THE FOEMS OF SOIL-WATBR 



167 



is clearly shown by the two loess silt loams. Perhaps most 

 important of all is the marked discrepancy between the actual 

 field capacity and the arbitrary and artificial laboratory 

 method. The normal water-holding capacity of a mineral soil, 

 varying with texture and organic matter, seems to range from 



50 %WATER 



Fig. 30.— Diagram showing the distribution of moisture in capillary 

 columns of soil of different textures. The end of each column 

 rests in free water. (Buckingham, E., Bur. Soils, Bui. 38, 1907.) 



about 10 to 50 per cent, based on dry soil. Muck and peat of 

 course run much higher, 400 per cent being not uncommon.^ 



^ Briggs and McLane have perfected a method of comparing soils on 

 fche basis of their capacity to hold water against a definite and constant 

 centrifugal force of one to three thousand times the force of gravity. 

 The soils, in thin layer, are placed in perforated brass cups which fit 

 into a centrifugal machine capable of developing the above force, and 

 are whirled until equilibrium is reached. The resultiant moisture per- 

 cetitage is designated as the moisture eqmvalent. It really represents 

 the capillary capacity of a soil of minimum column length when subject 

 to a constant and known force or pull. The finer the soil, the greater 

 of course is the moisture equivalent. The authors found that 1 per cent. 

 of clay or organic m-atter represented a retentive power of about .62 



