308 



ANNUAL REPORT OF THE 



Off. Doc. 



II. SOILS, THEIR NATUEE AND ORIGIN. 



Rocks form the solid crust of the earth. These when brought under 

 the intluence ©f the atmosphere, frost and pej'colating waters, etc., 

 are broken and disintegrated, forming a layer of loose materials con- 

 stituting the soil. The nature of these disintegrations and their re- 

 sulting products vary with each mineral, and hence the character 

 of the soil is largely dependent upon the mineral composition of the 

 underlying rocks. 



Let us take as an example a region underlaid by some rock of the 

 granite family, such as is found under a considerable portion of 

 southeastern Pennsylvania. Such rock will contain the following 

 minerals: Quartz, orthoclase feldspar, plagioclase feldspar, biotite, 

 hornblende, and accessory apatite and magnetite. The composition 

 of these several minerals will be represented in the following table: 



Table I. 



Name of Mineral. 



Chemical Composition. 



Quartz 



Orthoclase 



Plagioclase 



Biotite 



Hornblende 



Apatite 



Magnetite 



Silica, 

 do. 

 do. 

 do. 

 do. 



Phosphoric 

 acid. 



From the table it is seen that in the granite rock under consider- 

 ation most of the mineral elements necessary to plant growth exist, 

 hence a soil formed from its decay will contain the basis of fertility. 



The process by which this rock becomes converted into soil is some- 

 thing as follows: The orthoclase, the plagioclase, the biotite and 

 the hornblende in the above list are compounds of silica and alum- 

 ina, various alkalies and earthy materials as potash, soda, magnesia, 

 lime and iron. These latter compounds are slowly dissolved out of 

 their respective minerals by surface waters, rain and atmospheric 

 moisture, more or less charged with carbonic, nitrous, nitric and 

 various organic acids until there is left behind only the silica and 

 alumina, which combined with water form clay. The hard minerals 

 just mentioned, bound together into a rocky mass are thus converted 

 into a soft plastic material. The quartz on the other hand, remains 

 iindecomposed, but its grains are set free by the disintegration of the 

 other minerals, and there results more or less sand, which, mixed 

 with the cliiv, tends to loosen the latter and give it the character of 

 an arable soil. It is not to be understood that all of the above min- 

 erals undergo dissolution uniformily. The feldspars begin to die- 



