384 Journal of Agricultural Research voi.xx.No. s 



the curves for total solids and freezing-point depressions is on the whole 

 excellent, considering the technical difficulties involved. Chief among 

 these is the uncertainty concerning the free and unfree water in the soil, 

 which, as Buoyoucos (1) has clearly shown, markedly affects the concen- 

 tration of the soil solution. While all the values have been calculated 

 to the same moisture basis, it is not to be expected that this can be done 

 with a high degree of accuracy, since the percentage of unfree water may 

 vary with different moisture contents and perhaps with different concen- 

 trations of the soil solution. In both groups of soils there is a somewhat 

 marked divergence between the curves for total solids and freezing- 

 point depressions at a period beginning about 10 weeks after planting 

 the crop. This can reasonably be explained on the basis of certain 

 observations reported in former articles (6, 9). It was shown in these 

 that a larger quantity of very slightly soluble material was dissolved 

 from a soil by a given proportion of water when the soil solution had 

 reached a low concentration as a result of absorption of solutes by the 

 plant. At a certain period, therefore, the cropped soil will yield a higher 

 percentage of dissolved material (not part of the actual soil solution) as 

 compared with earlier periods. This means that the extractions of the 

 cropped and uncropped soils are not on exactly the same basis at all 

 times, and it might be predicted that at the period of low concentration 

 in the cropped soil the proportion of dissolved substances would increase. 

 The inference is substantiated by the experimental data. This generally 

 neglected phenomenon of the effect of the solutes already present in the 

 soil solution in depressing the solubility of substances dissolved from the 

 soil mass by water is thought to be of considerable importance in all 

 studies on soil equilibria by means of water extracts. Finally, it should 

 be emphasized that at no time is there any indication that conclusions 

 based on the water extracts would lead to an erroneous estimate of the 

 general relation between the soil solutions of cropped and uncropped 

 soils. As the authors have pointed out before, the actual differencse 

 would tend to be of greater magnitudes than those calculated from the 

 results on water extracts. 



When a 1 to 5 extract of soil is made with distilled water, the quan- 

 tity of total solids is from 1.5 to 5 times that present in the soil solution, 

 as calculated by the freezing-point method. By the latter method we 

 can calculate the total concentration in the soil solution; but this does 

 not enable us to determine whether or not the relation between the ele- 

 ments in the soil solution is at all similar to that in the soil extracts. 

 Another type of experiment is necessary to give evidence on this point. 

 It war suggested that such evidence might possibly be obtained by 

 determining the concentration and composition of a solution which 

 would remain unchanged when in contact with the soil mass. In other 

 words, if one passed through a sample of moist soil a solution having the 



