96 THE PEOPLE'S FARM AND STOCK CYCLOPEDIA. 



Thus was formed the soil on which we live, and it contains 

 three substances of great importance of such importance that 

 it will b.e well to repeat them : Phosphoric acid and potash, 

 obtained from decaying rocks, and nitrogen, brought down by 

 the rains from the air ; also, a substance of scarcely less import- 

 ance, carbon, gathered by the plants from the air, and added to 

 the soil on the death of the plant. This work is still going on. 

 The rocks still decay and furnish their share of plant food ; the 

 rains still bring down nitrogen, and the plants, when allowed to 

 decay upon the land on which they are grown, still add carbon. 



Source of Nitrogen in Soils. We have seen that the 

 original source of nitrogen in the soil was that brought down by 

 the rains in the form of nitric acid. Of course decaying plants 

 add much nitrogen to the soil, but this they obtained in the first 

 place from the soil, as they have no power to gather nitrogen from 

 the air, although their leaves are constantly bathed in an ocean of 

 it. Therefore crops by growing and decaying can not increase the 

 amount of nitrogen in the soil. 



The amount gained each year from the air is very small, not 

 more than from five to ten pounds on the surface of an acre. It 

 is therefore a question of both interest and importance to know 

 if there is any other source of nitrogen in the soil. On this 

 point scientific men have differed, and long and warm have been 

 the controversies over it. The writer of this article is of the 

 opinion that there is another source. 



All soils at least all good soils are porous, and the pores 

 of the soil, when not filled with water are filled with air, four- 

 fifths of which is nitrogen. Now we believe that under certain 

 circumstances this nitrogen contained in the pores of the soil 

 combines with oxygen, forming nitric acid, which remains in the 

 soil, adding to the total amount of nitrogen it contains. The 

 circumstances we believe to be essential are these: 



Warmth. 



A porous soil, moist but not wet, and containing a good pro- 

 portion of decaying vegetable matter. 



The presence of some alkaline substance, such as lime, in 

 the soil. 



