ORIGIN OF SOILS 59 



As a rule, silicates containing much iron are easily changed by 

 weathering agencies. 



Action of Animal and Vegetable Life. Animal and vegetable 

 life act on rocks, both by their living activities and the decay of 

 their remains. Vegetation acts both mechanically and chemically 

 upon the soil and rocks. Roots of plants penetrate the crevices 

 of rocks, and, as they grow, split even large rocks. The shelter 

 of growing plants keeps the rock surface moist, thus enabling the 

 water to act upon the rock, and the carbon dioxide excreted from 

 roots, adds its effect to that derived from other sources. Plants 

 take up material, which, under natural conditions, returns to the 

 soil in a modified form. 



Earthworms in some cases bring to the surface large quantities 

 of soil, most of which has passed through their intestines and 

 undergone mechanical and chemical changes. 



Vegetable or animal residues aid weathering in several ways : 



(a) By maintaining more moisture in the surface of the soil. 



(b) By supplying copious quantities of carbonic acid. The 

 following figures of Boussingault and Levy 1 exhibit the amount 

 of carbonic acid in the air of the soil under different conditions : 



Carbonic acid in 

 10,000 parts by weight 



Ordinary air 6 



Air in sandy subsoil of forest 38 



Air in loamy subsoil of forest 124 



Air in surface soil of forest 130 



Air from surface soil of pasture 270 



Air from surface soil rich in humus 543 



Newly manured sandy field in wet weather 1413 



(c) By direct action of organic acids such as acetic, propionic, 

 "humic," etc., which are found in vegetable matter or produced 

 in its decay. 



(d) By furnishing a medium for the activity of the lower 

 soil organisms, such as bacteria and molds. 



Products of Weathering. The general tendency of weathering 

 is towards the production of simpler compounds from more 

 complex ones. The oldest rocks (which are igneous in origin) 

 1 Jahresber. der Chem., 1852, p. 783. 



