Dec.2.i9i8 Relation of Inorganic Soil Colloids to Plowsole 511 



cm. high. Another portion of 50 cc. of distilled water was added to the 

 soil remaining in the flask, and the former treatment repeated. The 

 same treatment was repeated a third time, and on the third decantation 

 all the soil was washed into the test tube. The tube finally contained 

 about 180 cc. and was left undisturbed for 24 hours. The suspended 

 material was then carefully siphoned off and precipitated with neutral 

 ammonium sulphate. The precipitation was hastened by warming. 

 The precipitate was filtered, ignited, and weighed as the percentage of 

 " colloid suspension." 



Duplicate determinations were made, and the departure varied from 

 nothing to a maximum of about 5.5 per cent. When the departure did not 

 exceed 3.5 per cent the results were used. When the error was greater, 

 duplicate determinations were again made. 



It was found to be more difficult to obtain two uniform duplicate 5-gm. 

 soil samples than to make good duplicate determinations on a very uni- 

 form sample of soil " colloid" used for experimental purposes. "To insure 

 better duplicate sampling for colloid determinations, the soil as taken 

 from the groves was rolled and sifted through a 2-mm. sieve. 



It is realized that this method of measuring the colloidal content of 

 soils does not show the absolute amount of colloid in a soil. The colloidal 

 content by this method is necessarily affected by tie amount and charac- 

 ter of the electrolytes going into solution. The purpose of the measure- 

 ments was not to get a measurement of the absolute colloidal content, 

 but to study the relative colloidal content in the three layers of soil in 

 plowsole groves. Ashley (7) measured the colloid matter in clay by the 

 absorption of dyes in the study of the plasticity of clay. His work has a 

 special bearing on the ceramic industries. This method does not appear 

 applicable to the present work, since it is probable that not only the col- 

 loid matter remaining in suspension after 24 hours, but the colloidal mat- 

 ter not remaining in suspension as well would absorb dyes; and so in all 

 likelihood would the organic matter in the soil, which wa,s found to bear 

 no relation to the percentage of inorganic colloid suspension in the soil. 

 The purpose of the present study was to obtain a measurement of the 

 colloid matter that was free to move with the soil moisture, as it seemed 

 to do. 



That the method of determining the colloid suspension as above de- 

 scribed is entirely conventional is shown by the iollowing experiment. 

 Five-gm. soil samples from a composite sample were put into Erlenmeyer 

 flasks, distilled water was added, and the flasks were placed on the bath. 

 The total amount of water used and the total time of the sample on the 

 bath were the same for each determination. In each case, also, the sus- 

 pended matter as transferred to the test tubes stood 24 hours before 

 being siphoned off. The variable factor was the numbei* of decantations 

 and consequently the amount of water used for each. Table VI gives 

 the results of the determinations and the condition of each test. 

 88093°— 18 3 



