8. HUMUS IN THE SOIL. 



Illustrative material: A small quantity each of clay, sand, 

 and leaf mold. Swamp muck or dark-colored garden soil may 

 be substituted for the leaf mold, if the latter can not con- 

 veniently be obtained. 



Humus Defined If we dig up the ground at the bot- 

 tom of a hollow in the woods where the leaves have gath- 

 ered and decayed for centuries, we find the soil there very 

 dark-colored and very porous. It is dark-colored be- 

 cause it consists almost entirely of humus, a substance 

 that is always formed where vegetable matter decays in 

 the soil. Humus is the vegetable or animal matter in 

 which the process of decay is well advanced, but not com- 

 plete. 



Black Soil. The prairie lands of the United States are 

 very rich in humus, because the prairie grasses grew and 

 decayed on them for centuries before they were used for 

 farming. The soil of marshes is usually very dark-col- 

 ored, because, like the leaf mold of the woods, it consists 

 largely of humus. Whenever a farmer or gardener adds 

 vegetable or animal matter to his soil, and permits it to 

 decay there, he makes his soil richer in humus. The 

 more often a soil is manured with such matter, the darker 

 colored it is as a rule. 



Humus Helps Growth of Plants Humus in the soil 

 helps the growth of plants in several ways : it enables the 

 soil to hold more water than it otherwise would ; it tends 



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