The Physical Character of Soils 31 



Importance The larger part of the atmosphere is made 



of Nitrogen, ^j nitrosren, which, as we have shown. 



Aluminum, . , ?m i 



and Potassium ^^ ^"^^^ ^° dilute the oxygen so that we can 

 breathe it safely. Nitrogen rapidly leaves 

 the soil by the growth of plants and by leaching away 

 in the soil water, and is also driven back to the air 

 by the combustion of organic matter. In the soil it 

 is most abundant in the vegetable decay of humus that 

 makes soils dark in color. The decay of animal tissues 

 also contributes to this humus and consequently to the 

 store of nitrogen. Nitrogen exists, further, in the soil in 

 combination with lime, potash, and other bases formed 

 through the action of microscopic plant life on organic 

 matter. By this action anmionia, the hydride of nitro- 

 gen, is released, which is still further reduced by these 

 various bacterial forms to nitric acid. The nitric acid 

 seeks a base in the soil and forms the nitrates of Ume, 

 soda, or potash. Plants with green leaves use nitrogen | 

 only when it has become a nitrate, and hence the action! 

 of bacteria in reducing the organic matter and carrying 

 its nitrogen to a nitrate is an important work in nature's 

 process and goes far toward separating a Hving soil which i 

 abounds in microscopic Hfe from a dead soil in which \ 

 these minute forms are starved out by the lack of organic 

 matter. 



The metal aluminum is one of the most abundant 

 elements. It is of late becoming more familiar to all in 

 its metallic form, since modem chemistry has devised 

 ways for separating it from its combinations in clay. It 

 is the very foundation of all true clays, which are formed 

 by the decomposition of granite or feldspar; and while 



