320 SOIL CONDITIONS AND PLANT GROWTH 



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 

 considerably by human efforts, within limits fixed by the 

 properties of the unalterable 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 drain- 

 age. 



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 stand for, remain 

 untouched. A distinction is therefore made between the 

 temporary fertility or "condition" within the cultivator's 

 control, and the "inherent" fertility that depends on the 

 unalterable ultimate particles. Of course, the distinction is 

 very indefinite and, in practice, wholly empirical, no proper 

 methods of estimation having yet been worked out, but it is 

 of importance in compensation' and valuation cases. 



Serious soil exhaustion did not arise under the old 

 agricultural conditions where the people practically lived on 

 the land and no great amount of material had to be sold 



