NATURE OF SOIL 305 



out returning manure to the field become deficient in humus, 

 and the yield- is decreased. 



Effects of humus on the soil. The most easily noted 

 effect of humus is in the darkening of the soil. The so- 

 called "black" soils get their color from the abundance of 

 humus they contain. The coloring is accomplished by the 

 partially decayed and partially preserved black organic mat- 

 ter coating over the small particles of the soil. The depth 

 to which humus extends, ranging from a few inches to sev- 

 eral feet, can be noted at any wayside ditch by the line be- 

 tween the dark soil on top and the lighter soil beneath. 



Humus serves several very important uses in the soil: 



( 1 ) it acts as a storehouse for different kinds of plant food ; 



(2) it increases the capacity of the soil to hold water; (3) 

 it aids in both creating and conserving heat in the soil ; (4) 

 it favors the growth of bacteria helpful to plants ; (5) it im- 

 proves the physical condition of the soil, making it more 

 porous and more easily cultivated. 



Living organisms. Reference has already been made 

 to the influence of certain bacteria in the fixing of nitrogen 

 in the soil. The soil harbors many different kinds of bac- 

 teria and other organisms. It is fairly teeming with life, 

 some of which is hostile to plant growth, but more of which 

 is necessary to successful plant development. 



Other organic matter. Besides living organic matter 

 and that which has undergone chemical changes converting 

 it into humus, most soils contain a certain amount of veg- 

 etable matter in the form of roots and stems of plants 

 which have not yet begun the process of decomposition. 

 These affect the soil chiefly in making it more accessible 

 to air and light, and more permeable to water. 



