342 SOILS: PROPJEBTIES AND MANAGEMENT 



It will be noticed that the dissolved matter, both or- 

 ganic and inorganic, fell off markedly after the first ex- 

 traction, which was larger because of the matter in solu- 

 tion in the soil water. Later extractions were doubtless 

 supplied largely from the substances held by adsorption, 

 which gradually diffuse into the water extract as the 

 tendency to maintain equilibrium of the solution overcomes 

 the adsorptive action. With the removal of the adsorbed 

 substances, the equilibrium between the soil particles 

 and the surrounding solution is disturbed, solvent action 

 is increased, and more material gradually passes from the 

 soil into the solution. In this way the uniform and con- 

 tinuous body of extractives is maintained. 



245. Other factors influencing extraction. — For pur- 

 poses of soil analysis, the quantity of water used for extrac- 

 tion must be placed at some arbitrary figure, and this 

 is open to the objection that it does not represent accu- 

 rately the soil-water solution. Analyses of soils of different 

 types are not comparable, and the water extract cannot 

 be considered as measuring the concentration, or even 

 the composition, of the solution existing between the 

 root hair and the soil particles. However, for studying 

 some of the changes which go on in the soil and which are 

 detectable in the soil-water solution, the practice may be 

 followed to advantage. 



246. The soil solution in situ, — It has already been 

 pointed out that the interstitial spaces of any arable soil 

 contain more or less water all the time; that there is a 

 constant tendency for this water to assume the capillary 

 condition owing to the gravitational movement of free 

 water ; and that the normal evaporation of moisture from 

 the soil tends to reduce the capillary film to the condition 

 of hygroscopic water (par. 132). As the movement 



