22 FIRST PBINCIPLES OF AGBICULTUBE. 



order of agricultural plants, or those useful as food for 

 man. 



Earth "Worms. — Earth worms and other living crea- 

 tures living in the ground also aid in preparing the soil 

 for the growth of plants. They burrow deep into the 

 ground ; the passages thus made aid in drainage and 

 circulation of air, and facilitate the penetration of roots, 

 which increases their power to secure food. They also 

 drag into their underground passages considerable vege- 

 table matter, which in its decay aids in forming true soil. 



Movement of Soils. — While our present soil remains 

 almost constant in quantity, the parts removed by various 

 causes being supplied by the gradual decay of the rocks, 

 the same agencies which form soils, particularly weather 

 action, are wasting or carrying them away. Soil is almost 

 constantly moving; it is thinnest at the top of the hill, 

 and deepest in the valley. This is very noticeable in 

 mountainous or hilly districts; great furrows are formed 

 in the hillsides after heavy rains, the swollen and muddy 

 streams carrying the soil in suspension to lower parts 

 of the land. 



Soils in Place. — Soils which have been formed from 

 the decay of the underlying rock are called sedentary 

 soils, or soils in place; they have not been moved from 

 the place in which they originated. These partake of the 

 nature and composition of the rock underneath; though, 

 from the method of their formation, viz., the growth 

 and decay of plants, they contain considerable vegetable 

 matter obtained from the atmosphere. 



Transported Soils. — Transported soils are those 

 which have been moved from the place where they 



