REVERSIBILITY OF THE COLLOIDAL CONDITION OF SOILS 517 
Chemical nature of soil colloids 
During the course of the experiments in which the soil was allowed to 
stand with an excess of water, it was frequently observed that a colloidal 
material of the appearance of ferric oxide was obtained. In some cases 
the similarity to ferric hydrate was very marked, and in other cases the 
material had only a rusty stain. 
It was felt that this was, in part at least, colloidal ferric hydrate, and 
that it was caused by bacterial decomposition of the organic matter of the 
soils, and solution and hydration of the released salts. 
Dye-adsorption tests failed to show any significant differences. It was 
thought that diamine sky-blue, which had been found to be strongly 
adsorbed by artificially prepared colloidal ferric oxide, would show differ- 
ences. It did not, however. It is very likely that the organic matter 
in the soil is more or less adsorbed by the ferric oxide and that it acts as 
a protective colloid, preventing the adsorption of an acid dye. 
It was decided to-try the solubility of this material in weak hydrochloric 
acid. Accordingly, the soils were shaken with approximately N/30_ 
hydrochloric acid, allowed to stand for a while, and then filtered. In the 
filtrate iron was determined by the colorimetric method given in Bulletin 
31 of the United States Bureau of Soils. The results are given in table 25: 
TABLE 25. Errecr OF DIFFERENT TREATMENTS ON THE SOLUBILITY OF [RON IN WEAK 
Hyprocuioric ACID 
(Expressed in parts per million) 
Soil standing in 200 per cent of 
water 
Soil : pba hest ts 
: Air- Oven- dried dried 
Moist dried dried and 
sterile 
Dunkinkesurtacese 4a acess se cese oste se Trace 135 |e 5526 4.6 3.1 
WV eISUTRACE race Uh oc.8 sides cosinor ogee 26.6 26.4 | 42.75 | Trace | Trace 
Wergennes subsoil, 0.02. so. Trace | Trace | Trace | Trace | Trace 
Wecilesulsoilemm ese nc eke anggicnraice St sects wen [ene one SPO se ne Trace 
It is seen that oven-drying these soils previous to their standing in 
water increased the amount of easily soluble iron, and that sterilization 
