FERTILITY OF THE SOIL 29 



Effect of humus. Humus is valuable in securing 

 the proper texture of soil. A gravelly soil or sandy 

 soil may be too loose for the plants to get a proper hold 

 and enough water. Humus will improve it by binding 

 the coarse particles together and by increasing the 

 capacity for film water. A heavy clay soil may be 

 too compact to allow proper drainage of surplus water 

 and free access of air. Such a soil can be made lighter 

 by the addition of humus. Thus, humus lessens the 

 difference between a heavy clay soil and a light sandy 

 soil by making them more loamy. The great value of 

 barnyard manure is that it not only furnishes substances 

 which are plant foods, but it also adds humus to im- 

 prove the soil texture. 



Effect of sand. Sometimes sand is mixed with a 

 tough, heavy clay to make it easier to till and to give 

 greater circulation of the soil water and soil air. 



Effect of lime. Lime is sometimes put in a heavy 

 clay soil to make it more porous and easier to work. 

 At times a soil becomes sour by the presence of an acid, 

 and its fertility is reduced. The application of lime will 

 destroy the acid and thus sweeten the soil. Lime is 

 more often applied for this than for any other purpose. 



QUESTIONS 



1. What are the two things that determine the fertility of 

 the soil? 



2. What are some of the most important elements that plants 

 require from the soil ? 



