152 Food from the Soil 



had taken sixty-six times as much from the soil. The 

 quantity is still very small, of course, only two grains 

 and a fifth; but it strikingly illustrates the immense 

 importance of small, and even minute quantities, since 

 it made the whole difference in the growth of the 

 plants. 



The dwarf sunflowers obtained their small fraction 

 of nitrogen from the air ; but this is no contradiction 

 to what has been previously said, for they did not take 

 pMYc nitrogen, but ammonia, which is a compound of 

 nitrogen and hydrogen. 



There is always some very minute quantity of 

 ammonia in the air — about one part in a million — 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 quantity 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 

 combination with hydrogen in the course of the year 

 seems to be from about 2 lb. to nearly 21 lb. per 

 acre ; but the average is about 4J lb. to the acre — 

 4I lb. spread over an acre of ground ! Considering 

 that there are 7,000 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 45 J lb. of nitrogen ; while an acre of 

 clover uses 108 lb. 1 



And even this does not at all represent the whole of 

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



