362 J our7ial of Agricultural Research voi. xii, no. 6 



This conclusion is notably at variance with the earlier statements of 

 Whitney and Cameron (61). It is believed that the experimental method 

 which has been followed brings out the relationship between different 

 soils with unusual clarity and effectiveness. 



The influence of the crop on the soluble nutrients is another point 

 which has been denied by the same authors. This is shown by the 

 following quotation from Bureau of Soils, Bulletin 22 : 



At the same time we have detected no constant decrease in the amount of soluble 

 salts which could be easily detected with the methods used during the advance 

 stages of growth of the crop, notwithstanding the considerable withdrawal there must 

 be by the plants {61, p. 60). 



The contrast between the curves of the planted and unplanted soils is 

 evident at the most cursory glance. It is also clearly shown that the 

 soil did not immediately recover and yield the same quantities of soluble 

 nutrients as the uncropped plot, even after the crop was recovered. At 

 the last observation recorded, on February 12, there was still an appre- 

 ciable difference between the duplicate containers. In this connection 

 it is pertinent to refer to the data obtained in a preliminary study in 

 which a cropped soil was extracted with a diluted solution from an 

 uncropped portion of it. In that study it was shown that this diluted 

 solution exerted a depressing effect on the nutrients extracted from the 

 cropped soil. This has an important application. 



At the period of greatest growth of the crop, the water extract of the 

 cropped soil approached much more closely to distilled water in the case 

 of the planted soil than it did with the unplanted duplicate. In conse- 

 quence of this fact, a greater portion of the extract obtained from it was 

 from the solution of the actual soil minerals than was the case with the 

 uncropped soil. The real difference between the two duplicate con- 

 tainers was therefore greater than the graphs would indicate. They can 

 only be taken as the minimum difference which actually existed. 



The data obtained by Hoagland, using the freezing-point method, 

 corroborate this difference between the planted and unplanted soils. 

 He has also observed that soils 9, Kimball fine sandy loam, and 12, 

 Arnold fine sandy loam, are notably lower than any of the other soils in 

 the concentration of their solutions. This coincided with the observa- 

 tions made from a study of the water extracts. Considering soil 3, 

 Yolo silty clay loam, the next lowest soil in water extract, he could not 

 draw any definite conclusion. 



Jensen {32) in his studies on sugar beets has described a decrease in 

 water-soluble nutrients and has also found this less noticeable in the case 

 of phosphates. King's {35) figures also showed some decrease in soluble 

 compounds as the crop advanced in growth. Harris and Butt {24) have 

 observed differences in soluble salts and nitrates between cropped and 

 fallow soils. It is believed, however, that the use of the unplanted dupli- 

 cate soil and the periodic observations made upon it and the cropped 



