152 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



The glacial debris is heterogeneous in composition and texture, 

 and the depth of its deposit is highly variable. Drainage is imper- 

 fect, but melt water from the receding ice is plentiful. Its early, 

 rapid, and haphazard flow results in the transporting and assorting 

 of a large amount of soil material, which, as drainage lines become 

 established, is deposited to form topographic and soil features as- 

 sociated with glacio-fluvial activity. The water-assorted soils de- 

 posited in the valleys of glacial streams or carried from terminal 

 moraines to form outwash plains are characteristic. 



Although glacial deposits may include weathered rock and some 

 organic material, these are usually not abundant in the beginning. 

 Weathering and the establishment of vegetation at first proceed 

 slowly on glacial soil, but as they progress, a generally good, pro- 

 ductive soil is formed. The soils of the northeastern United States 

 and most of Canada are almost entirely of glacial origin. 



Soil Texture.— One of the most useful bases for classifying soils 

 is that of size of particles. The local variations discussed above are 

 all reflected in soil texture, which in turn has much to do with soil 

 moisture, aeration, and productivity. 



The standard classification in the United States is that of the 

 United States Department of Agriculture, which recognizes the 

 following sizes of soil particles by name: 



Name Diameter, mm. 



Fine gravel 2.00 -1.00 



Coarse sand 1.00 —0.50 



Medium sand 0.50 -0.25 



Fine sand 0.25 -0.10 



Very fine sand 0.10 -0.05 



Silt 0.05 -0.002 



Clay < 0.002 



The percentage weight of these size classes in a soil sample is 

 determined by mechanical analysis. The larger classes may be 

 separated satisfactorily by means of sieves, but the fractions of 

 small size are determined by the pipette method 182 or, better still, 

 the use of a hydrometer. 26, 27 > 28 Both methods are based upon the 

 differential rate of settling of particles in water. 



After mechanical analysis, accurate textural description is pos- 



