IX.] CAUSES OF STERILITY 281 



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 

 generally caused in this country by the absence of 

 calcium carbonate from the soil. When this happens 

 on light sand)' land it will become evident by the 

 tendency of black mild humus to accumulate, by the 

 paucity of leguminous plants in the herbage, and by the 

 prevalence of fungoid diseases like " finger-and-toe." 

 On strong lands, and when accompanied by water- 

 logging, black acid peat accumulates : the soil shows 

 an acid reaction, oxide of iron forms below the surface, 

 and the soil water contains soluble iron salts, as is seen 

 by the iridescent scum which spreads over any water 

 standing in the ditches. 



Another source of sterility is the presence of un- 

 oxidised iron salts in the soil : many clay subsoils are 

 coloured dark blue or green by double ferrous silicates 

 like glauconite, or by finely disseminated iron pyrites. 

 Until these materials become oxidised to ferric hydrate, 

 the soil remains sterile : particularly is this the case 

 with iron pyrites, which in the form of marcasite easily 

 oxidises to yield both ferrous sulphate and sulphuric 

 acid. Voelcker has recorded three instances of soil 

 sterile through these causes: one was land reclaimed 

 from the bed of the Haarlem Lake, which contained 0-71 



