow enough food to feed most of the world's population

higher amounts of food and better nutrition increase fecundity (the rate at which humans conceive and bear children)

better pre-natal and post-natal nutrition decrease infant mortality, therefore the world's population increases

to feed the increasing population we must use more and more fertilizer, which is making the world's environmental condition deteriorate

Should we continue to use fertilizers which may destroy the earth, or should we let the children of the earth starve to death?