6 



THE SOIL AND THE PLANT 



have taken place in the transformation of a micaceous gneiss to a 

 soil, by the decomposition processes which occurred in situ under 

 weathering agencies of a humid climate. 



The general changes in the chemical composition during the 

 process of rock weathering and the formation of the earth's crust 

 consist in the separation of the silica and of the bases, the oxida- 

 tion of the compounds of iron, the removal of bases by processes of 

 leaching and replacement, the general hydration of the remaining 

 silicates, aluminum and iron, and a very appreciable addition of 

 organic matter coincident with the invasion of plants and of 

 microorganisms. 



In order to characterize soils as to texture they may be ana- 

 lyzed mechanically, to separate the particles into groups of cer- 

 tain sizes including various sands, silt and clay. Typical mechan- 

 ical analyses of two soils are given in Table 4. 



TABLE 4 



Classification of Soil Particles and Mechanical Analysis of two 



Soils* 



Fraction 



Fine gravel . . . 

 Coarse sand . . 

 Medium sand. 

 Fine sand . . . . 

 Very fine sand 



Silt 



Clay 



Size 



Millimeters 

 2.00-1.00 

 1.0 -0.50 

 0.50-0.25 

 0.25 0.10 

 0.10-0.05 

 0.05-0.005 

 0.005 and below 



Fine sandy 

 loam 



Per Cent 



1 



2 



3 

 22 

 35 

 24 

 13 



Clay 



Per Cent 



1 



2 



2 



6 

 11 

 41 

 37 



* On the basis of methods used at the U. S. Dept. of Agriculture. 



Effect of Climate upon Chemical Composition of Soil. — 

 Climate is the most important factor determining the type of 

 changes brought about in the transformation of the rock con- 

 stituents and therefore in the development of the soil. Among 

 the various climatic factors, temperature and precipitation are 

 of major importance. Differences in climate affect the rate of 

 change, the course of mechanical and chemical transformation, 



