Page:Popular Science Monthly Volume 62.djvu/145

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NITROGEN-FIXING BACTERIA.

minor importance as regards the maintaining of the store of combined nitrogen. We should remember that a fair crop of hay will remove from the soil more than 50 pounds of nitrogen, that at times there is removed from the soil 100 to 150 pounds of nitrogen per acre in one season, and remembering that, we can easily appreciate how entirely inadequate the 3 or 4 pounds of nitrogen per acre that are brought down from the atmosphere by dew, rain or snow are for supplying the nitrogen requirements of even a very meager crop.

There must be, then, another factor, or other factors, that are concerned in the supply of the vast quantities of combined nitrogen that are consumed from day to day. The mineral portion of plant food, that portion which constitutes the ash of plants, containing the calcium, magnesium, potassium, sulphur, iron, phosphorus, etc., is derived from the common rocks of the earth's crust. It is otherwise with nitrogen; to be sure, small quantities of it are contained in primitive rocks in iron deposits, in meteoric iron, etc.; yet, speaking generally, nitrogen is not a normal constituent of rocks. It is the atmosphere, and the atmosphere only, which must remain its source for plant and animal life. It is idle to speculate in what condition that nitrogen existed when the earth's crust first began to solidify. It is not likely that it existed as ammonia, for the hydrogen having a greater affinity for oxygen would have combined with the latter. It is not improbable, however, that it existed in combination with oxygen when the temperature of the earth's atmosphere had become sufficiently low. Be it as it may, when the surface rock began to disintegrate and lower plant life first appeared, there was no soil nitrogen. As rock disintegration proceeded, as the rock fragments became finer and offered a more favorable dwelling place for plants and bacteria, the store of nitrogen in the soil began to accumulate. And now we come to those agencies that are of the greatest importance in this gradual increase of the nitrogen store. Small amounts of combined nitrogen formed through electric discharges and brought down to the soil by precipitation would be sufficient in themselves through countless centuries to give rise to vast accumulations, provided that there was a gain only and no loss. But we have already seen that nitrogen is constantly being leached out of the soil. Analytical data at hand show that there is drained away from the land as much as 75 pounds of nitrogen per acre in the form of nitrates, and this certainly is lost to the soil. On some soils the loss is much smaller, on other soils it is even greater, but this, taken together with the amount removed in the crops taken off from year to year, shows clearly that unless there are other means in the economy of nature for drawing upon the great store of atmospheric nitrogen, the present store would soon be exhausted, in fact, it could never have accumulated.