73 



Physics of the Soil, 



ingredients, thus giving rise to a soil very different in chem- 

 ical composition from the parent rock. 



If, on the other hand, the rock is made up of minerals of 

 nearly equal solubilities, or if in any way the soil results 

 from a mechanical breaking up of the rock, then the soil 

 may hays much the same relative amounts of ingredients as 

 the parent rock shows. In the table which follows are 

 given the composition of some rocks and of soils derived 

 directly from them: 



Composition of rocks and residual soils** 



The two limestones, it will be seen, have given rise to a 

 soil containing almost as much silica, alumina and iron 

 oxide combined as is contained in the three soils from the 

 other three kinds of rock, the per cents, standing, in round 

 numbers, 83, 75, 84, 85 and 87. In other words there is 

 a strong tendency to bring all soils approximately to one 

 composition. Indeed it may be said that in any soil the 

 essential ingredients of plant food make up but from 3 to 

 8 per cent, of the total dry weight. It will be observed 

 that in the case of the soil derived from the Bermuda lime- 

 stone, not less than 98 pounds of every 100 pounds of rock 



i Rocks, Rock Weathering and Soils. Merrill. 



