254 POWERIOF THE SOIL TO ABSORB SALTS [chap. 



Both the humus and the zeolitic double silicates take 

 part in the retention of the potash salts, the reactions 

 being exactly similar to those taking place with the 

 ammonium salts. In some of Way's experiments with 

 pure clays the application of potash salts was followed 

 by the appearance of the corresponding sodium salts in 

 the percolating water, though with most soils it is 

 calcium that is turned out of combination. Potash salts 

 applied to the soil also react to a certain extent with the 

 calcium carbonate, giving rise to a little potassium 

 carbonate, the bad effect of which upon the tilth of the 

 soil will be considered later (p. 293)- Dyer has examined 

 the soils of the Rothamsted wheat plots which had then 

 been continuously manured in the same way for fifty 

 years, with the view of tracing the fate of the mineral 

 manures applied. The following table shows a com- 

 parison of the amounts of potash soluble in strong hydro- 

 chloric acid, in lbs. per acre, found in the top 9 inches 

 of soil from four of the plots; one (No. 11) received 

 nitrogen and phosphates, but no potash, every year, the 

 others were variously manured, but all received 200 lbs. 

 per acre of sulphate of potash. Estimates are also given 

 of the total amount of potash applied as manure and 

 removed in the crops over the whole period, so that in 

 the last two columns a comparison can be made between 

 the actual surplus of potash in the manured over the 

 unmanured soils, and the surplus calculated from the 



