430 .' ON FALLOW. 



in acids with great ease, while the silica is separated in its gela- 

 tinous and soluble form. Common potter's clay forms generally 

 very sterile soils, although it contains within it all the conditions 

 for the luxuriant growth of plants ; but the mere presence of 

 these conditions does not suffice to render them useful to vegeta- 

 tion. The soil must be accessible to air, oxygen, and carbonic 

 acid, for these are the principal conditions to favor the develop- 

 ment of the roots. Its constituents must be contained in a state 

 fit to be taken up by plants. Plastic clay is deficient in all these 

 properties, but they are communicated to it by a gentle calcina- 

 tion.* 



The great difference between burnt and unburnt clay may be 

 observed in places where burnt bricks are used for building. 

 In Flanders, where almost all the houses are constructed with 

 burnt bricks, the surface of the walls, after exposure for a few 

 days to the action of the weather, becomes covered with an efflo- 

 rescence of salts. When these salts are washed away by the 

 rain, a new efflorescence again appears ; and in some cases, as 

 the gateway of the fortress at Lille, this may be observed, even 

 though the walls have stood for centuries. The efflorescence 

 consists of carbonates and of sulphates with alkaline bases — salts 

 that are known to play a most important part in the economy of 

 vegetation. Lime exercises a striking effect upon these saline 

 efflorescences, for it may be observed, that they first appear in 

 those parts where the mortar and bricks come in contact. 



It is obvious that mixtures of clay and lime contain all the con- 

 ditions necessary for the decomposition of the silicate of alumina, 

 and for rendering soluble the alkaline silicates. Lime dissolved 

 in water by means of carbonic acid acts upon clay in the same 

 way that milk of lime does. This fact explains the favorable 

 influence of marl upon most soils, marl being a clay rich in lime. 

 Indeed there are certain marly soils surpassing in fertility, for all 

 plants, soils of any other kind. Burnt marl must be in a very 



* The author saw an example of this in the garden of Mr. Baker, at 

 Hardwick Court, near Gloucester. The soil consisted of a stiff clay, and, 

 from a state of complete sterility, had b;en made remarkably fertile, by 

 simple burning. The operation, in this case, was carried on to a depth of 

 three feet, — certainly not an economical, although a completely successful 

 •experiment. 



