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“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 supported. The denitrifying bacteria, on the other hand, are continually nibbling at the basic store of our food; decomposing the nitrogen compounds and freeing the nitrogen from them in the form of the pure gas which is useless to us from the point of view of food.”