22 THE SOIL SOLUTION 



dries out again below the critical amount, there is again a shrink- 

 ing. As it is always either raining or not raining, soils are al- 

 ways either getting wetted or are drying. Consequently the 

 individual grains are continually moving about among them- 

 selves. A heavy object, such as stone, when left on the ground 

 gradually sinks into it. 1 Earthworms, burrowing animals and 

 insects are continually at work in most arable soils. The action 

 of frost in "heaving" a soil is familiar to everyone. Not so well 

 known, however, is the fact that the apparently superficial cracks 

 which occur to a greater or less extent in every soil, under 

 drought conditions, are in reality quite deep, extending well into 

 the subsoil. By the edges breaking off, and by wind- and water- 

 borne material being carried in, considerable surface soil is thus 

 brought into the subsoil. Through these various agencies, 

 therefore, the solid components of the soil are continually sub- 

 ject to much mixing; subsoil is becoming surface soil, and to 

 some extent vice versa. An important result of these various 

 processes is the bringing into the surface soil of degradation and 

 decomposition products from underlying rocks. The processes 

 involved are essentially dynamic. 2 



The soil solution is also a dynamic problem. When the rain 

 falls on the soil, a part, the "run-off," flows over the surface and 

 finds its way into the regional drainage; a part immediately 

 evaporates into the air, and is designated as the "fly-off;" a 

 third part, the "cut-off," enters the soil. 3 The cut-off water 

 penetrates the soil by way of the larger openings and interstices, 



1 On the small vertical movements of a stone laid on the surface of 

 the ground, by Horace Darwin, Proceedings of the Royal Society of 

 London, 68, 253-261 (1901). On the other hand, geological literature 

 would probably furnish numerous references to the heaving out of 

 boulders, probably as the result of successive freezings and thawings of 

 the soil. The shape of the stone as well as the specific nature of the 

 movements of the soil particles evidently has an important influence in 

 determining whether the stone sinks into the soil or vice versa. 



1 It is clear that as the soil is continually changing through physical 

 agencies, the chemical analysis of it cannot be expected to furnish evi- 

 dence as to the mineral constituents removed by crops or by leaching. 



* This terminology has been suggested by Dr. W. J. McGee. 



