WATER PEN^ETRATIOK IN GUMBO SOILS. 11 



That this slow movement in the dry subsoil is due to the com- 

 pacting of the wet soil above is further shown by laboratory experi- 

 ments on dry soil under field conditions. It was found that where 

 the soil was allowed to swell freely the water would move at least 

 an inch in five minutes in the heaviest section of the soil. Since the 

 rate of movement under field conditions is so mucli slower, there is 

 no doubt that the compacting of the wet soil in the field is so great 

 that it renders any movement of water through it almost impossible. 

 Water movement in the dry subsoil is therefore limited by the rapid- 

 ity with which the water can make its way through the wet soil above. 



SUMMARY. 



Water movement in the gumbo soils of the Belle Fourche Recla- 

 mation Project may be summed up as follows: 



On a dry soil, penetration takes places rapidly to a depth of about 

 2 feet because of the cracked condition of the soil near the surface. 

 After the layer of easily penetrated soil becomes wet, it becomes 

 so swollen and compact that it is nearly impervious, and further 

 water movement is very slow. 



The fact that moisture can move only verj^ slowly in the wet sur- 

 face soil would make it necessary to run water over the soil for a very 

 long time in ordei' that any considerable portion might be absorbed. 

 This is not practicable, for the experiment with a dry subsoil showed 

 that water from the surface penetrated almost as deep in a few 

 minutes as it did in 10 days, so that the increase in the amount of 

 moisture absorbed where the water stands for any considerable 

 length of time over that taken in when the soil is simply covered 

 would be so small as to be negligible. After a field has once been 

 covered with water little benefit can result from having water con- 

 tinue to stand on or flow over the soil. 



It is interesting to note the radical difference in water absorption 

 between this .soil and the saiidy loam soil at Scottsbluff. The maxi- 

 mum rate of absorption is obtained on the wet soil at Scottsbluff and 

 on the dry soil on tlie Belle Foiirclie project. These diametric differ- 

 ences apparently are due to the physical differences between the two 

 .'-oils and show clearly that a satisfactory practice on one type of soil 

 may not be cfjually successful under other soil conditions. 



The results of these experiments and observations can easily be 

 applied in field practice, and recommendations for methods and prac- 

 tices may be based upon tlicin. 



The following points relative to the application ol' water by irri- 

 gation to these gumbo soils are clearly shown : 



(1) Wntv.r Hh<tnl<l he ;iitr)li<!(l only when llic siirfiicc is dry. 



(2) The f)iiiiritity of wafor ahs<»rh»!fl will dc'iu'iid iijton (lie dryiieHs mid con- 

 >«Hjuoiit cDK-ki'd coridil idii of ihc siirCiicc soil. 



