THE SOIL. 71 



It will be recollected that, when vegetable matter 

 decomposes in the soil, it produces certain gases (car- 

 bonic acid, etc.), which either escape into the atmos- 

 phere, or are retained in the soil for the use of plants. 

 The production of these gases is always accompanied 

 by heat, which, though scarcely perceptible to our 

 senses, is perfectly so to the growing plant, and is of 

 much practical importance. This will be examined 

 more fully in speaking of manures. 



Another important part of the organic matter in 

 the soil is that which contains nitrogen. This forms 

 but a very small portion of the soil, but it is of 

 very great importance to vegetation. As nitrogen 

 in food is of absolute necessity to the growth of 

 animals, so nitrogen in the soil is indispensable to the 

 growth of cultivated plants. It is obtained by the 

 soil in the form of ammonia (or nitric acid) from the 

 atmosphere, or by the application of animal or vege- 

 table matter. In some cases, manures called nitrates* 

 are used ; and, in this manner, nitrogen is given to 

 the soil. 



We have now learned that the atmospheric mat- 

 ter in the soil performs the following offices : 



Organic matter thoroughly decomposed is chiefly 

 carbon, and has the various effects ascribed to this 

 substance on p. 68. 



Organic matter in process of decay produces car- 



* Nitrates are compounds of nitric acid (which consists of ni- 

 trogen and oxygen), and alkaline substances. Thus nitrate of 

 potash (saltpetre), is composed of nitric acid and potash ; nitrate 

 of soda (cubical nitre or cubic-petre), of nitric acid and soda. 



