14 



compound. The highly concentrated fertilizers of commerce 

 only in part meet this requisition, as they only contain the most 

 necessary ingredients of plant food, and many of the physical 

 uses of farm manure would be lost if they were used alone, such 

 as heat, humus, porosity. 



CHAPTER III. 



THE SOIL. 



ORIGIN OF SOILS. 



The greater part of the surface of the land is covered by a 

 mixture of stones of various sizes, clay, sand, and other mineral 

 substances, together with a variable quantity of decaying vege- 

 table matter this is the soil. In some places it exists onl}- in 

 patches and thin layers on the surface of rocks, and affords only 

 a scant growth of mosses, lichens, etc., while elsew here it will be 

 found as a deep mass of vegetable mould, so fertile that the 

 crudest cultivation will produce heavy crops of ail the crops 

 necessary for man and animals. Between these extremes are 

 found all the varieties with which we are familiar. 



Soils are classified agriculturally, according to their composi- 

 tion and texture, into sandy, containing only ten per cent, clay, 

 sandy loam, containing 30-40 p. c. clay loam containing 40-70 p. c. 

 of clay. When it contains 70-85 p. c. of clay, it is called a clay 

 loam; from 85-90 of clay a strony day, fit for bricks, and if it 

 contains no sand, it is called pipe clay. If a soil contains more 

 than five per cent, of lime it is called &marley soil ; if more than 

 twenty per cent, a calcareous soil, and if the soil consists almost 

 entirely of vegetable matter, it is called a peaty soil or bog earth. 



Geologically, soils are divided into two kinds, soils of disinte- 

 gration and soils of transport. The.former are found lying on the 

 rocks from which they were formed by the mechanical and 

 chemical action of the atmosphere, and having the same com- 

 positions as the rocks ; while the latter have been transported 

 by the agency of winds, waters, or glacial action, to a distance 

 from their source, and are found on rocks having a far different 

 composition from the overlying soils. 



All soils originate in the disintegration of rocks which form 

 the surface of the earth, chiefly by the action of the atmos- 

 phere. This action, as we have said, is of two kinds, mechanical 

 and chemical. The mechanical action consists in the power ex- 

 erted by volcanoes, floods, ice, and snow; also the abrading 

 action of winds, which by forcing small particles of sand 

 against the surface of rocks, wears them away as with a file. 

 The chemical action is exerted by the elements of the atmos- 



