C. PLANT NUTRIENTS IN THE SOIL 



I. SOLUBILITY OF THE SOIL THROUGH NATURAL 



PROCESSES 



Fortunately for mankind, only an exceedingly small 

 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 insolubility that gives the soil its perma- 

 nence, for, otherwise, in humid regions, it would be 

 rapidly carried away in the drainage water. The portion 

 soluble in the various natural solvents with which it 

 comes in contact furnishes the mineral-food materials for 

 plants. The great mass of soil which is relatively in- 

 soluble is constantly subjected to natural processes 

 which very slowly bring the constituents into solution. 

 Those agents concerned in the decomposition of rock 

 also act upon the soil to bring about its further disin- 

 tegration, and thereby render it more soluble, while 

 added to those are the operations of tillage, which con- 

 tribute to the same end. 



The surfaces of the particles alone come into contact 

 with the decomposing agents, and hence it is these por- 

 tions of the particles that are rendered most soluble. 

 The factors that determine how rapidly solution shall 

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

 we have 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 

 process, there would soon be no mineral food available 



(267) 



