Dec. i, 1920 Relation of the Soil Solution to the Soil Extract 393 



may easily be increased by the addition of soluble salts. Bouyoucos and 

 Laudeman (j) have shown, moreover, that this increase of concentration 

 occurs immediately and in most cases is not altered over a long period 

 of time. 



Theoretically, also, it is very apparent that the soil solution or extract 

 may be increased in its concentration of a given element by the addi- 

 tion of a soluble salt. A simple case will illustrate this fact. A saturated 

 solution of slightly soluble silicates of potassium, for example, can be 

 prepared by shaking the finely divided minerals with water. The con- 

 centration of potassium in the solution is limited by the solubility of 

 the components of this particular system. However, the addition of 

 another component of different solubility, such as potassium chlorid, 

 will increase the concentration of potassium in the solution, although 

 the solubility of the potassium silicate may possibly be diminished 

 because of the increased concentration of the potassium ion. In the 

 same way the soil solution is saturated only with respect to the particular 

 system existing at any given moment. In general it is not saturated 

 with respect to any particular ion, so from theoretical considerations 

 there is no reason to accept the earlier statements of Cameron that the 

 chemical equilibria would require the precipitation of added salts with 

 a tendency to maintain a constant composition in the soil solution. 

 The fact that water extracts of soils become more dilute with each 

 increase in the proportion of water used gives evidence to show that 

 the solubility of the original soil minerals is not the chief factor govern- 

 ing the concentration of the soil solution. 



Presumably in the actual soil solution the increase of concentration 

 due to the addition of soluble salts will in part be limited by the removal 

 from the dissolved to the absorbed phase. When an excess of water 

 is employed, however, as in making an extract, nearly all of the added 

 solutes will appear in solution or be represented by equivalent quantities 

 of other substances, as is shown, for example, in the well-known exchange 

 of bases. The total quantity of absorbed substances would be a func- 

 tion of the concentration of the surrounding solution, which would 

 vary with the moisture content of the soil or volume of water used in 

 making an extract. In extraction procedures there would occur, of 

 course, a very great dilution of the soil solution. While the latter 

 would be increased in concentration by the addition of soluble salts, 

 the evidence at hand does not indicate that all the added salt would 

 necessarily be effective in increasing the concentration of this soil solu- 

 tion even when the water extracts contained the total or equivalent 

 quantities of the elements added. It is reasonable to assume, however, 

 that the "adsorbed" substances are capable of easily replenishing the 

 soil solution when its concentration is decreased as a result of withdrawals 

 by the plant, new soil solution-adsorption systems being formed con- 

 tinuously during the season. 



