4« PLANT SUCCESSION AND CROP PRODUCTION 



forest, many of which are on the edge of their ranges. The drain- 

 age systems are to a large degree responsible for this condition. 



B. The Soils 



Soils represent the action of plants upon finely divided min- 

 eral materials. In the broadest sense the term soil means the 

 unconsolidated mixture of mineral and organic substances in which 

 green plants grow. This forms a covering, varying from a few 

 inches to many feet in thickness, over the rock surface of the 

 earth's crust. In a more restricted agricultural sense the soil is 

 only the surface eight or ten inches of this material. It is the 

 upper part which contains the largest proportion of the roots of 

 plants and which has become dark in color from the organic con- 

 tent. All strata below this surface layer are termed subsoil. 

 After the mineral matter of the subsoil has been exposed to the 

 action of the weather and has become the home of plant and ani- 

 mal microorganisms, it gradually becomes transformed into soil. 



The mineral part of Ohio's soils were, for the most part, de- 

 rived from the geological formations underlying them. (Fig. 9). 

 One of the results of glacial action was the transportation of mate- 

 rial from north of Lake Erie. As the ice melted, the stones, clay, 

 and sand thus transported were left in a great mantle of drift on 

 top of similar finely ground material. Part of this transported 

 material became mixed with the soil existing at that time. Since 

 then much of the original drift has been carried away by stream 

 erosion, first as the ice melted, and second after it entirely dis- 

 appeared, by the normal stream flow. Some streams have cut 

 through to the original bedrock in many places. Others have 

 been forced by the accumulation of drift material, to cut entirely 

 new channels. The soils then, in the glaciated region, consist of bed- 

 rock material, surface and subsoil, plus a little transported soil 

 material and the organic remains of plant life which has not been 

 washed away since the beginning of post-glacial time. Coffey 

 (1912) gives a full discussion of the soil types of the state. 



In the unglaciated region of southeastern Ohio, the mineral 

 part of the soils has been derived from the underlying bed rock 

 exclusively. Since the mixing effects of glaciation were lacking, 

 the composition of the soil material corresponds closely to the 

 composition of the rock except as weathering and organic action 

 have altered it. The dissection of the Allegheny Plateau by 



