50 SOILS AND FERTILIZERS 



of rocks, obtaining food from the air, and leaving a 

 certain amount of vegetable matter which undergoes 

 decay and is incorporated with the rock particles, pre- 

 paring the way for higher orders of plants which take 

 their food from the soil. When this vegetable matter 

 decays, it enters into chemical combination with the 

 pulverized rock, forming humates. 18 The disinte- 

 grating action of plant roots and vegetable matter 

 upon rocks has been an important factor in soil forma- 

 tion. The action of vegetable remains in soil produc- 

 tion is discussed in Chapter III. 



56. Combined Action of the Various Agents. In 



the decay of rocks the various agents named water 

 acting mechanically and chemically, heat and cold, 

 air, micro-organisms, and vegetation have been act- 

 ing jointly, and have produced more rapid disinte- 

 gration than if each agent were acting separately. 

 Wind also has been an important factor in the pro- 

 duction and modification of soils. The denuding 

 effects of heavy wind storms are well known. Large 

 areas of wind-formed soils are found in some coun- 

 tries. . Sand dunes are transported by winds and often 

 their subjugation by soil-binding plants is necessary 

 in order to prevent their encroaching upon valuable 

 farm lands and inundating villages. Soils formed by 

 the action of winds are called aeolian soils. 

 DISTRIBUTION OF SOILS 



57. Sedentary and Transported Soils, The place 

 which a soil occupies is not necessarily the place 

 where it was produced ; that is. a soil may be pro- 



