126 THE FERTILITY OF THE SOIL [CH. ix 



through the loams to light blowing sands. The loams 

 are very fertile : the extremes of clay and of sand are 

 infertile. But there is no sharp end-point : a number 

 of soils near the limit may be productive under one 

 system of management and not under another, and 

 may be in cultivation during times of prosperity and 

 derelict when prices fall. 



In applying these general principles to any par- 

 ticular case a considerable amount of balancing of 

 probabilities is necessary. Means taken to alter the 

 physical condition of the soil may react on the micro- 

 organisms, the chemical composition, etc., and vice 

 versa. Above all, as the soil is cultivated for profit, 

 economic considerations come in at every turn. Thus 

 fertility problems are usually more complex than they 

 appear at first sight, and require to be studied in the 

 laboratory, the pot culture house and the field before 

 they can be regarded as solved. 



BIBLIOGRAPHY 



HALL, A. D. The Soil. (Murray, 1911.) 



The Book of the Rothamsted Experiments. (Murray, 1905.) 

 HOPKINS, CYRIL G. Soil Fertility and Permanent Agriculture. 



(Boston, 1910.) 

 PROTHERO, ROWLAND E. English Fanning past and present. 



(Longmans, 1912.) 

 RUSSELL, E. J. Soil Conditions and Plant Growth. (Longmans, 



1912.) 



