CHAPTER III 



THE ORGANIC MATTER OP THE SOIL AND 

 THE CHANGES IT UNDERGOES 



The organic matter of the soil represents, as we have 

 seen, the remains of previous generations of plants and 

 animals and can roughly be sorted out into two divisions 

 according to age : some of it is as old as the soil, having 

 been deposited along with the mineral particles when the 

 soil was first made ; some of it is newer, representing the 

 residues of recently living plants. It is not known 

 whether the original organic matter has any particular 

 effect on the soil, but one generally supposes that it has 

 not. Its properties have been studied by examination 

 of the subsoil at a depth below the range of the surface 

 vegetation. 



The more recent material is of supreme importance to 

 the soil. It is subdivided into (a) the newest of all, the 

 undecomposed roots or stubble which still retain some 

 of the structure of the plant; and (6) the partly decom- 

 posed material, dark brown or black in colour, which 

 has fallen entirely to powder and become completely 

 intermingled with the soil ; this part is commonly called 

 "humus." The undecomposed part serves two pur- 

 poses: it is the source from which the humus is derived, 

 and it keeps the soil open and porous, maintaining 

 passages through which water can drain away and air 

 can enter, and preventing the mineral particles from 

 settling down too compactly. In glasshouse practice it 

 is always desirable to keep up the supply and this is 



