6 THE BIOLOGY OF FLOWERING PLANTS 



too wet becomes puddled, quite unfit for plant growth, and 

 may require prolonged treatment before it is again fit to 

 bear crops. 



The details of this structure are far from being fully 

 elucidated but its general nature may be sketched. The 

 coarser grains of sand, ranging up to particles of 3 mm. 

 diameter, form a skeleton round which are built up the 

 crumbs by the glueing action of colloidal particles of clay 

 and humus fully imbibed with water, and by the surface 

 tension of fine water films ; the crumbs are aggregates of 

 the larger particles with intermixture of the finer grades of 

 sand and silt and of clay and humus. The precise state of 

 the colloidal constituent is not known ; it may consist of 

 the clay and humus particles as a whole, or of a gelatinous 

 and indefinite layer surrounding these particles. 



We do not know whether there is a fundamental unit 

 size of soil crumb, nor, if there is, how it varies in different 

 soils. The crumby structure is evidently the effect of 

 natural causes of soil formation, the earthworm being 

 probably the most important agent ; but the maintenance 

 of this structure under the special conditions of agriculture — 

 the keeping of the soil in good tilth — is the chief aim of 

 cultivation. 



The moisture of the soil is not, of course, pure water. 

 It is a solution of salts, organic compounds, and gases. The 

 inorganic salts are of special importance in forming the 

 source of supply of the essential mineral elements of the 

 plant. The soil water is conveniently referred to as the soil 

 solution. 



Soil Solution. — The composition of the soil solution is 

 not known in any single case, the reason being that there 

 at present exists no method of separating it from the soil 

 in its original state. In fact a little consideration shows 

 that even for a single soil the composition of the soil solution 

 must vary with changing conditions in the soil and probably 

 very rapidly. The solution tends to come to an equilibrium 

 concentration of solutes ; but this equilibrium depends not 

 only on the nature of the soil minerals, and on the solvent 



