EXPERIMENT STATION BULLETINS. 697 



been observed in the first place, that after the soils have stood in the 

 tubes for 50 or 00 days aljiae begin to ai)i)ear on the sides of the tubes 

 above the soil. The column of Avater standing- on the top of the soil, 

 after the latter settles, becomes sometimes turbid and occasionally a 

 red precipitate appears in this column of water of some of the soil 

 or salt treatments. At about this stage, the solubility becomes more or 

 less constant and in some cases a revei-se action takes place, i. e., there 

 is a sudden or gradual reduction in tlie freezing point depression, indi 

 eating that some material in solution is going out of solution. Tlie red 

 precipitate occurs very generally in tlie untrealcd soil and only occa- 

 sionally in the treated soils, and when it occurs it stains the glass of the 

 tube and this stain becomes fixed as it cannot be washed with water. 

 AVhile it is not definitely known what the cause of this red precipitate 

 is, it seems to be brought about by living organisms. The reverse action 

 takes place sometimes even when there is no precipitate of any kind. 

 This happens invariably in the case of the soil treated with (NH4)oS04. 

 The concentration of the solution continues to increase in this case for 

 about 40 or 50 days and then quite suddenly it decreases. The reverse 

 action took place only in those soils treated with (NHJoSO^ and not in 

 the other salt treatments. 



Another factor which seems to limit or prevent further solubility as 

 indicated by the progressive increase of concentration is the concen- 

 tration of the solution itself. It was found, for instance, that when a 

 soil was treated with a salt and the salt was only partially washed with 

 water the original depression remained quite stationary or constant, 

 while if all the salts were washed away, the original depression was 

 greatly increased. Evidently the presence of a high concentration pre- 

 vents the soil from ;\'ielding to the solution even the amount of the 

 material that it would in an untreated condition. This conclusion is 

 further sujiported by the results of a former investigation on the meas- 

 urement of the velocity of the reaction between soils and chemical agents 

 and the behavior of equilibrium. It was shown in this research that the 

 velocity of the reaction between soils and salts was extremely rapid if 

 not almost instantaneous, and the equilibrium attained remained con- 

 stant for long times, even one hundred days. Now if the soils were yield- 

 ing material to the solution tliis equililtrium certainly could not be main- 

 tained constant for such long periods of time. 



If concentration, therefore, tends to inhibit solubility then the further 

 solubility of the salt-treated soil was retarded or completely stopped by 

 the concentration of its own solution. The solubility went on until a 

 certain concentration was reached and then this degree of concentration 

 had an opposite effect upon the solubility and it either completely stopped 

 it or greatly retarded it. 



In what manner the concentration affects unfavorably the solubility 

 of the soil cannot be stated at present. It may be argued that the 

 solubility still goes on but that the soil read.^orbs some of the material 

 in the solution, or that the soluble materials react between themselves 

 and form new compounds, or precipitate one rinother, and these pro- 

 cesses are so balanced that the total concentration remains the same for 

 a long time. In view of the effect of total solids upon the depression, 



(6) Mich. AgT. Expt. Sta. Tech. Bnl. .".7 (1917). 



