The explosive known as ammonal is a mixture of T.N.T., aluminium powder, and ammonium nitrate; the function of the latter substance is to supply more oxygen to render the combustion of the carbon of T.N.T. complete.
Nitrates and the Food Supply. Chemical analysis shows that compounds of nitrogen enter largely into the composition of the living tissues of all plants and animals; hence, either nitrogen itself or some of its compounds must be assimilated by all living organisms to provide for growth and development, and to repair wastage. Air, since it contains approximately four-fifths of its volume of free nitrogen, is the most obvious source of supply. At every breath, a mixture of oxygen and nitrogen is inhaled by animals, but only part of the oxygen is used. Practically the whole of the nitrogen is returned to the atmosphere unchanged; it serves only to dilute the oxygen. From this it is clear that animals do not build up their nitrogenous constituents from elementary nitrogen.
With plants it is very much the same, for, although they obtain their principal food, namely, carbon, from the carbon dioxide which is present in air, it is only in a few exceptional cases that free nitrogen is assimilated. The exceptions will be considered first, because it was through these that we first began to learn something definite about the great importance of nitrogen in agriculture.
Virgil, who was born in 70 B.C., wrote a poem in praise of agriculture. Almost in the opening lines he deals with the treatment of corn land. He advises that, in alternate years, this should either be left fallow or sown with pulse, vetch, or lupin; but not with flax or oats, because they exhaust the land. From this we learn that rotation of crops was one of the established principles of good husbandry even at the beginning of the Christian era.
It was not until the later years of the nineteenth century that any explanation as to why rotation of crops is beneficial was put forward. Let us first state the facts more precisely. Peas, beans, vetches, clover, and other members of the natural order called Leguminosae, which includes about 7,000 species, produce fruits rich in complex nitrogen compounds without being dependent in any way upon nitrogen compounds in the soil. Moreover, they do not exhaust the land as far as these compounds are concerned; hence wheat and other grain can be grown on the same land the following year.
It is now known that leguminous plants assimilate atmospheric nitrogen with the help of certain bacteria. If anyone will dig up a lupin root, he will observe[2] conspicuous wrinkled swellings or nodules at various points on the roots. These, when examined with a high-power microscope, are found to contain colonies of bacteria. It is these minute vegetable organisms which assimilate nitrogen and pass on nitrogen compounds to the larger plant. Other plants cannot assimilate what we might call raw nitrogen; they require soluble nitrates. These they build up into complex organic nitrogen compounds suitable for the feeding of animals which can assimilate neither free nitrogen nor nitrates.
The Nitrogen Cycle. The supply of nitrates in the soil needs continually to be renewed by the addition of decaying vegetable matter, stable or farmyard manure, or Chili saltpetre. The natural manures contain organic nitrogen compounds which were built up during the life of some animal or plant. They are not immediately available as food for other plants, because they are, as it were, the end products of life, and are not soluble in water. But Nature provides for this. The manures decay, forming humus, and ultimately ammonia, one of the simplest of inorganic nitrogen compounds. Ammonia is then transformed to nitrites by certain bacteria present in the soil, while other bacteria change nitrites into nitrates. Both of these organisms are quite distinct from the root nodule bacteria of the Leguminosae.
The nitrates pass into the plant in solution, and then begins again that wonderful cycle of changes which we have described. This is perhaps made clearer by the following diagram.
Fig. 6. THE NITROGEN CYCLE