154 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



carbon dioxide and water upon many silicates containing lime 

 and magnesia. Both compounds are slightly soluble in water, 

 and more soluble in water containing carbon dioxide. All waters 

 which have been in contact with the earth contain lime and 

 magnesia. Large limestone deposits have been formed by shell 

 fish and other organisms, which withdraw carbonate of lime from 

 solution. Many of our most fertile soils are derived from lime- 

 stone deposits, or contain two per cent, or more of carbonate of 

 lime. Carbonate of lime is an important soil constituent. Its 

 presence flocculates clay and makes clay soils less sticky and more 

 easily worked. In calcareous soils, the phosphoric acid and 

 potash is generally held in more available forms. Carbonate of 

 lime unites more or less slowly with soluble phosphates which 

 may be present or introduced into the soil, forming compounds 

 which, while less soluble than before and of less value to plants, 

 are more soluble and apparently of greater value to plants than 

 the soil compounds of phosphoric acid with iron or aluminium. 

 Other actions of lime will be referred to later. 



Sulphate of lime is found in small quantities in many soils, in 

 large quantities in a few soils. 



Iron Minerals. By the decomposition of silicates containing 

 iron, various compounds of iron are produced, mostly oxides and 

 carbonates. These bodies ordinarily occur in the soil, often 

 giving the soil a red, brown, or yellow color, depending on the 

 stage of oxidation. Hematite is anhydrous ferric oxide Fe^CX, 

 red when finely powdered. Magnetite Fe ; ,O 4 is black oxide of 

 iron. Limonite is hydrated ferric oxide 2Fe 2 O,3H 2 O, and has a 

 yellow or brown color. Siderite is carbonate of iron, and is gray 

 or brown in color. It is affected with difficulty by cold acids, 

 easily by hot acids. The bulk of the phosphoric acid of ordinary 

 soils is probably in combination as basic phosphates of iron or 

 aluminium, or in basic silicates of these elements. Such forms of 

 combination are apparently not as valuable to the plant as the 

 calcium phosphates. Limonite is deposited from water contain- 

 ing iron. Oxides of iron are reduced by decaying vegetable 

 matter and combine with carbonic acid to form ferrous car- 



