“That isn’t what I mean at all,” Wotherspoon interrupted, “I want you to look at it in another way. I suppose you had your usual style of dinner today. Just think of the items: soup, fish, meat, bread, and so on. Your soup was made from bones and vegetables; your fish course was originally an animal; so was your joint; your sweet was probably purely vegetable; and your dessert certainly was a plant product. Now don’t you see what I mean?”
“No, I confess I don’t.”
“Haven’t I just shown you that everything you ate comes from either the animal or vegetable kingdom? You don’t bite bits out of the crockery, like the Mad Hatter. Everything you use to keep your physical machine alive is something which has already had life in it? Isn’t that so? You never think of having a meal of pure chemicals, do you?”
“It never occurred to me; and I doubt it I shall begin now. It doesn’t sound very appetising.”
“It would be worse than that; but follow my argument further. Take the case of your joint. Presumably that came from an ox or a sheep. Where did the animal, whatever it was, get its food? From the vegetable kingdom, in the form of grass. Isn’t it clear that everything you yourself eat comes, either directly or indirectly, from the plants? And aren’t all animals on the same footing as yourself—they depend ultimately on the vegetables for their sustenance, don’t they? A fox may live on poultry; but the chickens he kills have grown fat by eating grain; and so you come back to the plants again. If you like to look on it in that way, we are all parasites on the plants; we cannot live without them. Our digestive machinery is so specialised that it will assimilate only a certain type of material—protoplasm—and unless it is supplied with that material, we starve. We can convert the protoplasm of other animals or of plants to our own use; but we cannot manufacture protoplasm from its elements. We have to get it ready-made from the vegetables, either directly or indirectly.
“Now the foundation-stone of protoplasm is the element nitrogen. The plants draw on the store of nitrogenous compounds in the soil in order to build up their tissues; and then we eat the plants and thus transfer this material to our own organisms. What happens next? Do we return the nitrogen to the soil? Not we. We throw it into the sea in the form of sewage. So you see the net outcome of the process is that we are gradually using up the stores of nitrogen compounds in the soil, with the result that the plants have less and less nitrogen to live on.”
“Well, but surely four-fifths of the atmosphere is nitrogen? That seems to me a big enough reserve to be drawn on.”
“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 Chile 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.
“So you see that outside our bodies we have bacteria working for or against us. The nitrifying bacteria are helping to pile up further supplies of nitrogen compounds upon which the plants can draw and whereon, indirectly, we ourselves can be
