68 WILTING COEFFICIENT FOE DIFFERENT PLANTS. 



relationship between the wilting coefficient and the moisture-holding 

 capacity is then expressed by the formula : 



Moisture-holding capacity — 21 . 



2 9 _,_q qq = wilting coefficient. 



RELATION OF THE "WILTING COEFFICIENT TO SOIL TEXTURE AS EXPRESSED BY MECHAN- 

 ICAL ANALYSIS. 



Soil texture has been used for the quantitative description of soils 

 more extensively than any other physical property, and unfortu- 

 nately it has been one of the most difficult to interpret from the 

 standpoint of moisture retentiveness. Texture is quantitatively 

 expressed by means of the mechanical analysis, which shows the 

 composition of the soil when the particles are separated into groups 

 according to size. The accuracy with which the texture of the soil 

 can be expressed by this means is dependent on the number of groups 

 into which the particles are separated. But the difficulty of effecting 

 a complete separation of the finer particles into the desired groups 

 places a practical limit upon the number of groups, which is usually 

 limited to seven. 1 



The use of mechanical analysis as a basis for determining the 

 moisture retentiveness of a soil is further complicated by the fact that 

 soils having a high clay content will show great differences in the 

 amount of colloidal material, which greatly affects the moisture 

 retentiveness. Furthermore, the particles constituting a given group 

 may lie much nearer one limit of the group than the other, so that a 

 given group does not always have the same properties. Consequently 

 the particles constituting a given group in the mechanical analysis do 

 not always have the same moisture retentiveness per unit mass. It is 

 also possible that the specific retentivity of a group when measured 

 alone is modified to some extent by admixture with particles from 

 other groups. 



Briggs and McLane, 2 using the method of least squares, have estab- 

 lished a quantitative relationship between the mechanical composition 

 and the moisture equivalent, based on data covering 104 types of soil. 

 The resulting probable error of the coefficients in the relationship 

 established is 1.7 per cent. 3 In attempting the correlation of the 

 mechanical composition with the wilting coefficient the same relative 

 values for the sand, silt, and clay coefficients that were obtained by 

 Briggs and McLane in their moisture-equivalent correlation have 

 been used in our computations. The actual values of the coefficients 



1 Briggs, L. J., Martin, O. F., and Pearce, J. R. The Centrifugal Method of Mechanical Soil Analysis. 

 Bulletin 24, Bureau of Soils, U. S. Dept. of Agriculture, 1904, p. 33. 



2 Briggs, L. J., and McLane, J. W. The Moisture Equivalent of Soils. Bulletin 45, Bureau of Soils, U. S. 

 Dept, of Agriculture, 1907, p. 18. 



3 This value should not be confused with the probable error of a single determination, as given by Briggs 

 and McLane. 



230 



