Ill] INJURIOUS FACTORS 37 



to no hypothesis), it can be remedied by drainage, 

 lime, and good cultivation. 



Cases are recorded of infertility arising from 

 excess of iron or of manganese in the soil, but no 

 satisfactory evidence is afforded in proof. Drainage, 

 lime and good cultivation are here also found to be 

 beneficial. 



"Sickness" of soil has in the past been attributed 

 to the presence of a toxin, but more recent work 

 indicates that it is biological in nature and remedied 

 by partial sterilisation. 



The alkali soils of dry regions owe their sterility 

 to excess of soluble salts; they may be treated by 

 drainage (which is a first essential), irrigation, and 

 addition of gypsum where much sodium and potas- 

 sium carbonates are present. Much interesting work 

 remains to be done in elucidating the causes of 

 infertility of certain special soils. 



Looking back over these various fertility factors 

 we see that they are not a mere tangle of unrelated 

 things, but are very closely connected one with the 

 other. The water supply, air supply, and temperature 

 are to a large extent mutually interdependent, and 

 changes in any of them are reflected in the food 

 supply. We can simplify matters by selecting three 

 as the leading factors in normal cases: the water 

 supply, food supply, and stock of calcium carbonate ; 

 when these are satisfactory it will usually be found 



