242 CAUSES OF FERTILITY AND STERILITY [chap. 



want and the excess of water due to texture and 

 situation, deficient aeration, the absence of calcium 

 carbonate, and the toxic action of certain com- 

 pounds, such as the salts of magnesia, iron pyrites or 

 ferrous salts generally, 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 perma- 

 nent 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 

 common heath land, never having been worth the 

 expense of enclosing. Allusion has already been made, 

 under the head of drainage, to the evils which ensue in 

 a waterlogged soil : from time to time clays are met 

 with of so close a texture that the vegetation suffers in 

 an analogous manner through deficient aeration. On 

 certain areas of the Oxford Clay and London Clay, and 

 the Boulder clays derived therefrom, pastures degenerate 

 after a few years into a mass of creeping rooted plants 

 like bent grass, and the land must be broken up afresh 

 in order to aerate it before any crop can be grown. 



Sterility due to chemical causes is perhaps most 



