“So it would be, if the plants could tap it directly; but they can’t do that except in the case of some exceptional ones. Most plants simply cannot utilise nitrogen until it has been combined with some other element. They can’t touch it in the uncombined state, as it is in the atmosphere; so that as far as the nitrogen in the air goes, it is useless to plants. They can’t thrive on pure nitrogen, any more than you can feed yourself on a mixture of charcoal, hydrogen, oxygen and nitrogen; though these elements are all that you need in the way of diet to keep life going. “No, Flint, we are actually depleting the soil of these nitrogen compounds at a very rapid rate indeed. Why, even in the first decade of the twentieth century South America was exporting no less than 15,000,000 tons of nitrogen compounds which she dug out of the natural deposits in the nitre beds of Chili and Peru; and all that vast quantity was being used as artificial manure to replace the nitrogenous loss in the soil of the agricultural parts of the world. The loss is so great that it even pays to run chemical processes for making nitrogenous materials from the nitrogen of the air--the fixation of nitrogen, they call it. “Well, that is surely enough to show you how much hangs upon this nitrogen question. If we go on as we are doing, there will eventually be a nitrogen famine; the soil will cease to yield crops; and we shall go short of food. It’s no vision I am giving you; the thing has already happened in a modified form in America. There they used up the soil by continual drafts on it, wheat crops year after year in the same places. The result was that the land ceased to be productive; and we had the rush of American farmers into Canada in the early days of the century to utilise the virgin soil across the border instead of their own exhausted fields.” “I suppose you know all about it,” I said, “but where do these come in?” I pointed to the pinkish disks of the cultures. “These are what are called denitrifying bacteria. Although the plants can’t act upon pure nitrogen and convert it into compounds which they can feed upon, some bacteria have the knack. The nitrifying bacteria can link up nitrogen with other elements so as to produce nitrogenous material which the plants can then utilise. So that if we grow these nitrifying bacteria in the soil, we help the plants to get more food. The denitrifying bacteria, on the other hand--these ones here--act in just the opposite way. Wherever they find nitrogenous compounds, they break them down and liberate the nitrogen from them, so that it goes back into the air and is lost to us again.