ESTIMATION OF COLLOIDAL MATERIAL IX SOILS. 31 
ammonia adsorption ratios re-present the quantity of colloidal 
material in the soil. 
If all. or the larger pari of. the colloidal material is separated from 
the soil, the unextracted colloidal material in the washed soil residues 
can be estimated with considerable accuracy by mean^ of the petro- 
graphic microscope as described on page 22. The total quantity 
of colloid in the soil is thus the sum of the quantity actually separated 
and the quantities determined microscopically in the soil residues. 
In Table 10 the quantities of colloid determined in this way are 
compared with the quantities determined by the uncorrected adsorp- 
tion ratios, by the adsorption ratios corrected for possible alteration 
in the colloid produced by extraction, and by the water-adsorption 
ratio, using an average factor for the water adsorptive capacity of the 
colloid. 
Table 10. — Colloidal content of soils determined by different metliods. 
Percentage of colloidal material in the soil, indicated by— 
Quantity Adsorption ratio, cor- Water 
Uncorrected adsorption ratio of colloid] rected for alteration adsorp- 
adsorption per gram soil extracted 1 in adsorptive capac- tion 
TAT^^fi^r ^^ ,T™-r ^~^ir™-/iX100. plus the ity of colloid pro- ratio. 
S^ AdS ^^nce a ^l^ttL- unex - duced by extraction, using 
Mtt>tance adsorbed being. lracte d substance adsorbed factor 
colloid being: 0.3 for 
esti- ! adsorp- 
" mated ' tive 
micro- cap 
Dve. H-0. NH 3 . scopi- Dve. H 2 0. NH>. njthe 
callv. colloid. 
' 
Cecil clay loam, soil 9. 3 18. 1 20. 6 , 1& 4 11. 11. 2 15. 9 1 14. 7 
Cecil clav loam, soil (du- 
plicate" determination 1 . 8.0 15.6 IS. 3 9.7 12.0 14.7 
Huntingtonloam.sc.il... 25. i 24.0 38.5 s 2-5.9 25.5 20.4 27.3 17. 8 
Hunting! on loam. 'subsoil 19.8 26.4 36.7 24.5 21.4 23.5 23.9! 26.4 
Sassafras silt loam, sub- 
soil 19.7 20.8 25.3 23.5 21.1 19.8 20.2 18.9 
Sharker clay, soil 59.4 66.3, 64.7 54.4 58.2 
Sharkey clay. soil 
(colloid graded by su- 
percentrifuge) 59. 2 65. 3 69. 6 (}) 45. 49. 6 51. 5 ! 66. 1 
Vega Baja clay loam, -oil 
("colloid graded bv su- 
nercentrifugei....*. 60.7 78.2 72.4 74.0 66.2 75.9 
1 The quantity of colloid extracted plus the quantity determined microscopically amounted to 59.7 per 
cent in this sample. It include^ the same class of material as the sample above and is, therefore, a dupli- 
cate determination. The adsorption ratios given for the Sharkey soil (colloid graded by the supercentri- 
fuge; should not necessarily agree with the ratios for the first sample of Sharkev soil as "thev mav refer to 
slightly different classes of material, the two sets of raiios being based on dinerent colloidal'material; i.e.. 
on material less than 1 micron and on material less th2n 0.3 micron. 
A comparison of the data in columns 2 to 4 with those in columns 
6 to 8 shows that discrepancies between the dye, water, and ammonia 
adsorption ratios disappear for the most part when a correction is 
applied for the probable alteration in the adsorptive capacity of the 
colloid produced by extraction. Also, the corrected dye. water, and 
ammonia ratios agree well in most cases with the quantities of colloid 
determined by mechanical separation and microscopic observation. 
This would indicate that the chief element of inaccuracy in the ad- 
sorption method of determining the colloidal content of a soil proba- 
bly lies in the difficulty of separating a sample of colloidal material 
from the soil without changing its adsorptive capacity. 
