THE OEIGIN OF SOILS 41 



has not previously been accounted for. The substances 

 provided by the decomposition of the plants, after death, 

 and the nitrogenous compounds brought down in the rain 

 water (p. 17) add to the possibilities of further growth 

 and the organic matter is gradually increased. The pene- 

 tration of the roots of plants, the solvent action of their 

 acid juices and the carbonic acid produced by the decom- 

 position of the organic matter, all tend to loosen the par- 

 ticles, allow air and water to enter, and so hasten the 

 conversion of rock into soil. 



The action pf burrowing animals, such as moles, in 

 loosening and stirring the soil is familiar. That of earth- 

 worms is perhaps less noticeable but more important. 

 These creatures swallow the finer particles of soil for the 

 double purpose of making their burrows and of obtaining 

 nourishment from the organic matter. Most of this 

 material is carried up and deposited at the surface. It 

 has been calculated that there are about 25,000 earth 

 worms in an acre of average soil, and that, by their com- 

 bined action, about ten tons of finely pulverent material 

 are brought to the surface every year. According to this 

 calculation each animal would have to move about a 

 twenty -fifth part of an ounce per day. 



Now all these agencies, mechanical, chemical, and bio- 

 logical, are constantly at work producing physical and 

 chemical disintegration of minerals and the ..growth and 

 decomposition of organic matter. 



Mechanical pulverisation converts the rock into a suit- 

 able medium for the growth of plants and accelerates the 

 chemical changes in the minerals and organic matter 

 whereby their constituents are reduced to a condition in 

 which they can be assimilated. These plant foods are sub- 

 ject to constant abstraction by crops, and in other ways, 

 and they are constantly regenerated by the operation of 

 the various forces on the fragments of rock and undecom- 



