Food from the Soil 



125 



There is always some very minute quantity of ammonia 

 in the air — about one part in a miUion — and there seems 

 to be no doubt that plants can and do take this up by 

 their leaves, for they thrive all the better when the quan- 

 tity is artificially increased. But they take it up also, and 

 in larger quantities, by their roots, when it has been 

 absorbed by the soil, or brought down to it by rain, snow, 

 and dew. 



The quantity of nitrogen thus washed down in combi- 

 nation with hydrogen in the course of the year seems to 

 be from about two pounds to nearly twenty-one pounds 

 per acre; but the average is about four and one-half 

 pounds to the acre — four and one-half pounds spread over 

 an acre of ground! Considering that there are seven 

 thousand grains in a pound, and that the sunflowers, even 

 when they had more within reach, took up little more than 

 a grain of nitrogen apiece, perhaps the quantity may not 

 sound so very small. 



But an acre of wheat, yielding twenty-eight bushels, 

 takes up about forty-five and one-half pounds of nitrogen; 

 while an acre of clover uses one hundred and eight 

 pounds! 



And even this does not at all represent the whole of 

 what is required; for the roots, however many, cannot 

 possibly be in close contact with all parts of the soil at 

 once, and they can no more make use of all the nitrogen 

 than they can of all the mineral matter, or all the moist- 

 ure; so that of this, as well as of the rest, they need much 

 more than they can actually use. 



An acre of soil, one foot deep, weighs some four mil- 

 lion pounds; and just a few pounds of nitrogen equally 

 mixed in this would be almost as difficult to find as a 



