THE BIOLOGY OF SOIL. 57 



the solutions they yield to water in contact, the 

 manner in which they retain some of these solu- 

 tions and allow others to pass easily, and the 

 remarkable double decompositions which go on in 

 them. Moreover, I must assume as known the 

 chief physical properties of ordinary soils with 

 respect to the phenomena of capillarity, absorption 

 of heat, action of frost, and so forth. 



But all ideas as to the nature of soil based merely 

 on the study of its chemistry and physics are mis- 

 leading, and it is in just the establishment of this 

 truth that modern discoveries in Agricultural and 

 Forest Botany have played so important a part. 



From the facts that organic debris is found 

 chiefly at the surface of the earth, and that the 

 smallest particles are held in suspension by the 

 water near the surface, it is comprehensible why 

 such organic remains abound in the upper parts 

 of the soil, where the rootlets with their absorbing 

 root-hairs are also found, because they must have 

 oxygen. The rule is, therefore, that an ordinary 

 soil consists of upper strata, rich in organic 

 materials and in oxygen, and a subsoil, poorer 

 in these substances. 



Among these organic materials are countless 

 myriads of living beings, especially fungi and bac- 

 teria, which require oxygen and organic materials 

 for their subsistence, and it depends on the 

 open or close, moderately moist or damp, warm 

 or cold nature of the soil, and on some obviously 

 connected factors, how far down these aerobic 

 organisms can thrive. As we go deeper down they 



