220 SOILS: FB0FEETIE8 AND MANAGEMENT 



subtracted from the total water will give the capillary 

 water present in the sample. The capillary water at 

 various points in a soil column may be obtained by sub- 

 tracting the hygroscopic coefficient from the various 

 percentages of moisture present, since the thin hygroscopic 

 iSlm is not influenced by height of column or ordinary 

 structural conditions. In ordinary soils, however, the 

 differences in hygroscopicity are not so great but that 

 the total water retained in a soil column against gravity 

 serves as a very good measure of relative capillary 

 capacity. 



148. The moisture equivalent of soils. — Briggs and 

 McLane ^ have perfected a method of comparing soils 

 on the 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 

 resultant moisture percentage is designated as the mois- 

 ture equivalent. 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 also found that 1 per cent of 

 clay or organic matter represented a retentive power of 

 about .62 per cent, while 1 per cent of silt corresponded 

 to a retention of .13 per cent. Representative data 

 which show the correlation of the moisture equivalent 

 to the textural properties of the various types are given 

 in the table on the following page. 



1 Briggs, L. J., and McLane, J. W. The Moisture Equiv- 

 aleats of Soils, U. S. D. A., Bur. Soils, Bui. 45. 1907. 



