24 SOILS. 



its productiveness, the mineral parts themselves require to be 

 mixed together in certain proportions, and in certain states of 

 division, in order to produce the greatest degree of fertility. 



Silica (sand) and alumina (clay) form the principal mineral 

 parts of the soil. If one or other of these earths be in excess, 

 the soil is defective in its composition. If the alumina prevail, 

 the soil is too adhesive if the silica prevail, it is too loose; a 

 medium is therefore best: and although the precise proportions 

 in which these two earths should exist have not been deter- 

 mined, it is safer that there should be a tendency to an excess 

 of clay than of sand. Further, the fertility of the soil depends 

 on the state of mechanical division of these minerals; that is, 

 the more thoroughly the parts are separated or pulverized, the 

 greater will be the increased fertility of the soil. 



Sometimes therefore we have the means of improving the 

 constitution of soils, by mixing sand with clay, or clay with 

 sand. But in practice, the very extensive mixing of these two 

 substances is rare, because the state in which sand and clay are 

 usually available for this purpose, it seldom happens that the 

 matter of both is in that state of minute division which is fa- 

 vourable to fertility. 



It is otherwise with the earth LIME. This can, in all cases, 

 be reduced by heat to that state of minute division which is 

 favourable to the productiveness of soils; and hence it can 

 always be supplied with benefit to those soils in which its pre- 

 sence is required. 



The composition of soils may be improved, as we have 

 shown, by the addition of animal and vegetable matter; and 

 also, in many cases, by the addition of those earths in which 

 they may be deficient; and in an especial degree of lime, which 

 we can always employ in the form of minute division, best 

 suited to improve. the composition of the soil. This is the 

 first of the means referred to of adding to the productive 

 powers of soils. 



The second mode referred to of increasing the productive 

 powers of soils, is that of altering their texture, depth and pro- 

 perties, by tillage and other means. The mere effect of that 

 separation of the parts of the soil which it undergoes in the 

 common operations of tillage, is seen to have a very beneficial 

 influence on its productive powers. Whether it imbibes from 

 the atmosphere any thing besides aqueous vapour or not, it is 

 known that the exposure of the matter of the soil to the at- 

 mosphere, and the separation or pulverization of its parts by 

 tillage, add permanently to its fertility. 



The first object of pulverization is to give scope to the roots 

 of vegetables, for without an abundance of roots or fibres no 



