OEIGIN AND FORMATION OF SOILS. 25 



formation of nitrates. It exerts a favorable physical 

 effect upon soils; its presence helps to separate the adhe- 

 sive particles of clay, and makes heavy soils loose and 

 friable, which permits the easy passage of water through 

 them. Lime also increases the absorbing and retaining 

 power of sandy soils, by causing the particles to adhere 

 more closely to each other. 



Humus. — The decaying vegetable matter in soils, 

 which is made up of carbon, oxygen, hydrogen, and 

 nitrogen, is called ^^ humus.'' In virgin soils it is derived 

 from the dead roots and leaves of a former vegetation. 

 It has a dark brown or blackish color. Leaf mould, 

 found in forests, is largely composed of humus. It was 

 believed at one time that humus served as a direct food 

 for plants, but this idea has been proved to be incorrect; 

 it is the final products of its decay, chiefly carbonic acid, 

 ammonia, and water, that serve as plant-food. 



Humus is a very useful ingredient in all kinds of 

 soils, though a soil may contain too much decaying 

 organic matter. Humus improves sandy soils, not only 

 on account of the nitrogen and other plant-food con- 

 stituents which it contains, but because it increases their 

 absorbing and retaining power. Humus will absorb and 

 retain more moisture than any other ingredient of soils. 

 Clay soils are improved by it, however, on account of its 

 property of loosening and aerating them. 



The four principal ingredients of soils are useful, there- 

 fore, not altogether because they furnish plant-food, but 

 because they give to soils certain physical properties 

 which enable them to retain heat, moisture, and plant- 

 food. These properties give to soils what is called phys- 



