Analysis of Soilsi. a^o 407 



gravity of a soil, but of this I could never feel conviction. 

 That a peat soil, that is, one containing a great excess of 

 vegetable matter, is much lighter in weight than such as 

 contain more of earthy constituents, is certain ; but such 

 do not require their specific gravity to be taken to detect 

 them. If a soil is but rather above or under the average 

 specific gravity, I do not see how the knowledge of that can 

 determine whether the excess of weight arises from silica or 

 carbonate of lime ; or the deficiency of weight, from vegetable 

 matters, alumina, or other light constituent. The specific 

 gravity of silica is 2-66; of carbonate of lime 2*7 ; of alumina 

 only 2. The unproductiveness of a soil usually arises from 

 the excess of some one of the usual constituents which are enu- 

 merated in the foregoing imaginary analysis, rather than from 

 the admixture of any foreign substance prejudicial to vegetation. 

 In a previous communication (Vol. III. p. 270.), I have 

 given the constituents of a fertile soil in detail, and to what I 

 have stated there I have little to add. I have also stated, in 

 another place, that a soil too retentive of moisture is seldom 

 met with, that cannot be rectified by the mechanical remedy of 

 underdraining. If it is purposed to ameliorate a soil which 

 contains too much alumina, by a surface application, much 

 judgment is necessary. The most obvious application is 

 sand, either from the sea-shore or drift, road scrapings, coal 

 ashes, &c. : but if these are not applied largely, the soil is 

 rendered even worse and more difficult of cultivation ; for I 

 have seen such soils, which have had a slight dressing of 

 silicious matters as above enumerated, rendered thereby so 

 appi'oaching in constitution to brick earth, that in dry weather 

 they have become so hard as to defy any power but that of a 

 volcano to break them up. A soil is not rendered sterile by 

 an excess of alumina, unless it contains nearly 50 per cent of it ; 

 and, to such, nothing short of 40 tons of sand per acre would 

 be of unalloyed benefit. 



If a soil is unproductive, from containing too nmch silica, 

 the obvious application to improve its staple is clay and chalk. 

 Four hundred parts of soil of Bagshot Heath contain 380 

 parts of silicious sand. It is completely barren. Yet 

 Sir Humphry Davy, who made this analysis, found that a 

 good turnip soil in Norfolk contained 8 parts out of 9, 

 silicious sand. Such light soils, however, are more manage- 

 able, for they are always capable of tillage ; and the cultivator 

 can render them more absorbent and retentive of moisture, by 

 means of vegetable manures, chalk, &c. Such soils are 

 termed hungry, for the yard manure applied to them is soon 

 exhausted, and for this reason, that its mucilaginous and 



D D 4 



