GEOLOGICAL CLASSIFICATION OF SOILS 51 



the ocean to be deposited and stratified, possibly after being 

 pounded and eroded by the waves for years. At last came 

 the emergence above the sea and the action of the forces 

 of weathering in situ. The latter effects are of great moment 

 since they determine not only the topography but the fertility 

 of the new soil as well. The availability of the nutritive ele- 

 ments, and especially the amounts of organic matter, are de- 

 termined by recent and still active forces. 



The marine soils of the United States, while younger than 

 most of our residual soils, are usually more worn and gener- 

 ally carry less of the nutrient elements. Their silica eon- 

 tent is very high and they are often sandy, especially along 

 the Atlantic seaboard. Sands, sandy loams, and loams pre- 

 dominate, although silt loams and clays are by no means un- 

 usual, especially in the Atlantic and Gulf coastal flatwoods 

 and the black prairies and interior flatwoods of Alabama and 

 Mississippi. The organic content of the sandy soils is gen- 

 erally low, but on the heavier types it may almost equal delta 

 and flood plain soils. 



A direct comparison ^ between typical coastal plain and 

 residual soils usually shows the former to be considerably 

 higher in silica but lower in iron and aluminium. The marine 

 soil is, on the other hand, lower in phosphoric acid and 

 potash. The nitrogen, organic matter, and lime are so vari- 

 able in both soils that no reliable deductions can be drawn. 

 The following data from Eastern United States substantiate 

 the above generalizations.^ (Tables VIII and IX, page 52.) 



The soils of the Atlantic and Gulf coastal provinces, formed 

 as vast outwash plains and occupying 11 per cent, of the area 

 of the United States, are very diversified, due to source of 



* When soils are compared on the strictly chemical basis great caution 

 should be observed in drawing conclusions as to relative productivity. 

 The amount of a nutrient present is by no means a measure of its 

 availability. A chemical analysis usually throws but little light on the 

 fertilizer needs of a soil. 



*See also Walker, S. S., Cliemcal Composition of Some Louisiana 

 Soils as to Series a7id Texture; La. Agr. Exp. Sta., Bui. 177, Aug., 1920. 



