LAND AND OTHER AGENTS OF PRODUCTION 173 



markets and suitability to his conditions of soil and climate. To a 

 certain extent it is possible to bridge the gap between plant require- 

 ments 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 infer- 

 tility 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 distinc- 

 tions 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. We 

 have seen that the compound particles can be altered considerably by 

 human efforts, within the limits fixed by the properties of the unalter- 

 able ultimate particles. In trying to improve the 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 deter- 

 mine what this is. 



4. The food supply can be increased by the addition of fer- 

 tilizers, the plowing-in of green leguminous crops, feeding cake on 

 the land, etc. 



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

 (England) to the removal of organic matter, calcium carbonate, and 

 some of the food (often the nitrogen compounds), and 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 method of estimation having 



