Feb. II, 1918 Freezing-Point Method and Soil Solution 393 



tion, then its concentration in the cropped soil should not be strikingly 

 diminished over a long period of time after the crop is removed, which in 

 fact is the case. Also the soil solution in a leached soil has a greatly 

 diminished concentration which is not markedly increased by long 

 standing. In brief, the evidence is quite opposed to the theory that 

 there is an immediate restoration of equilibrium when the soil solution is 

 diminished in concentration by the plant or other agency. 



One explanation of the fluctuations in the soil solution lies most prob- 

 ably in the varying nature of the solvent, especially in its content of 

 carbon dioxid, which may well be one of the preponderating factors. 

 The effect of carbon dioxid on the solution of soil minerals has been 

 demonstrated by Mitscherlich, and the present investigation has indi- 

 cated the effect of carbon dioxid in increasing the depression of the freez- 

 ing point. It is generally conceded that the partial pressure of carbon 

 dioxid in the soil atmosphere is greater than that of the outside air. 

 Russell (7) has shown that the content of carbon dioxid in the soil may 

 vary greatly at different times of the year, according to the intensity of 

 bacterial action. In addition to carbon dioxid, other products of bac- 

 terial activity may have a further influence, the extent of which is still 

 problematical. 



It has been a general teaching of agricultural art that soil fertility is 

 increased by those operations which tend to bring about optimum 

 biological conditions in the soil. The experiments recorded in this 

 investigation seem to afford direct evidence that the soil solution may 

 be greatly affected by the activity of microorganisms and indirectly 

 therefore by cultivation, temperature, organic matter, etc. Finally, the 

 results of the experiments reported in this paper have considerable 

 significance to the plant physiologist, since comparisons may be made 

 between osmotic pressures in nutrient solutions and in the soil solution 

 as it actually exists in the soil under conditions favorable to crop 

 growth. It will be observed that in none of the soils did the plants 

 obtain their nutriment from a highly concentrated solution. The 

 general range of concentrations was from a maximum of 0.5 or i.o 

 atmosphere to a minimum of o.i or 0.2 atmosphere. Under the condi- 

 tions of approximately optimum moisture contents there was no sharp 

 distinction between the silty clay loams and the fine sandy loams. By 

 lowering the moisture content of the clay loams considerably, it is true 

 that a very high concentration of the soil solution results, but under 

 moisture conditions favorable to plant growth the solution is dilute. 

 From certain of the soils stored in the bins a different idea of the con- 

 centration of the soil solution might be obtained, since this treatment 

 has in some cases greatly increased the soluble material. The true 

 conception of the nature of the nutrient media in soils can only be 

 attained when soils are studied throughout the season under conditions 

 suitable for crop growth. 



