ABSOEPTION BY SOIL CONSTITUENTS. 5 



natant liquid is practically the same as that of a check solution 

 containing 10 cubic centimeters of the aforementioned dye solution 

 in a total volume of 50 cubic centimeters. This is to insure the 

 absorbed dye being in equilibrium with practically the same con- 

 centration of nonabsorbed dye in each determination. The suspen- 

 sions are then shaken one hour in an end-over-end shaker to insure 

 complete absorption of the dye by the soil. This time is probably 

 longer than necessary in most cases. Sufficient normal sodium 

 chloride solution to coagulate the colloid is then added to the sus- 

 pensions; usually 5 cubic centimeters is sufficient. After a few 

 minutes settling the suspensions are centrifuged in order to throw 

 out any solid material and the supernatant liquid is read colori- 

 metrically against the check solution. 



ESTIMATION OF NONCOLLOIDAL ABSORPTION FROM SOIL FRACTIONS. 



METHODS OF SEPARATING SOIL COLLOIDS. 



In separating the colloidal matter from the larger soil grains two 

 difficulties are encountered that are inherent in all systems of me- 

 chanical analysis : First, the difficulty of obtaining a complete defloc- 

 culation of the ultimate soil particles; second, the difficulty of sorting 

 out the particles of different size after they have been deflocculated. 



In the system of mechanical analysis heretofore practiced in the 

 Bur-eau of Soils (10), treatment with a few drops of ammonia followed 

 by long shaking have been relied on to bring about deflocculation. 

 Williams (26) adopted long continued boiling supplemented by 

 rubbing as the most efficient method of deflocculation. Atterberg 

 (S) in deflocculating the soil for his system of mechanical analysis, 

 utilizes rubbing and treatment with an alkaline solution of bromine, 

 with hydrochloric acid and sodium hydi'oxide, or with concentrated 

 nitric acid, according to the nature of the soil. Konig and Hasen- 

 baumer {14) suggested boiling for a short time and repeated rubbing. 



While all these methods probably deflocculate and form good 

 suspensions of part of the colloidal matter, it is not certain that any 

 of them deflocculates all of it. Some colloidal matter adhering to 

 mineral particles may be difficult of separation, and we have evidence 

 in our work that the colloidal matter which is present in the dry soil 

 as more or less indurated aggregates is especially resistant to dis- 

 persion. In our attempts at separation we found the use of ammonia 

 and rubbing with a rubber pestle to be especially effective. For 

 instance, two soils when rubbed with distilled water yielded only 

 5 per cent and 29 per cent of colloid; but when the residues were 

 rubbed with distilled water containing a trace of ammonia further 

 yields of colloid, 3.5 per cent and 13.5 per cent, respectively, were 

 obtained. Other results show the effect of rubbing. A soil that 

 had been agitated 15 times with water containing ammonia failed 

 to give further colloidal suspensions. However, after the pasty 

 residue had been gently rubbed, succeeding treatments with ammonia 

 water yielded additional amounts of colloid. 



Separation of the finer particles from the coarser, after defloccula- 

 tion has been produced, is brought about by subsidence in raost 

 methods of mechanical analysis, A much quicker separation, 

 however, can be made by use of a centrifuge. In this particular 

 investigation we made use of both subsidence and centrifuging. 



