16 INTRODUCTION. I. 



Large quantities of magnesium compound are found asso- 

 ciated with potassium compounds in the Stassfurth deposits. 



Magnesium is always present in plants, but as a rule there 

 is far more present in a soil than is necessary for the crops' 

 requirements. Consequently it is not of much importance as 

 a fertiliser. 



Iron is very abundant in nature, generally in the state 

 of combination. Native metallic iron is occasionally found, 

 but in insignificant amount. As oxides (Fe. 2 8 and Fe H O 4 ) and 

 carbonate (FeCO H ) immense quantities occur in the minerals 

 known respectively as hcematite, magnetite, and spathic iron ore. 

 These compounds form the most valuable ores of iron. In com- 

 bination with other substances iron is also abundant. It is the 

 main cause of the red or yellow colour of soils. Two series of 

 compounds are known, ferrous salts, in which iron is divalent, 

 and ferric compounds, in which it is trivalent ; only the latter 

 are suited to the requirements of plants. 



Iron is essential as a plant food, but a very small quantity 

 suffices. It is rarely advisable, therefore, to use iron com- 

 pounds as manures. 



Manganese resembles iron in most of its properties. It is 

 always found in union with other elements, often with oxygen, 

 e.g., as pyrolusite, MnO 2 , psilomclane, BaO. 2Mn0 2 . It is ap- 

 parently not essential as a plant food, but is often found in the 

 ashes of plants. 



Aluminium is never found in the free state in nature. It is 

 extremely abundant and is one of the most important con- 

 stituents of most mineral silicates. In the form ot felspar and 

 mica, it enters largely into the composition of many igneous 

 rocks. By the action of water and carbon dioxide upon felspar 

 (K20.Al. 2 3 .6Si0 2 ) the potash is to a great extent removed and 

 a residue of kaolin or china clay, AL 2 8 .2Si0 2 .2H 2 0, is even- 

 tually obtained. Ordinary clay consists of a mixture of kaolin 

 with some incompletely decomposed felspar and therefore is 

 rich, in potash. Clay constitutes an important ingredient of 

 soils, to which it imparts valuable properties, especially as 

 regards retentive power for water and other substances. 



