MAINTAINING SOIL FERTILITY 319 



of animals. This is used to explain the value 

 of a rotation of crops. We have been accustomed 

 to believe that the reason why a rotation of crops 

 results in increased yields is because the different 

 feeding habits of the crops bring a larger area of 

 soil under tribute, and equalise the demand upon 

 it; because it improves the texture of the soil; be- 

 cause it alleviates weediness, disease and other 

 difficulties. The new explanation is that the bene- 

 fit of rotating crops is not due so much to those 

 factors although their importance is not denied 

 as to the fact that a rotation puts a new kind of 

 plant into a soil that has become clogged with the 

 excretions of the old crop and which has therefore 

 become so "unsanitary" that the old plants can- 

 not grow well. The new plant is not injured by 

 the excretions of its predecessor and so makes a 

 vigorous growth. 



The second radical change of view that the new 

 theory introduces is in regard to the action of 

 manures and fertilisers. We have been believing 

 that the value of supplying manures and fertilisers 

 to the soil is that they actually enrich it with the 

 plant food they contain and that this plant food 

 that we apply is actually needed by the crop and 

 is used by it. The new conception is that manures 

 and fertilisers are valuable chiefly because they 

 aid in renovating the soil, or in cleansing it of the 

 plant excretions, or. "toxic" matter, although they 

 do supply plant food. In other words, fertilisers 

 are chiefly beneficial not because they enrich soil but 

 because they purify it. They act not upon the plants 

 but upon the soil; they purify the soil from the 

 excreta of the crop that nas oeen grown and so 

 affect the growth of the crop that is to be grown. 



No soil physicist would champion a theory that 



