Feb. II. 1918 Water Extractions of Soils and Crop Production 307 



Very small differences at critical points may reflect significant differences 

 in the performance of soils (fig. i). 



All of the poor soils appear among the three lowest in two out of three 

 characters. If we examine the medium soils, we find that Nos, 4 and 10 

 are among the three lowest in one character. If we draw a line over the 

 three lowest characters as a tentative indicator of the existence of sub- 

 optimal conditions, we find that five of our six medium and poor soils 

 fall below in one or more characters; that the remaining medium soil, 

 No. 7, approaches it in one character; that none of the good soils fall 

 below in any character. It is true that several of the good soils, No. 

 14, 6, and i, approach the line very closely, and we do not wish to be 

 understood as attaching too much importance to slight differences in the 



Fig. 1. — Graphs showing soils arranged with reference to yield and important characters. 



magnitudes of these characters. It would appear, however, that the 

 yields of soils 7, 3, 12, and 9 are closely correlated with their nitrate 

 content, but the mediocre yields of soils 4 and 10 can only be accounted 

 for by a reference to other characters. 



We conclude that the nitrate content of uncropped soils is the most 

 valuable single criterion for appraising the crop-producing power of 

 soils; that the amount of basic ions in cropped soils is indicative of the 

 extent to which soils tend to maintain their concentrations when 

 subjected to depletion by crops; that the differences in basic-ion content 

 between cropped and uncropped soils may reflect the ability of the soil 

 to meet the demands of the crop without dangerous diminution of con- 

 centration and without drawing upon the reserve (iasoluble) supply 

 of the soil. 



