THE SOIL IN RELATION TO PLANT GROWTH 151 



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 

 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 away from the farm. Phos- 

 phate exhaustion was the most serious occurrence, and as the original 

 supplies were not as a rule very great, it must have become acute by 

 the end of the eighteenth century in England, for remarkable improve- 

 ments were, and still are, effected all over the country by adding phos- 

 phates. Then began a process, which has gone on to an increasing 

 extent ever since, of ransacking the whole world for phosphates ; at 

 first the search was for bones, even the old battlefields not being 

 spared if we may believe some of the accounts that have come down ; 

 later on (in 1 842) Henslow discovered large deposits of mineral phos- 

 phates to which more and more attention has been paid. 



The crowding of the population into cities, and the enormous 

 cheapening of transport rates, led during the nineteenth century to the 

 adoption in new countries, particularly in North America, of what is 

 perhaps the most wasteful method of farming known : continuous arable 

 cultivation without periodical spells of leguminous and grass crops. The 



