LECT. V.] THE ROOT. SOILS. 213 



of the components of sap cannot be obtained, 

 owing- to that fluid, as it advances, even in the 

 vessels of the root, dissolving some already di- 

 gested vegetable matter, which had been depo- 

 sited there in the preceding autumn, we are 

 obliged to form conclusions certainly not free from 

 error ; and to content ourselves with an approxi- 

 mation only to the truth. From the knowledge, 

 however, which we do possess I will endeavour 

 first to point out to you the known general com- 

 ponents of natural soils ; secondly, what part of 

 these is taken up as food by plants ; and thirdly, 

 in what manner, and by what means, soils are 

 improved and rendered more productive ; or to in- 

 vestigate the general nature of manures. 



Every soil fit for yielding nutriment to vege- 

 tables may be supposed to consist of earth, water, 

 air, a small proportion of metallic oxyds, and de- 

 composing vegetable or animal matters, in which 

 are included salts, gases, and vegetable extracts. 



Earth, which is the essential basis of all soils, 

 is, as it is commonly spoken of, a compound of 

 different earths ; the most general of which are 

 Calcareous earth, Argillaceous earth, Siliceous 

 earth, Magnesian earth, and Ferruginous earth. 



1. CALCAREOUS EARTH comprehends lime, usu- 

 ally combined with carbonic acid, in the state of 

 limestone, chalk, shells, and marl which is a 

 mixture of 'carbonate of lime with clayey and 



p3 



