130 SOIL CONDITIONS AND PLANT GROWTH 



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 : 



1. The water supply may be increased by deepening the soil, e.g., 

 by breaking a " pan," by enriching the lower spit, or other device, while 

 the air supply can be increased by drainage. 



2. The compound particles may be built up by proper cultivation 

 and the addition of organic matter (e.g., dung, green manuring, etc.) 

 and of calcium carbonate. 



3. Sufficient calcium carbonate must be added for the needs of the 

 crop and the micro-organisms nothing but a field trial can determine 

 what this is. 



4. The food supply can be increased by the addition of fertilisers, 

 the ploughing-in of green leguminous crops, feeding cake on the land, 

 etc. 



Conversely the " exhaustion " of soil is limited in our climate to the 

 removal of organic matter, calcium carbonate, and some of the food 

 (often the nitrogen compounds), and the destruction of the desirable 

 compound particles ; the ultimate particles, and all the possibilities they 



