Food from the Soil 153 



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 moisture ; 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 

 4,000,000 lb. ; and just a few pounds of nitrogen 

 equally mixed in this would be almost as difficult to 

 find as a needle in a haystack ; and even if there were 

 actually as much as the crop required, the roots could 

 not reach it. 



A heavy wheat-crop needs, therefore, nearly 300 lbs. 

 of nitrogen to the acre, or about six times as much as it 

 actually takes up. And this it certainly cannot get from 

 the ammonia in the air, or from that which is washed 

 from it into the soil. 



But when organic matter decays, whether it be 

 animal or vegetable, the nitrogen contained in it 

 combines with other gases to form not only ammonia, 

 but also nitric acid. 



Much of the ammonia streams off into the air, but 

 the nitric acid remains and combines with potash, 

 -soda, lime, magnesia, or iron, with which it forms 

 nitrates. These nitrates are easily dissolved, and it is 

 from them that the plants obtain their nitric acid — 

 very much diluted, of course, as is all the food which 

 they take from the soil. 



As already remarked, plants have the power of de- 

 composing such salts, taking one ingredient and leaving 

 the other. The sunflowers which throve so well in the 

 experiment described, were supplied Avith potassium 

 nitrate, and from this they were able to extract the 

 nitric acid which they needed. 



