Mineral Nutrients 



159 



The vast array of facts which have been developed with 

 respect to the amount of nutrients in soils is, after all, of 

 somewhat limited application. This is due in large part 

 to fundamental difficulties in obtaining a satisfactory 

 basis for a computation of effectiveness. If, for ex- 

 ample, the total quantities of the nutrients contained 

 in the first few inches of soil are made a basis, then, to 

 modify the calculations of the amounts of the nutrients, 

 there are such conflicting factors as the following : 



(1) Roots are not commonly limited to the first 7 or 

 10 inches of soil (Fig. 9). 



(2) To a certain extent there is in the soil a movement 

 of the soluble nutrients from higher to lower levels and 

 also the reverse. There is further a variable loss from 

 leaching. 



(3) The roots do not actually come in contact with all 

 of the soil, and the special solvent action (discussed later) 

 is greatest in the immediate vicinity of these structures. 



(4) No absorbing organs are able completely to "ex- 

 haust " or remove all the nutrients from any soil, and the 

 amounts readily removable depend upon complex chemi- 

 cal and physical factors. 



