4 CIRCULAR 5 63, U. S. DEPARTMENT OF AGRICULTURE 



The three plots or methods studied were A, plowed in the spring 

 each year and continuously cropped to spring wheat; B, plowed in the 

 fall each year and continuously cropped to spring wheat; and C and D, 

 a pair of plots alternately fallowed and cropped to spring wheat, so 

 that a crop was grown on fallowed land each year. 



The minimum point to which spring wheat could exhaust the water 

 in the soil of these plots (the lower limit of available water) had been 

 determined in another study. 3 With this basic information, it is 

 a simple and fairly accurate matter to determine whether the water 

 content of any foot section indicates wetness or dryness at the time 

 of observation. The data and charts presented by the authors in 

 another publication (4) show the annual or biennial h^drologic role 

 under the prescribed conditions of cropping, and also indicate that 

 soil and subsoil moisture conditions are such that the depth of moist 

 soil in the spring is in most cases a good measure of the quantity of 

 available water stored in the soil. 



In the total number of years studied, very few offered serious ques- 

 tion as to placement in the three categories: (1) Soil wet to 1 foot 

 or less; (2) soil wet to 2 feet; and (3) soil wet to 3 feet or more. In 

 determining the placement in any year, a certain margin was allowed. 

 If the second or the third foot contained only 3 percent of avail- 

 able water in coarse-textured soils or 4 percent in fine-textured soils, 

 that section was not recognized as being wet. Such a condition 

 meant that only a few inches of the upper part of the foot section in 

 question was wet. Correct expression of the classes would be wet to 

 about 1 foot or less, wet to about 2 feet, and wet to 3 feet or more. 



Many of the determinations were nearer the time of emergence 

 than seeding. Under the climatic conditions obtaining in the north- 

 ern Great Plains, the trend of this would be to increase the depth of 

 wet soil over that which obtained at actual seeding time. Condition 

 at seeding time as used in this study is a rather broad expression mean- 

 ing a spring condition before the crop began to exhaust soil water. 



In a very few cases where there was no determination of soil mois- 

 ture at a satisfactory date but there were determinations earlier or 

 later, or both, collateral evidence of precipitation and a knowledge of 

 soil moisture were used in fixing the status at seeding time. 



Although crop rotation and cultural work have been conducted at 

 the central-Montana substation, Moccasin, Mont., since 1908, that 

 station could not be included in this study, because coarse gravel in 

 the soil and subsoil prevented sampling by standard methods for soil 

 moisture. 



Similar investigations were conducted on the Edgeley (N. Dak.) 

 substation from 1906 to 1921. A local condition of shallow soil under- 

 lain by shale limits definitive determinations of water content and use 

 to 2 feet. Because of the abnormality of the soil and the very limited 

 area represented by it, the station was also omitted from this study. 



RESULTS AT INDIVIDUAL STATIONS 



The conditions at each station and the results obtained will be dis- 

 cussed briefly, with special consideration being given to the size of the 

 sample, the reasons for short records or for the omission of certain 



Unpublished data in the files of the Division of Dry Land Agriculture. 



