THE COMPOSITION OF THE SOIL 93 



However, the soil particles are not wholly unalterable. 

 The rain water and its dissolved carbonic acid exert a slight 

 solvent action, and the soil water always contains small 

 amounts of calcium and magnesium compounds, silica, and 

 other substances in solution. Each individual particle only 

 loses a very minute amount of substance to the soil water, and 

 its life is extraordinarily long ; nevertheless dissolution is 

 perpetually taking place. The surface soil contains less of the 

 smallest, and, therefore, most easily attacked, particles than 

 the subsoil. 



In any region where the rainfall and temperature conditions 

 are favourable, soil rapidly covers itself with vegetation ; even 

 a bare rock surface is not without its flora. The first vegeta- 

 tion must obviously have obtained its mineral food from the 

 dissolved material of the soil particles, but when it died and 

 decayed all the substances taken up were returned to the soil, 

 so that subsequent vegetation has food from two sources : from 

 the substances dissolved direct out of the soil particles during 

 the life of the plant, and from those dissolved out at earlier 

 times and taken up by previous races of plants. Thus in the 

 natural state, and where the vegetation is not removed, the 

 mineral plant food can be used over and over again and indeed 

 tends to accumulate as fast as it is extracted from the soil 

 particles by the rain water. 



The plant, however, returns to the soil more than it takes 

 away ; during its life it has been synthesising starch, cellulose, 

 protein, and other complex unstable and endothermic material, 

 much of which falls back on the soil when it is dead. This 

 added organic matter introduces a fundamental change because 

 it contains stored-up energy ; the difference between the soil 

 as it now stands and the original heap of mineral matter is 

 that the soil contains sources of energy while the mineral 

 matter does not. Hence it soon becomes the abode of a 

 varied set of organisms, drawing their sustenance and their 

 energy from the accumuluted residues, and bringing about 

 certain changes to be studied later ; some, as we shall see, are 

 capable of fixing gaseous nitrogen and so increasing the supply 



