38 



SOILS: FIIOFERTIES AND MANAGEMENT 



as a soil amendment, and often where it is found pure 

 enough in quality and in sufficiently large quantities it 

 is handled commercially. When it contains large amounts 

 of phosphorus, as it does in some cases, it may be used 

 as a fertilizer. The following analyses ^ show the general 

 character of this soil : — 





1 



2 



SiO" 



25.28 



3.02 



37.52 



.12 



.22 



.25 



.40 



29.02 



4.17 



5.65 



AI2O3 . FeiOg 



CaO 



MgO 



K2O 



NaaO 



P>05 



3.30 



48.51 



1.96 



.23 



.30 



Trace 



CO2 



Igiiitioa 



39.80 

 .25 





.*-'^ 



28. Colluvial soils. — This class of soil is not of great 

 importance, first because of its small area and its inac- 

 cessibility, and secondly because it is usually a coarse, 

 loose soil, rather unfavorable for plant growth. It is 

 formed, as its name indicates, in regions of precipitous 

 topography, and is made up of fragments of rocks de- 

 tached from the heights above and carried down the 

 slopes by gravity. Talus slopes, cliff d6bris, and other 

 heterogeneous rock detritus are examples of colluvial 

 soil. Avalanches are made up largely of such material. 

 As the physical forces of weathering are most active in 

 the formation of these soils, the amount of solution and 

 oxidation is small. The upper part of the accumulation 



* 1 Kerr, W. C. Geology of North Carolina, Vol. I, p. 195. 

 1875. 



