204 SCIENCE PROGRESS. 



The same soil treated for a short time with strong hydro- 

 chloric acid yielded potash equal to 6269 lbs. per acre. 



Plot 4 A has received during the thirty-eight years 

 about 4100 lbs. of potash per acre as manure; the total 

 potash of the two plots mentioned in the table differs, how- 

 ever, by more than this amount, probably from some 

 inaccuracy in sampling the soils. On the plot manured 

 with potash there are found 753 lbs. of potash soluble in 

 one per cent, citric acid. This result is in striking contrast 

 with the 57 lbs. yielded by the plot in which potash 

 exhaustion has commenced. The proportion of the potash 

 manure remaining soluble in citric acid, and therefore appa- 

 rently still available to the crop, is however far smaller than 

 the proportion of phosphoric acid remaining available in the 

 experiments already described. The cause of this requires 

 further investigation. We must either assume that the 

 potash has entered into combinations which are not decom- 

 posed by the weak citric acid, or that it has passed into the 

 subsoil. 



The examples quoted sufficiently show that the quantities 

 of phosphoric acid and potash extracted from these heavy 

 loams by a one per cent, solution of citric acid are plainly 

 related to the quantities of available phosphoric acid and 

 potash present, as shown by the barley crops grown on the 

 land. When, however, we look a little more nearly at the 

 figures yielded by the treatment with citric acid, we see 

 quite plainly that the action of this acid by no means exactly 

 represents the action of the barley roots ; that, in fact, the 

 one per cent, solution of citric acid is a much better solvent 

 for soil phosphates than it is for soil potash. If we take the 

 mean of Dr. Dyer's determinations of phosphoric acid in 

 eight plots of the barley field, to which no phosphates had 

 been applied for thirty-eight years, we find 199 lbs. per acre 

 as soluble in one per cent, citric acid. On all these plots 

 the crop was greatly reduced from the deficiency of available 

 phosphoric acid in the soil. If we now take the mean of the 

 determinations of potash in eight plots to which no potash 

 had been applied, we find 98 lbs. per acre soluble in the one 

 per cent, solution of citric acid. Yet this far smaller amount 



