LECT. V.] THE ROOT. SOILS. 215 



poses of vegetation ; and soils which contain much 

 of this earth are, therefore, barren and unpro- 

 fitable. In the form of sand it retains 25 per cent, 

 only of water ; while 100 parts of it, as it occurs 

 with clay in an arable field, retain 280 per cent, 

 of water. 



4. MAGNESIAN EARTH is not so commonly 

 found as the earths we have already noticed. The 

 magnesia it contains is combined with carbonic 

 acid, and mixed with siliceous particles. It ap- 

 proaches nearest to the nature of the clayey earths 

 in its power of retaining moisture ; that power 

 enabling it to retain 4^ times its own weight of 

 water. This renders it, when it predominates, very 

 prejudicial to vegetation ; while it increases, when 

 added in moderate proportion, the fertility of a 

 dry sandy soil. 



5. FERRUGINOUS EARTH consists of those oxyds 

 of iron known by the names of ochres and pyrites 

 mixed with siliceous matter. These oxyds, in parti- 

 cular the pyrites, when in any considerable quantity 

 in a soil, if it contains little calcareous matter, are 

 extremely injurious to vegetation. The pyrites is a 

 compound of sulphur and iron, and is converted by 

 exposure to air and moisture into sulphate of iron, 

 which destroys plants by over-stimulating them. 



Vegetable earths have the least specific gravity, 

 and sandy soils the greatest, whether they be dry 

 or moist: the vegetable earths contain, besides 



p4 



