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“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.