94 RELATIONS OF CLAY AND CALX TO PUTRESCENT MANURES. 



Aluminous earth also exerts some chemical power in attracting 

 and combining with putrescent manures, but too feebly to enable 

 a clay soil to become rich by natural means. [For though clays 

 are able to exert more force than sands in holding manures, their 

 closeness also acts to deny admittance beneath the surface to the 

 enriching matters furnished by the growth and decay of plants. 

 And therefore, before being brought into cultivation, a poor clay 

 soil would derive scarcely any benefit from its small power of com- 

 bining chemically with putrescent matters. If then it is con- 

 sidered how small is the power of both silicious and aluminous 

 earths to receive and retain putrescent manures, it will cease to 

 cause surprise that such soils cannot be thus enriched, with profit, 

 if at all. It would indeed be strange and unaccountable, if earths 

 and soils thus constituted could be enriched by putrescent manures 

 alone. 1835.] 



Davy states that both aluminous and calcareous earth will combine 

 with any vegetable extract, so as to render it less soluble (and con- 

 sequently not subject to the waste that would otherwise take place), 

 and hence " that the soils which contain most alumina and carbo- 

 nate of lime, are those which act with the greatest chemical energy 

 in preserving manures/' Here is high authority for calcareous 

 earth possessing the power which my argument demands, but not 

 in so great a degree as I think it deserves. Davy apparently places 

 both earths in this respect on the same footing, and allows to 

 aluminous soils retentive powers equal to the calcareous. But 

 though he gives evidence (from chemical experiments) of this 

 power in both earths, he does not seem to have investigated the 

 difference of their forces. Nor could he deem it very important, 

 holding the opinion which he elsewhere expresses, that calcareous 

 earth acts " merely by forming a useful earthy ingredient in the 

 soil/' and consequently attributing to it no remarkable chemical 

 effects as a manure. I shall offer some reasons for believing that 

 the powers of attracting and retaining manure, possessed by these 

 two earths, differ greatly in their degrees of force. 



The aluminous and calcareous soils of this country, through the 

 whole of their virgin state, have had equal means of receiving 

 vegetable matter ; and if their powers for retaining it were nearly 

 equal, so would be their acquired fertility. Instead of this, while 

 the calcareous soils have been raised to the highest condition, many 

 of the tracts of clay soil remain the poorest and most worthless. 

 It is true that the one laboured under acidity from which the other 

 was free. But if we suppose nine-tenths of the vegetable matter 

 to have been rendered useless by that poisonous quality, the re- 

 maining tenth, applied for so long a time, would have made fertile 

 any soil that had the power to retain the enriching matter. 



[Many kinds of shells are partly composed of gelatinous animal 



