ABSORPTION BY SOIL CONSTITUENTS. 



11 



Table 3 shows, for the four soils studied, that part of the total 

 •absorption of the soil which must have been due to the unaltered 

 minerals present. In compiling this table the absorption of each fine 

 ffraction was calculated from the mineralogical composition given 

 above and from the absorption data for particles of this size given 

 in Table 2. The absorption of each coarse fraction was calculated 

 from that of the corresponding fine one, assuming that the two frac- 

 tions were of the same mineralogical composition and that the 

 absorptive values were therefore inversely proportional to the size 

 of the particles. 



Table 3. — Part of soil absorption due to unaltered minerals. 



Type of soil. 



Absorption of the unal- 

 tered minerals present 

 in 1 gram of soil. 



Absorption of 1 gram 

 of soil.i 



Part of soil absorption due 

 to unaltered minerals, in 

 case of— 



Mala- 

 chite 

 green. 



H2O. 



NH,. 



Mala- 

 chite 

 green. 



H2O. 



NH3. 



Mala- 

 chite 

 green. 



HsO. 



NH,. 



Euntington loam, 

 soil 



Oram. 

 0. 0002 



Trace. 



.0005 

 .0007 



Gram. 

 0. 0002 



Trace. 



.0006 

 .0009 



Oram. 

 Trace. 



Trace. 



Trace. 

 Trace. 



Gram. 

 0.0234 



.0193 



.0286 

 .1880 



Gram. 

 0.0508 



.0712 



.0530 

 .1712 



Oram. 

 0.0089 



.0064 



.0060 

 .0312 



Per cent. 

 0.8 



Trace. 



1.7 

 .4 



Per cent. 

 0.4 



Trace. 



1.1 

 .5 



Per cent. 

 Trace. 



JBLuntington loam, 



Trace. 



Sassafras silt loam, 

 subsoil 



Sharkej' clay, soil... 



Trace. 

 Trace. 



1 Calculated from the absorption given for fractions in Table 1. 



It is evident from Table 3 that the absorption of the noncolloidal 

 minerals is less than 2 per cent of the total soil absorption in each of 

 the four soils described and is therefore negligible as compared with 

 the absorption of the colloid. This value for the noncolloidal absorp- 

 tion is very much lower than that shown by the first method of 

 investigation and is, we believe, much more accurate. 



The preceding calculations show the relative absorptions of the 

 colloids and noncoUoids in four soils only, but a rough estimate can 

 be made for soils in general. The average absorptions per gram of 

 the unaltered common soil minerals of silt and clay size for malachite 

 green, water, and ammonia, as shown in Table 2, are, respectively, 

 0.0088, 0.0095, and 0.0008, while the averages for ultra clays ex- 

 tracted from some 40 different soils are respectively 0.200, 0.298, 

 -and 0.040.^ The average absorptions per gram of unaltered minerals 

 just above the colloidal size are thus only 4 per cent, 3 per cent, and 

 2 per cent as high as those of the colloids for malachite green, water, 

 and ammonia. 



Since there are such wide variations in the absorptions of the 

 different minerals, a proper estimate can not be made of the relative 

 absorptions of the colloidal and noncolloidal parts of the soil unless 

 consideration is given to the relative quantities in which the different 

 minerals are present. In a former bulletin of the Bureau of Soils (19) 

 mineralogical analyses are given of the finer sand and silt groups in 

 26 representative American soils. The average mineralogical com- 

 position of the silt group in these 26 soils is as follows: Quartz 51 per 



» The extreme variations in the absorptions per gram of the different ultra clays are: 0.431 and 0.070 gram 

 of malachite green, 0.348 and 0.240 gram of water, and 0.076 and 0.019 gram of ammonia. 



