30 



Discussion. The averages for the five fields on the different types 

 (Table 20) show little variation from drift to drift. On the Kansan 

 the Fargo silt loam appears to have a finer texture in the two upper 

 sections than in the corresponding ones on the Late Wisconsin, but this 

 is probably due to the higher content of organic matter on the former. 



Table 20. — Moisture equivalents of samples from the different types on the 

 tzvo drifts. The data are averages for the five fields reported in tables 15 to 18 

 inclusive. 



The Carrington loam shows a more pronounced variation in tex- 

 ture from drift to drift than is found with the other types, this being 

 greatest in the third foot-section. However, the averages for the five 

 fields on the two drifts are very similar. 



To obtain the true moisture equivalent a correction for the coarse 

 gravel would need to be made, but this does not afifect the actual water- 

 holding capacity, since the rock fragments neither increase or decrease 

 the amount of water held by the fine-earth. 



The uniformity in texture on the same type on both drifts shows 

 that the U. S. Bureau of Soils Surveyors were fully justified in their 

 classification. Other differences are not so apparent in the field and are 

 brought out only by a laboratory investigation. 



The moisture equivalents, computed from the mechanical analysis 

 (Table 1) by formulas proposed by Briggs and Shantz (8, p. 73) and 

 Alway and Russel (5, p. 842) do not agree satisfactorily with those 

 obtained by direct determination, although the values found by the 

 Alway and Russel formula for the two cleared fields on the Marshall 

 silt loam resemble very closely those found by direct determination. 

 This is to be expected, since this formula was developed for loessial 

 soils only, and it was pointed out (5, p. 843) that different formulas 

 would be required for various soil types. The most marked difference 

 between the computed and actual values is found with the surface soil 

 on the Fargo silt loam, where the former are much too low. 



