298 Journal of Agricultural Research voi. xii, no. 6 



the absolute amounts present in the soil. We heartily concur in this 

 opinion; but the impossibility of removing for investigation the true and 

 unmodified soil solution, even from soils whose optimum water content 

 is high, is too well recognized to require discussion. It is true that by 

 the method of Bouyoucos and McCool ^ we can determine the total con- 

 centration of the soil solution, but are left in the dark as to the concentra- 

 tion of the individual solutes in that solution. 



In the present paper, therefore, our data are presented in terms of 

 the amounts of solutes extracted from soils and not in terms of concen- 

 trations. We shall endeavor to use this material in such a way as to 

 justify certain conclusions as to the presence or absence of power on the 

 part of soils to supply the needs of crops and to maintain optimum 

 concentrations in the soil solution. Hoagland,^ in this laboratory, has 

 shown that, while the total amounts of material extracted from soils by 

 an excess of water are invariably greater than those contained in the soil 

 solution, they are of the same general order of magnitude (as 2 is to 4 

 or 5), depending on the type of soil. Obviously, then, if the amounts of 

 soil constitutents extracted from cultivated soils can be shown to be 

 relatively high or relatively low, we may legitimately infer that the 

 amounts in the soil solution are correspondingly high or low. We do 

 not mean to imply that figures so obtained would necessarily indicate 

 the existence of an adequate supply of any element, because even low 

 figures might constitute adequacy. The question of adequacy or in- 

 adequacy may or may not be answered by such data, but it would cer- 

 tainly reflect the relative magnitudes of the present supply although it 

 might not indicate the soils' power of renewal. 



Data presented by Stewart ^ in figures 8 to 20 show that normal seasonal 

 fluctuations in the amounts of the essential elements extracted at different 

 times by water (i to 5) from a cultivated soil (cropped or uncropped) are 

 likely to be as great as the variations between different soils. He also 

 shows that soils under crop (barley) invariably contain smaller amounts 

 of nitrate, potassium, calcium, and magnesium than their uncropped 

 duplicates, apparently reflecting the inability of soils to maintain their 

 initial concentrations and at the same time supply the needs of growing 

 plants. Since the fluctuations in the amounts of water-soluble substances 

 in soils are known to be quite large and the effect of withdrawal important, 

 it follows that the limitations of soils can best be shown by data obtained 

 during the period in which the withdrawal is actually taking place. 



A comparison of the charts referred to above shows that, even though 

 good (uncropped) soils may contain considerably more solutes than poor 



1 Bouyoucos, G. J., and McCool, M. M. The freezing point method as a new means of measuring 



THE CONCENTRATION OF THE SOn, SOLUTION DIRECTLY IN THE SOIL. Mich. Agr. Exp. Sta. Tech Bui. 



24, p. 592-631, 2 fig. I916. 



* HOAGLAND, D. R. Op. cit. 



3 Stewart, G. R. the effect of season and crop growth in modifying the sou, extract. In Jour. 

 Agr. Research, v. 12, no. 6, pp. 311-368, 24 fig., pi. 1918. 



