108 SOILS AND MANURES 



of internal surface (p. 55), and that the power of attrac- 

 tion must be very great. It is probable, however, that 

 such purely mechanical retention would affect the salts 

 as a whole, :and one .salt nearly as much as another./ 

 Mechanical causes, though they doubtless contribute to 

 the retention of salts in the soil, must, therefore, be 

 regarded as of minor importance. 



Organic Matter. The organic matter of the soil is 

 mainly of vegetable origin and is derived from the remains 

 of previous generations of plants. In the case of soils 

 under cultivation the crops are removed, but, as a rule, 

 a portion of the plants sometimes the roots, sometimes 

 the leaves is left in or on the land. The removal of the 

 crops is at least partially compensated by ploughing under 

 weeds and green manures, and by the application of farm- 

 yard manure. The organic matter rapidly decays owing 

 to the action of fungi, bacteria and ferments of various 

 kinds ; the organised structure is destroyed and it becomes 

 intimately mixed with the other particles of soil. When 

 organic matter accumulates under water the vegetable 

 structure is, to a large extent, preserved and peats are 

 formed. The disappearance of the organised structure in 

 the open soil has been attributed to the action of worms and 

 larvae, but it seems more probable that it is due simply to 

 more rapid oxidation, and its preservation under water 

 to the exclusion or limiting of the air supply. The decom- 

 position of organic matter certainly proceeds most rapidly 

 on soils that are most exposed to oxidation open sandy 

 soils and the presence of clay favours its accumulation. 

 After passing through various intermediate stages, the 

 organic matter finally becomes completely oxidised, the 

 oxides of carbon and hydrogen are volatilised, and the 

 nitrogen and ash constituents become available to growing 

 plants. 



Humus is not a definite compound but a complex mix- 



