15G SOIL 



they are killed back to the level of the snow so that they are kept 

 constantly in the condition of shrubs, their height being limited by 

 the depth of the snow. It is well known to farmers that fall-sown 

 wheat stands the winter better if it is protected by a covering of 

 snow. 



A covering of leaves and withered grass or other plant remains 

 serves the same function as a covering of snow though not as effi- 

 ciently. A living cover serves to a certain extent in the same way 

 as a non-living one but is more important in what it takes from and 

 what it adds to the soil. It also, of course, serves to stabilize the 

 soil and prevent it from being blown or washed away, as was brought 

 out in paragraph 99. If the living cover consists of mosses it may 

 be as efficient as a non-living cover in protecting against loss of water 

 from the soil since the moss plants take very little moisture from the 

 soil themselves and they form a very compact cover. 



101. Edaphic Factors and the Distribution of Plants.— Ecological 

 factors which are entirely local in their effects are called edaphic 

 factors in contrast with climatic factors which are usually much less 

 limited in their areas of activity. All soil factors are local in their 

 effects and for this reason the term edaphic, as applied to factors, 

 has come to refer almost exclusively to soil factors. 



Plant distribution within limited areas is determined almost 

 entirely by edaphic factors. This fact is well expressed by what we 

 may call Schimper's Second Law which states that "The local dis- 

 tribution of plants and of plant communities is determined chiefly 

 by the nature of the soil, either directly, or in its relation to other 

 factors." 



REFERENCES 



Alway, F. J., McDoLE, G. R., and Trumbull, R. S.: Relation of Minimum 



Moisture Content of Subsoil of Prairies to Hygroscopic Coefficient, Bot. 



Gaz., 67, 185-207, 1919. 

 Atkins, W. R. G.: Some Factors Affecting the Hydrogen-ion Concentration 



of the Soil and Its Relation to Plant Distribution, Notes Bot. School, Trinity 



Coll. (Dublin), 3, 133-177, 1922. 

 AuTEN, J. T.: Porosity and Water Absorption of Forest Soils, Jour. Agric. 



Res., 46, 997-1014, 1933. 

 Burgess, Paul S., and Pember, F. R.: "Active" Aluminum as a Factor 



Detrimental to Crop Production in Many Acid Soils, Rhode Island Agric. 



Exp. Sta. Bull., 194, 1-40, fig. 1-5, 1923. 

 Clark, G. R. : Soil Acidity and Its Relation to the Production of Nitrate and 



Ammonia in Woodland Soils, Oxford Forest, Mem., 2, 1-27, 3 fig., 1924. 

 Cutler, D. W., and Crump, L. M. : Problems in Soil Microbiology, New York, 



Longmans, Green & Co., 1935. 



