48 THE MECHANICAL ANAL YSIS OF SOILS [chap. 



recent origin or the humus which is characteristic of all 

 soils proper. 



The term humus is applied to the black or dark 

 brown material of vegetable origin which gives to 

 surface soil its characteristic darker colour as compared 

 with the subsoil. It is essentially a product of bacterial 

 action ; there are a number of bacteria working in the 

 absence of air and universally distributed, which attack 

 the carbon compounds of plant tissues, especially the 

 carbohydrates, with the production of marsh. gas or 

 hydrogen, carbonic acid, and humus. In the presence 

 of air the characteristic humus-forming fermentation 

 is continued by one which results in the complete 

 combustion of the organic matter to carbonic acid. For 

 this reason more humus is found in a pasture than in a 

 continually aerated arable soil, more again in clays than 

 in the lighter soils through which air is always being 

 drawn as the rain percolates, and the accumulation 

 of humus reaches its maximum where considerable 

 rainfall and an impermeable stratum combine to make 

 the soil so water-logged that all access of air is cut off, 

 as in swamps and bogs. The presence of chalk in the 

 soil also assists in the destruction of humus, since it 

 neutralises the acids which largely compose the humus, 

 and which tend to inhibit the further action of bacteria. 



The chemical composition of humus is indefinite ; 

 it is a variable mixture of several substances, themselves 

 of very complex constitution ; it always contains more 

 carbon and less hydrogen and oxygen than the vege- 

 table tissues from which it was formed. The following 

 figures show the composition of grass and of the top 

 brown layer of turf in a peat bog, also of the same peat 

 of greater age at depths of 7 and 14 feet, the mineral 

 matter and moisture being excluded in calculation in 

 each case : 



