CHEMISTRY OF SOILS 125 



recorded history, and showed that it affords a much clearer 

 indication of the manurial requirements of the soil than 

 can be obtained from the total acid extract. This is well 

 illustrated by comparison of the results obtained by the 

 two methods when applied to samples of soil from the 

 plots at Bothamsted, which differed only in the manurial 

 treatment to which they had been subjected. The plot, 

 which has been continuously unmanured for thirty-eight 

 years, yields smaller crops than those to which phosphatic 

 and potash manures have been regularly applied for the 

 same length of time, owing to the partial exhaustion of 

 these constituents in the first. It was to be expected, 

 therefore, that smaller proportions of phosphoric acid 

 and potash would be found in the unmanured plot than 

 in those to which the manures had been applied. As a 

 matter of fact this proved to be the case, and a certain 

 difference was revealed even in the total acid extract, but 

 the difference in the proportions extracted by the dilute 

 acid is much greater and, therefore, more easily distin- 

 guished. The results obtained are shown in the tables 

 (p. 126). 



It will be seen that while the amounts of potash and 

 phosphoric acid extracted by dilute citric acid are in all 

 cases much less than those extracted by strong hydro- 

 chloric acid, they are, nevertheless, a much better guide 

 to the manurial requirements of the soil. This becomes 

 more apparent when the ratios of the percentages are con- 

 sidered. Thus, the dilute acid extracted from the plot 

 treated with phosphatic manure nearly eight times as much 

 phosphoric acid as from the unmanured plot, whereas 

 strong hydrochloric acid extracted less than twice as much. 

 The dilute acid also extracted ten times as much potash 

 from the plot treated with potash manures as it did from 

 the unmanured plot, whereas strong hydrochloric acid ex- 

 tracted only 1*7 times as much. It will be noticed that 



