28o CAUSES OF FERTILITY AND STERILITY [chap. 



the ring to yield the results there shown. Doubtless 

 an investigation of the nature and distribution of the 

 bacteria and micro-fungi in and about a " fairy ring " 

 would throw further light on the varying fertility of 

 such closely neighbouring areas of soil, but no data are 

 at present available. 



Sterility of Soils. 



Few soils occurring in this country can be described 

 as absolutely barren, yet from time to time land is met 

 with which yields such poor crops that it may fairly 

 be designated as sterile. The causes of sterility are 

 various; amongst them may be enumerated both the 

 want and the excess of water due to texture and 

 situation, deficient aeration, the absence of calcium 

 carbonate, and the toxic action of certain salts, such as 

 the sulphates of soda and of magnesia, and common 

 salt itself. An acid reaction of the soil, which is highly 

 prejudicial to vegetation, is generally brought about by 

 one or other of the causes enumerated above. 



The sterility brought about by a deficiency of water 

 is only seen in this country when the soil is so entirely 

 composed of coarse sand that it possesses no retentive 

 power for the rainfall ; even then the absolutely bare 

 condition does not persist long, and may be attributed 

 as much to the lack of nutriment as to the want of 

 water. Little by little vegetation is found to creep over 

 recent deposits of coarse sea-sand and shingle, until a 

 turf is established. As a rule, such deposits have per- 

 manent water at a comparatively short distance below 

 and by this the vegetation is maintained ; but where a 

 coarse, open-textured sand occupies the uplands, as on 

 the Bagshot and Lower Greensand formations of the 

 south of England, or the Bunter beds of the Midlands, 

 the soil is kept so poor that it has largely remained 



