THE CHEMICAL PROCESSES OF SOIL FORMATION. jg 



the cracks and thus to increase the surface exposed to attack. 

 Since ferrous compounds, when soluble in water, are injurious 

 to plant growth, this oxidation is of no little importance, and 

 in soils must be carefully maintained against a possible reversal. 

 It is hardly necessary to insist that the action of all these 

 chemical agents continues in the soils themselves, and that 

 owing to the fineness of the material, resulting in an enor- 

 mously increased surface exposed to attack, such action ac- 

 quires increased intensity. This is the more true as in soils 

 bearing vegetation there are always superadded the effects 

 of the humus-acids resulting from the decay of vegetable 

 matter, as well as of the acid secretions of the living plants. 



Action of Plants and their Remnants in Soil Formation. 



(a} Mechanical action. The direct action of plants in forc- 

 ing their roots into the crevices of rocks and minerals and thus 

 both widening them by wedging, and by exposing new sur- 

 faces to weathering, has already been alluded to. That the 

 mechanical force exerted by root growth is very great, may 

 readily be judged from their effects in forcing apart, even to 

 rupture, the walls of rock crevices ; but actual measurement has 

 shown the force with which the root, e. g., of the garden pea 

 penetrates, to be equal to from seven to ten atmospheres, say 

 from 200 to over 300 pounds per square inch. Such a force, 

 exerted under the protection of the corky layer protecting the 

 root tips, often produces surprising effects. 



(b) Chemical action. Vegetation takes a most important 

 part, from a chemical point of view, both in the first formation 

 of soils and in their subsequent relations to vegetable life. The 

 lower forms of vegetation are usually the first to take posses- 

 sion of rock surfaces; foremost among these are the lichens. 

 In humid climates we find these crust-like plants incrusting 

 more or less all exposed rock surfaces, sometimes with a solid 

 mantle that can be peeled off in wet weather, showing the 

 corroded rock-surface, and the beginnings of soil clustering 

 amid the root-fibrils beneath. A microscopic examination of 

 the substance of these lichens often shows as a prominent in- 

 gredient, crystals of oxalate of lime, the lime having of course 

 been derived from the rock, while the oxalic acid has been 



