SOIL FORMATIONS 



19 



in the growth of plants. The following table shows the 

 partial composition of an Arkansas limestone and of the 

 clay soil formed from it, also the percentage of each of the 

 constituents lost in the process : 



Table 2. — Partial Composition op Limestone Rock and Its 

 Residual Clay 



It will be seen from the above table that lime, magnesia 

 and potash have disappeared in large quantities, as has also 

 iron, but that silica has lost little or none of what was orig- 

 inally present, and now constitutes by far the larger part 

 of the soil. Silica although not of great importance as a 

 plant nutrient is, nevertheless, of value in crop production, 

 because it contributes to the formation of the absorptive 

 compounds before mentioned. 



The great age of residual soils has also led to changes in 

 the composition of iron compounds, producing usually those 

 of a red or yellow color, these colors being characteristic of 

 residual soils. The long period of weathering has frequently 

 resulted in wearing down the particles to such a degree of fine- 

 ness that heavy soils of the nature of clay, clay loam or silt 

 are produced. 



Analyses of two typical residual soils from Virginia, that 

 have been formed from gneiss and limestone respectively, 

 are given in the following^table : 



