5IO 



Journal of Agricultural Research 



Vol. XV, No. 9 



The organic and inorganic colloidal material in these humus extracts 

 evidently moves with the electric current, as the following experiment 

 shows : 



Two tubes were connected together with rubber tubing into the form 

 of a U-tube, and filled with humus extract. Electrodes from a battery 

 of six new dry cells connected in series were inserted into the tops of the 

 U-tube touching the extract, and left undisturbed for seven days. The 

 liquid in the two tubes was then separated by closing the rubber-tube 

 connection. A colorimetric humus reading was made of the liquid in 

 each tube. The inorganic colloids were precipitated with nitric acid 

 and ammonium carbonate, ignited and weighed. Table V gives the 

 results obtained. 



Table V. — Inorganic colloids and humu? in humus extracts of soil in positive and 



negative sides of the U-tube 



Electric charge in U-tube. 



Positive. . 

 Negative . 



Percentage 



of inorganic 



colloids in 



solution. 



O. 0380 

 .0230 



Percentage 



of humus in 



solution. 



o. 0625 

 .0500 



In the preceding discussion it has been shown that there is evidently 

 no accumulation of water-soluble iron, calcium, magnesium, or silica in 

 the plowsole layers that could account for its formation; that the total 

 ammonia-soluble silica, iron, aluminium, or phosphoric acid could not 

 account for the formation of the plowsole; and that there was accumu- 

 lation of ammonia-soluble colloidal silica, iron, and aluminium, in the 

 plowsole, but not of colloidal phosphoric acid. 



The last-mentioned fact indicated the desirability of making a direct 

 study of the inorganic colloidal material in the soil.^ 



The term "colloid suspension" will be used frequently in what follows, 

 and for the purpose of clearness it will be used to mean simply the 

 inorganic material of a soil remaining in suspension at the end of 24 

 hours after the soil has been treated in a certain manner. 



The following conventional method was adopted for determining the 

 percentage of colloid suspension in the soils. Five gms. of soil were 

 placed in a 250-cc. Erlenmeyer flask, 50 cc. of distilled water was added, 

 the flask covered with a watch gfass and placed on the water bath. The 

 contents were shaken periodically, and after i }4 hours the suspended 

 material' was decanted into a test tube about 3 cm. in diameter and 25 



' A discussion of the general properties of colloids will not be entered into here . Whether colloids are 

 classified according to size, chemical constitution, or method of preparation will have but little bearing 

 on the nature of the present work. Excellent discussions on colloids are given by Ostwald (OsTWAiD, 

 Wolfgang. HANDBOOK OF coiLOiD-CHEMisTRY, . . . Trans. from 3d German ed. 278 p., 60 fig. 

 Philadelphia); and by Burton (Burton, E. F. physicax properties of colloidai, solutions, zoo p.. 

 18 fig. London, New York), not to mention many other earlier workers in the field. 



