21 



acid, which is readily absorbed by plant life. The mechanical condition 

 of a soil is much improved by its presence when in moderate quantities ; 

 but when present in excessive amount, it acts injuriously by deoxidiz- 

 ing ferric salts and in other ways.* 



THE INORGANIC OR MINERAL PORTION OF THE SOIL 



Is, with the addition of alumina, composed of the same substances as 

 make up the inorganic portion of plants, and which form their ashes 

 when burnt. The mineral soil constituents include the following sub- 

 stances: Silica, SiO 2 ; alumina, A1 2 O 3 ; lime carbonate, CaCO 3 ; ferric 

 oxide, Fe 2 O 3 ; phosphoric acid, P 2 O 5 (phosphoric anhydride) ; potash, 

 K 2 O ; Soda, Na 2 O ; magnesia, MgO ; chlorine, Cl ; sulphuric acid, SO 3 

 (sulphuric anhydride). 



These exist in very different proportions in different soils. The first 

 three, sand, clay, and lime, represent more than 90 per cent, of the 

 substance of most soils, and as one or the other predominates the soil 

 is said to be sandy, clayey, or calcareous. The most active constituents 

 of the feoil, phosphoric acid and the two alkalies, potash and soda, occur 

 in very small quantities, as do the other and less important constituents, 

 magnesia, chlorine, and sulphuric acid. 



Silica exists in different proportions in the various soils, mostly in an 

 insoluble state, and that most largely in the poorest sandy soils. Fer- 

 tile soils contain generally a very small quantity of it in a soluble form. 

 Sandy soils contain from 70 to 90 per cent, of silica, even stiff clay soils 

 from GO to 70 percent., and calcareous or lime soils and marls from 20 to 

 30 per cent. 



Its value, as a source of plant food, consists in being in the form of 

 soluble silicates. In its insoluble state, like quartz sand, its action is 

 merely mechanical, making the soil lighter for cultivation. Those soils, 

 derived from rocks of which feldspar is one of the constituents, will 

 contain some silica in a soluble form, whilst those derived from quartz- 

 ose rocks will contain it in the insoluble state. The hydrated silica, 

 in the analyses, represents that which is gradually available for plant 

 food. 



Alumina or clay is a silicate of aluminium, and is derived from the 

 disintegration of feldspathic rocks and other similar silicates; if ab- 

 solutely pure it would furnish nothing for plant food; as, however, 

 this is seldom the case, it furnishes a supply of potash frequently in 

 considerable quantities. Clay has the important property of absorbing 

 and retaining phosphoric acid, ammonia, potash, lime, and other sub- 

 stances necessary for plant food. Clay soils contain on an average from 

 6 to 10 per cent, of alumina. In sandy soils it varies from 1 to 4 per 

 cent. ; and in marls, calcareous soils, and vegetable molds from 1 to 

 6 per cent. 



* Versuchs Statioiieu Organ, vol. xiv, p. 248-300. 



