36 THE PRINCIPLES OF SOIL MANAGEMENT 



it will be seen that the total loss from the rock is 13.47 

 per cent. Column III of Table III represents a gneiss from 

 Albemarle county, Virginia, under almost the same 

 climatic conditions as the granite. But the soil is a red 

 clay of the Cecil series, and represents a loss in transition 

 from the rock of 44.67 per cent, or three and one-half 

 times as much as from the granite. The composition of 

 the two rocks is not greatly different. The differences 

 in the two soils illustrate the two types of rock-decay. 

 The granite soil, which is very sandy, probably does 

 not represent the same advanced stage of decay as the 

 gneiss soil, and apparently has been subjected most 

 largely to disintegration, or physical breakdown. On 

 the other hand, the gneiss soil represents both the disin- 

 tegration and an advanced stage of chemical change or 

 decomposition. 



In general, the productiveness of a soil depends 

 even more on its physical characteristics than on its 

 chemical composition. The physical characteristics of 

 a residual soil depend quite as much on the stage 

 and type of decay to which it has been subject as to 

 its chemical composition. Mechanical processes, such 

 as abrasion and fracture due to impact, temperature 

 changes and frost, never produce the same fine texture 

 which may result from chemical processes, and therefore 

 such material is usually very sandy. A sand composed of 

 aluminum silicate minerals in large proportion is increas- 

 ingly subject to chemical decay, which will reduce it to 

 a gritty clay of progressive coarseness from the surface 

 downward. These principles may be summed up in the 

 statement that the characteristics of a soil are determined 



