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LESSONS IN AGRICULTURE 



dissolving away and depositing particles of the rocks 

 to form soil. Frosts, winds, rains, running streams, 

 plant roots, burrowing animals, and other forces of 

 nature are constantly changing the rocks to soil. 



Kinds of soil. Since all soils come from rocks, the 

 kind of soil must therefore depend on the kind of rock 

 from which it was made. Sandy soil must come from 



Fig. 42 Drawing, showing how Fig. 42* Drawing, showing gla- 

 rock gradually breaks up and de- ciai (Irlft deposited on top of the 

 cays from the top downward. solid rock. 



sandstone, lime soil from limestone, clay soil from clay 

 stones, etc. 



Humus. Humus is the decaying and partly decayed 

 plant and animal life in the soil. It is usually dark 

 colored. Humus greatly improves the soil for plants, 

 because in its decay acids are set free which help dis- 

 solve plant food in the soil, and because it improves 

 the quality of the soil, making clay less and sand more 

 compact, thus increasing the water-holding capacity 

 for plants. The loss of humus makes hillside land wash 

 badly, and unless the farmer is careful, his sloping fields 

 are soon "worn out." All weeds, stubble, clovers, etc., 



