Feb. II, 1918 Water Extractions of Soils and Crop Production 303 



TabIvE IV. — Water-extr actable matters in cropped and uncropped soils. Seasonal 



averages — Continued 



Constituent. 



Yield in bushels per acre 

 Soil No 



Nitrate: 



Uncropped 



Cropped 



Difference 



Phosphate : 



Uncropped 



Cropped 



Difference 



Potassium: 



Uncropped 



Cropped 



Difference 



Calcium: 



Uncropped 



Cropped 



Difference 



Magnesium : 



Uncropped 



Cropped 



Difference 



Productivity. 



Medium. 



141 



33 

 108 



45 

 40 



5 



66 

 18 



26 



69.4 



118 

 34 



31 

 22 



9 



70. 6 



130 



39 

 91 



49 

 30 

 19 



Aver- 

 age all. 



129 



35 

 94 



9 



10 



— I 



42 

 31 



66 



44 



16 



56. 2 



88 

 30 

 58 



43 



37 



6 



32 



25 



7 



14 



4S-8 



54 



18 

 36 



44 



35 



9 



83 

 23 

 60 



30 

 20 

 10 



41 

 24 



17 



Aver- 

 age all. 



75 

 24 



51 



31 



25 



6 



39 

 28 



Examination of uncropped soiIvS. — ^We regard the magnitudes shown 

 as the resultant of the combined effects of previous withdrawals, the time 

 which has elapsed since the soil was last depleted, and the rate at which 

 the soil is capable of replacing solutes removed from the soil solution. A 

 soil might show high figures simply because previous withdrawals by 

 crops had been relatively low or remote in time. Nevertheless it is 

 significant that the average content of the more productive soils for each 

 constituent is relatively considerably higher than the average for the poor 

 soils. We infer from this that high figures ordinarily, but not necessarily, 

 indicate a relatively high rate of elaboration of solutes on the part of the 

 soil. A conspicuous example of an exception to this would be the case of 

 a so-called alkali soil, where large accumulations of solutes might have no 

 relation to the present elaborate power of the soil. 



Examinations of cropped soils. — While all averages of the good soils 

 under crop are higher than the averages of the poor soils, the magnitudes of 

 the differences are quite small. Apparently the crop by withdrawal or 

 indirect effects on the soil tends to reduce the initial amounts of solutes 



