126 The Great World's Farm 



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 three 

 hundred pounds of nitrogen to the acre, or about six 

 times as much as it actually takes up. And this it cer- 

 tainly 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 decom- 

 posing such salts, taking one ingredient and leaving the 

 other. The sunflowers which throve so well in the ex- 

 periment described were supplied with potassium nitrate, 

 and from this they were able to extract the nitric acid 

 which they needed. 



Nitrates are very soluble, and in damp soil they are 

 formed and dissolved so quickly as seldom to be visible. 

 But it is not so in regions where rain falls either at certain 

 seasons only, or very rarely. The most fertile soils of 

 Bengal, for instance, are often covered during the dry 

 season with a white crust of some of these salts, chiefly 

 potassium nitrate, otherwise called nitre and saltpeter. 

 The crust vanishes as soon as the rain comes, being dis- 

 solved and washed into the soil, which is so rich as to 

 bear two or three crops a year. 



