THE SOIL IN RELATION TO PLANT GROWTH 319 



enormous losses arise in virgin countries through continuous 

 cultivation (p. 181), and at this end in making dung (p. 193), 

 and especially through our methods of sewage disposal. It 

 seems inevitable that these losses must make themselves felt 

 some day, unless the movement for the conservation of natural 

 resources ever becomes a potent factor in international 

 life. 



Soil Fertility and Soil Exhaustion. 



From the preceding paragraphs it is clear that fertility is 

 not an absolute property of soils, but has meaning only in 

 relation to particular plants. Plant requirements vary ; a soil 

 may be fertile for one plant and not for another ; every soil 

 might conceivably prove fertile for something. But in practice 

 the agriculturist can find use only for a very limited number 

 of plants ; he, therefore, has to select those combining the 

 double features of saleability in his markets and suitability to 

 his conditions of soil and climate. To a certain extent it is 

 possible to bridge the gap between plant requirements and 

 soil conditions ; the former may be permanently altered by 

 breeding if suitable plants cannot be found by selection, and 

 the latter may be changed by such processes as draining, 

 liming, etc. When all has been done that is economically 

 possible there may still remain a divergency between the 

 conditions ideal for the plant and those it finds in the soil ; 

 this divergency is the measure of the infertility of the soil for 

 the crop. 



The problem has to be simplified by restricting attention 

 to the common agricultural crops and interpreting fertility to 

 mean the capacity for producing heavy crops regardless of any 

 subtle distinctions of quality. Four factors then come into 

 play : an adequate supply of air and water to the roots, a 

 sufficiently rapid production or solution of food material, a 

 sufficiently rapid movement of nutrient substances to the plant 

 roots, and absence of harmful agencies. These have already 

 been discussed in Chapters III. and VI., where also it is 

 shown that the three are not independent, but related to one 



