COMPOSITION OF RESIDUAL SOILS 37 



by two factors: (1) The original chemical and physical 

 composition of the rock. (2) The relative prominence of 

 physical and chemical processes in its formation. These 

 facts make possible the existence of a full series of soil 

 from any group of rocks. 



The composition of other rocks and soils than those 

 mentioned above are shown in Columns V to X of Table 

 II. For comparative purposes, Column XI is also of 

 great interest, as showing the average composition of 

 over 700 bulk analyses of igneous rocks as given by Clark. 

 This gives some idea of the relative abundance of the 

 several plant-food constituents in the rocks. It will be 

 noted that the least abundant elements, sulfur and 

 phosphorus, are present in amounts of several thousand 

 pounds per acre foot. 



Columns XII, XIII and XIV give the analysis of a 

 composite of many samples of sandstone, shales and 

 limestones. The first two may be considered as ancient 

 soils, and their average composition of the mineral 

 elements should be much the same as modern soils of 

 the same origin. 



Columns XIV to XVI give the composition of lime- 

 stones, and of a residual soil from such a rock in Ar- 

 kansas. From a comparison of the first two columns, 

 it will be found that the rock from which the soil is 

 derived is far from the average, especially in the amounts 

 of manganese and phosphorus it carries. A study of 

 the soil analysis also shows that, while it is derived 

 from a lime rock, it is not rich in lime, a condition not 

 uncommon. 



Turning now to Table III, there is given the propor- 



