36 Agricultural Chemistry. 



Minerals. The following minerals are abundant and of the 

 greatest importance to agriculture: 



Quartz is chemically a compound of silicon and oxygen. It 

 is estimated that it forms 35 per cent of the solid crust of the 

 earth. It is one of the hardest and most durable of substances 

 and is practically insoluble in water and but little affected by 

 the weather. Sea sand and the sands along the shores of our 

 fresh water lakes are often almost wholly made up of fine grains 

 of quartz, worn smooth by continuous agitation to which they 

 have been subjected. Fragments of quartz, consisting of crystals 

 rounded and worn by mechanical rubbing against each other, 

 form the largest constituent of many soils. Such sand is lack- 

 ing in plant food. 



Feldspars are probably the most abundant of all minerals and 

 constitute, it is estimated, 48 per cent of the earth 's crust. Chem- 

 ically, the feldspars contain silicon, oxygen and aluminum in 

 combination with either sodium, potassium, or calcium, and are 

 called by the chemist, silicates. 



The chief varieties of feldspars are 



Orthoclase potassium aluminum silicon oxygen 



Albite sodium aluminum silicon oxygen 



Labrador! te sodium aluminum silicon oxygen 



calcium 



Orthoclase or potash feldspar is the most important. It is a hard 

 mineral, often colored pink or green, though sometimes white. 

 Although hard, it is easily attacked by water and carbon dioxide, 

 the potassium being largely removed in solution while the final 

 residue is kaolin or China clay. Orthoclase furnishes a consider- 

 able quantity of the potash found in our soils. 



Mica is another abundant mineral and characterized by its 

 tciid'-ncy to split into thin elastic plates. It is essentially a com- 

 pound of aluminum, potassium, silicon and oxygen, though it 

 usually contains iron and often calcium or magnesium. Mica 

 also suffers decomposition under the influence of the weather, 

 but not so readily as the feldspars. It furnishes plant food in 



