96 



BULLETIN" 1059, U. S. DEPARTMENT OF AGRICULTURE. 



Table 4. 



-Moisture equivalents in several types of soil in relation to capillary 

 moisture and wilting coefficients. 



Description of group. 



9 Michigan and Nebraska 

 sands; humus not over 3 

 per cent 



5 Michigan and Nebraska 

 sands; humus 3 percent to 

 21 per cent 



9 Pikes Peak gravels; humus 

 2.1 per cent, clay 3.7 per 

 cent (maximum) 



3 granitic loams (spruce); 3 

 to 4 per cent humus 



3 granitic loams (spruce); 4 

 to 8 per cent humus 



4 granitic loams (spruce); 8 

 to 27 per cent humus 



1 prairie clay; 70 per cent silt 

 and clay, very little humus. 



Average of groups 



Mean variation be- 

 tween groups 



Mean 

 capil- 

 lary 

 mois- 

 ture. 



P.ct. 

 21. 82 



13. 78 



13.13 

 20.47 

 33.09 

 60.13 

 37.77 



Mean 

 mois- 

 ture 

 equiva- 

 lent. 



Mean 

 wilting 

 coeffi- 

 cient. 



P.ct. 

 5.49 



19.24 



4.92 

 12.62 

 20.76 



43.09 

 28.79 



P.ct. 

 1.73 



6.99 



2.58 

 3.36 

 6.36 

 14.86 

 8.90 



Mean 

 ratio 



M.E./C, 



0.253 



.377 

 .621 

 .632 

 .692 



.762 



.533 



.1624 



Mean 

 varia- 

 tion. 



0.055 



.090 



.040 

 .042 

 .054 

 .077 



Mean 



ratio 



W.C./M. 



E. 



0.320 



.357 



.530 

 .266 

 .307 

 .348 

 .310 



.348 

 .0543 



Mean 

 varia- 

 tion. 



0.035 



.038 

 .006 

 .030 

 .066 



Mean 



ratio 



W.C./C. 



0.080 



.200 

 .165 

 .193 

 .242 

 .236 



.181 

 .0419 



Mean 

 varia- 



within 

 group. 



0.016 



.023 



.030 

 .014 

 .010 

 .051 



1. There are three outstanding facts in connection with these data, 

 clearly shown by the diagram. The first of these is that the two 

 groups of sands show an extremely large proportion of the capillary 

 water removable by the force of 100-gravity, and correspondingly 

 low wilting coefficients. This speaks for the light hold which the 

 sands have on their moisture, when even approaching saturation. 



2. The second conspicuous fact is that, with the exception of the 

 granitic gravels, the wilting coefficients and moisture equivalents 

 rise and fall somewhat proportionately. The gravels have the 

 smallest capacity for capillary water, a very weak hold on a large 

 part of it, and a strong hold on the remainder. This is partly caused 

 by a small quantity of clay derived from the feldspar, but more 

 largely to the fact that the feldspar is itself somewhat permeable. 

 Coarse cleaned gravel of this type has been shown to have a capil- 

 larity of only 2.90 per cent, but a moisture equivalent of 1.70 per 

 cent. It seems likely that practically all of the latter would be non- 

 available. 



3. Another important point to be noted is the very small amount 

 of water removable from the prairie clay by the moderate centrifu- 

 gal force, and the correspondingly high wilting coefficient. 



4. Finally, although the influence of humus is somewhat obscured 

 by the fact that increasing amounts of it in one general soil type are 

 usually accompanied by increasing amounts of silt and clay, it seems 

 fairly certain that the humus does not yield up its moisture any too 

 readily and that it may tend to make the wilting coefficient relatively 

 high by preventing capillary movement to the roots. It must also 



