CONSTITUTION OF THE SOIL 249 



The amount of humus which accumulates in any soil depends upon the rela- 

 tive rates of the addition of organic residues and of its disappearance as a 

 result of oxidation. Soils with a low humus content may result from a sparse 

 contribution of organic matter, as in desert or semi-desert regions, or from a 

 rapid oxidation of organic material which prevents accumulation of humus 

 even when the supply of organic residues to the soil is large. This latter 

 condition obtains in many tropical and sub-tropical soils. 



Humus is essentially colloidal in its properties and possesses an even greater 

 imbibitional capacity than the colloidal clay particles of the soil. The plas- 

 ticity and cohesiveness of the colloidal organic matter, while considerably less 

 than that of the colloidal clay, is much greater than that of the non-colloidal 

 fractions of the soil. Humus is rather inert chemically, its influence on the 

 soil being largely a physical one. Its presence in soils favors a looser struc- 

 ture and hence better aeration, a higher water holding capacity and a reduc- 

 tion in cohesiveness as compared with heavy clay soils. 



Since the clay fraction and the humus are the two essentially colloidal 

 fractions of the soil, and are associated in an intimate physical relationship, 

 many soil scientists refer to these two soil fractions, considered jointly, as the 

 colloidal complex of the soil. A large part of this colloidal complex occurs, 

 in many soils, as films enveloping the larger soil particles. The relation of 

 these films to the maintenance of the crumb structure of the soil has already 

 been mentioned. Many other important soil properties are either due to or 

 greatly influenced by the colloidal complex. 



3. Soil Water and the Soil Solution. — Water is universally a component 

 of soils, although the amount present may vary from the merest trace to a 

 quantity sufficient to saturate the soil, i.e. completely fill all of the spaces 

 between the soil particles. Dissolved in the soil water are varying quantities 

 of numerous chemical compounds. These originate principally from the dis- 

 solution or chemical weathering of the rock particles, from the decomposition 

 of organic matter, from the activities of micro-organisms, and from reactions 

 between the roots of plants and the soil constituents. It is thus more accurate 

 to speak of the soil solution than of the soil water, although in discussions 

 of the water relations of soils it is often a common practice to disregard 

 the presence of solutes in the soil water. The concentration of the soil solu- 

 tion in any given soil varies with the proportion of water present. While 

 in most soils the soil solution is very dilute, in saline and alkaline soils it may 

 be so concentrated that only a few species of plants can survive with their 

 roots in contact with it. The principal cations found in the soil solution are 

 Ca+ + , Mg+ + , K+, Na+, A1+ + + , and Fe+ + + (or Fe+ + ); the principal 

 anions are HCO3-, PO4 , CI", NO3-, SO4 — , and SiO.--. Other 



