CHAPTER XV 



Al'AILABILITY OF PLANT NUTRIENTS AS 

 DETERMINED BY CHEMICAL ANALYSIS 



FORTUNATELY for mankind, only an exceedingly minute 

 proportion of the soil is at any one time soluble in water 

 or in the aqueous solutions with which it is in contact. 

 It is this great degree of insolubility that gives the soil 

 its permanence, for in humid regions, without this property, 

 it would be rapidly carried away in the drainage water. 

 The portion of the soil that is soluble in the various natural 

 solvents with which it comes in contact furnishes mineral- 

 food materials for plants. The great mass of soil, which 

 is relatively insoluble, is constantly subjected to natural 

 processes which very slowly bring its constituents into 

 solution. The agents that are concerned in the decom- 

 position of rock also act on the soil to bring about its 

 further disintegration, and thereby render it more soluble ; 

 while added to these are the operations of tillage, which 

 contribute to the same end. 



Only the surfaces of the soil particles come into contact 

 with the decomposing agents, and hence it is the surface 

 matter of the particles that gradually goes into solution. 

 The factors that determine how rapidly solution shall 

 proceed are : (1) the amount of surface exposed, which, 

 as has been seen, varies with the size of the particles ; (2) 

 the composition of the particles; (3) the strength of the 

 decomposing and solvent agencies. Were it not for this 



827 



