00 TIUC 1M.A.NT LIFE OF MAKYLASD 



tained well but not too forcefully. The humus material of soils is 

 sponge-like and serves therefore to improve the water-holding 

 capacity of sands, at the same time that it may also improve the 

 texture of clays by increasing their aeration. The amount of air 

 contained by soils is the reciprocal of the water content, so it follows 

 that the aeration of sands is excellent while that of clays is poor. 



The relative importance of the chemical and physical features of 

 the soil in determining the character and distribution of the vegeta- 

 tion is a matter that does not admit of an absolute answer. That 

 the soil should contain the necessary elements of plant food, and in 

 available form, and not in such abundance as to be toxic is of para- 

 mount importance. That the physical character of the soil should be 

 such as to make available to the plant constant supplies of water, 

 without at the same time depriving the root system of the necessary 

 air for respiration is likewise of fundamental importance. As a 

 differential factor in the distribution of vegetation, however, the 

 physical character of soils plays a more important role than the 

 chemical. 



Not only does the chemical nature of the rock underlying residual 

 soils determine the chemical character of the soil-water, but its 

 mineralogical nature determines primarily the physical character of 

 the soil. A homogeneous rock-exposure subjected to weathering 

 becomes reduced to ;i soil of homogeneous texture. Gabbro, for 

 example, weathers to a clay, limestone to clay, and sandstone to 3and, 

 while a rock composed of several mixed minerals, as granite, usually 

 gives rise to loam. It thus comes about that there is n (dose corre- 

 spondence between the distribution of soils of different physical 

 texture and the underlying rock formations in the part of Maryland 

 above the "Kail-line." The careful mapping of the geological forma- 

 tions of the state by the Maryland Geological Survey, together with 

 the soil surveys which have been made in seven counties* by the 

 Bureau of Soils, serve to give a definite basis for a study of the 

 relation of vegetation to the physical character of the soil. The soil 

 types described by the Bureau of Soils are distinguished on their 



•The counties in which soil surveys have been made are: 

 Kent, Cecil, Harford, Prince George's, Calvert and St. Mary's. 



