THE CONSTITUTION OF THE SOIL 53 



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 vegetation 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 par- 

 ticles 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 bodies, much of which fall 

 back on the soil when it is dead This added organic matter intro- 

 duces 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 

 accumulated 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 of protein-like compounds, whilst others can 

 assimilate carbon dioxide. 



Thus the complex that we speak of as the soil consists of four 

 parts : 



1. The mineral matter derived from rock material, which consti- 

 tutes the frame-work of the soil and is in the main unalterable, but it 

 contains some reactive decomposition products. 



2. The calcium carbonate and phosphate (the latter being usually 

 in much smaller amount), and organic matter derived from marine or 

 other organisms deposited simultaneously with the soil. 



3. The soil water, a dilute solution of carbonic acid containing 

 small quantities of any soluble soil constituent. 



4. The residues of plants that have grown since the soil occupied 

 its present position, consisting of the mineral plant food taken up from 

 the soil water and of part of the complex organic matter. As the 

 source of energy this may be regarded as the distinguishing character- 

 istic of soils. 



These four constituents are invariably present, but not in the same 



