150 SOIL CONDITIONS AND PLANT GROWTH 



countries that export large amounts of animal and vegetable produce. 

 But the transfer is prodigiously wasteful ; enormous losses arise in vir- 

 gin countries through continuous cultivation (p. 83), and at this end 

 in making dung (p. 91), 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 re- 

 sources 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 some- 

 thing. But in practice the agriculturist can only find use 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 con- 

 ditions 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 capa- 

 city for producing heavy crops regardless of any subtle distinctions of 

 quality. Three factors then come into play : an adequate supply of 

 air and water to the roots, a sufficiently rapid production or solution 

 of food material, and absence of harmful agencies. These have already 

 been discussed in Chapters III. and V., where also it is shown that the 

 three are not independent, but related to one another, inasmuch as they 

 are all directly bound up with the nature of the compound particles, 

 and, therefore, with the ultimate particles as revealed by mechanical 

 analysis, and with the amounts of calcium carbonate and of organic 

 matter. 



We have seen that the compound particles can be altered consider- 

 ably by human efforts, within limits fixed by the properties of the un- 

 alterable ultimate particles. In trying to improve a soil, therefore, four 

 courses are open : 



