NITROGEN-FIXING BACTERIA. 139 



minor importance as regards the maintaining of the store of com- 

 bined nitrogen. We should remember that a fair crop of hay will re- 

 move 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 sup- 

 plying the nitrogen requirements of even a very meager crop. 



There must be, then, another factor, or other factors, that are con- 

 cerned 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 cal- 

 cium, magnesium, potassium, sulphur, iron, phosphorus, etc., is de- 

 rived 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 sur- 

 face 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 dis- 

 cbarges and brought down to the soil by precipitation would be suffi- 

 cient in themselves through countless centuries to give rise to vast ac- 

 cumulations, 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. 



