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. [Illustration: Fig. 6. THE NITROGEN CYCLE]