6o 



Journal of Agricultural Research 



Vol. XV, No. 2 



There is, indeed, much evidence that the crop fertilized never takes 

 off potassium in amount equivalent to that contained in the usual fertilizer 

 dressing; nor, very often, half as much. The agricultural value of the 

 residual fertilizer potash is consequently a point of much economic 

 importance, especially in view of present prices for fertilizer potash. 



The final criterion of such value is, of course, in the crop yields and their 

 potassium content noted at successive periods after potash fertilizing. 

 There are of record few long-continued field or pot studies upon this 

 point. 



Chemical methods of soil examination are doubtless not strictly com- 

 parable with biological methods for determining the degree of "avail- 

 ability" of a plant food present in the soil, or for determining the 

 quantity of such material present in different degrees of availability. 

 They do serve, however, to determine the existence of differences in the 

 condition of solubility of such plant food, and in a roughly quantitative 

 way, the amounts present in the conditions contrasted. 



PREVIOUS WORK 



Numerous chemical studies of this sort have been made upon soils 

 representing comparatively brief periods of fertilizer treatment with 

 collectively small amounts of added fertilizer constituents. Few such 

 studies represent, however, long periods of contrasted treatments defi- 

 nitely maintained. Of these, that reported by Dyer (j) from the exami- 

 nation of the barley soils of the Hoos Field, Rothamsted, is in many 

 respects the most important. The treatment contrasted had continued 

 for 38 years, and not only the amount and composition of the fertilizers 

 but also the crop yields and the ash content and composition of the crops 

 removed, were known for two of the plots, soils from which were analyzed. 

 Of these two plots, No. 2 A had received no fertilizer potash. No. 4A, 

 4,100 pounds. The potash had not largely increased the crop yields, 

 but had increased the potash content of the barley; so that, while the 

 38-year yields from plot 2A contained 984 pounds of potash, those from 

 plot 4A contained 2,057 pounds. The net residuum of fertilizer potash 

 on plot 4A was, therefore, 2,043 pounds; whereas, plot 2 A had lost 984 

 pounds in the crops, making a net contrasted difference between the 

 two plots of 3,027 pounds. The results of the chemical examinations 

 of the two soils at the end of the 38-year period with respect to 

 potash are summarized as follows: 



