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CIRCULAR NO. 109, BUREAU OF PLANT INDUSTRY. 



was to be supplied. It was believed that this method of experi- 

 ment would eliminate all varying factors except the salt content of 

 the soil solution. In order to determine whether the plants reduced 

 the moisture content to the wilting coefficient, it was necessary to 

 provide against direct evaporation from the soil during the course 

 of the experiment. This provision also served to reduce to a mini- 

 mum variations in the concentration of the solution in different 

 portions of the soil mass. 



STOCK SOILS USED. 



Two natural soils from North Platte, Nebr., one of which had a 

 much higher salt content than the other, were employed. Each 

 was moistened sufficiently to make it easy to handle and was then 

 thoroughly mixed. That a high degree of uniformity was thus 

 obtained is shown by the close agreement of the determinations of 

 electrical resistance, moisture content, and moisture equivalent * 

 made on different portions of each stock soil. 



The two stock soils had a mean moisture equivalent of 25.5 and 

 24.1 per cent, respectively, indicating wilting coefficients of 13.8 

 and 13.1 per cent. They gave, when saturated, a mean electrical 

 resistance of 205 and 38 ohms, respectively. 2 The total water- 

 soluble material was 0.17 and 1.03, respectively, in percentages of 

 the dry weight of the soil. 



The chemical composition of the alkali salts in each of these stock 

 soils is given in Table I, which follows. The data were obtained by 

 analyses of extracts prepared in the ordinary manner for alkali 

 determinations — agitation for 30 minutes with 500 c. c. of water 

 per 100 grams of soil. 



Table I. — Chemical composition of the extracts from soils used in the experiment, in 

 percentages of the total solids in the extracts. 



The composition of the soluble matter was markedly different in 

 these two soils, that which had a low salt content (No. 78) having 



1 The moisture equivalent has been shown by Briggs and McLane (Bulletin 45, Bureau of Soils, 1907) 

 to be an accurate indicator of the moisture-holding capacity of the soil and to be more readily determinable 

 than the moisture capacity itself. As defined by Briggs and Shantz (Bulletin 230, Bureau of Plant Indus- 

 try, p. 56), it is "the percentage of water which it [the soil] can retain in opposition to a centrifugal force 

 1,000 times that of gravity." The method followed in determining moisture equivalents is described on 

 pp. 56 and 57 of Bulletin 230. From the moisture equivalent the wilting coefficient can be calculated 

 by means of a formula given on p. 58 of the same publication. 



2 In the standard container (capacity about 50 c. c. ) used in determinations of the salt content of the 

 soil by means of the electrolytic bridge. The apparatus is described by Dr. L. J. Briggs in Bulletin 15, 

 Bureau of Soils, 1899, pp. 32 to 35. See also "The electrical bridge for the determination of soluble salts 

 in soils," by R. O. E. Davis and H. Bryan, Bulletin 61, Bureau of Soils, 1910, wherein tables are given 

 for temperature correction and for computing from the resistance at 60° F. the percentage of salts present. 



[Cir. 109] 



