MINERALS UNESSENTIAL OR INJURIOUS TO SOILS. 75 



(B.) MINERALS UNESSENTIAL OR INJURIOUS TO SOILS. 



The minerals heretofore mentioned contribute to soil forma- 

 tion either one or several ingredients, important to plant growth 

 either by their mechanical or chemical action. It remains to 

 consider some not intrinsically desirable, but frequently pres- 

 ent in certain soils, which should be known to the farmer in 

 order that he may be enabled to counteract or remove their 

 injurious effects. Leaving aside such as are of only casual or 

 rare occurrence, the following may be mentioned as among 

 those which not unfrequently affect soils desirable for culture 

 to such extent as to make them unavailable for general farming 

 purposes : 



Iron Pyrites; sulphid of iron containing two molecules of 

 sulphur to one of iron; a mineral exceedingly common in de- 

 posits of metallic ores, and whose deceptive gold-like color has 

 caused it to be mistaken for gold so often as to cause it to be 

 designated as " fool's gold " among miners. While it fre- 

 quently does contain some gold and is often associated with 

 valuable ores, it is practically valueless when occurring outside 

 of mineral veins, in rock masses ; and more especially in sedi- 

 mentary rocks, such as sandstones, limestones, shales and clays. 



When present in soils it sometimes becomes a source of 

 trouble to the farmer, because in contact with air it is soon 

 transformed into ferrous sulfate or copperas, which, like the 

 carbonate referred to above, is injurious to plants. Sometimes 

 indeed iron pyrites is actually formed in badly-drained soils 

 alongside of the carbonate of iron, when much sulfate (such 

 as gypsum) is present; and then its injurious effects subside 

 more slowly than do those of the carbonate (see above, p. 46). 



Recognition of Iron pyrites. The mineral is easily recognized by its 

 golden or brass-yellow tint ; the latter color being the one most com- 

 monly shown in the " sulphur balls " occurring in marls or soft lime- 

 stones. A very easy test is to pulverize it and then heat it on a shovel 

 over a fire, when it will soon itself take fire, burning with a blue sulphur 

 flame, and upon more complete roasting, leaving behind a red powder, 

 viz., " Venetian red" or red ochre; that is, ferric oxid. In clays it 

 commonly occurs in large, well-defined cubes, which do not readily 

 form copperas but rather become covered with a crust of limonite 

 or brown iron ore. 



