Acid and Potash Contents of Wheat Soils at Mothamsted. 13 



from the surface soil into the subsoil than on the chemically manured 

 plots, probably accompanied by fixation of some portion in an unavail- 

 able state. 



Strong hydrochloric acid, as a solvent for potash in soil analysis, is 

 shown to be practically useless as a gauge of potash fertility where 

 there is an abundance of total potash in mineral combination, as sili- 

 cates, &c. No concordant results are obtainable except by working 

 under the strictest arbitrary conditions, and the results, even when 

 concordant, have little meaning apart from an independent knowledge 

 of the history of the soil. With this knowledge the results are interest- 

 ing, but in its absence are of little use except in extreme cases. 



The results obtained by citric acid, however, are strikingly instruc- 

 tive and consistent. To illustrate this, it may be stated that the ratio 

 of the average quantity of hydrochloric acid soluble potash in the sur- 

 face soil of three potash-dressed plots to the average quantity found in 

 seven plots not dressed with potash was 1*20 : 1. The citric acid 

 soluble potash ratio, however, was 6*75 : 1. The plots dressed with 

 dung for fifty years and nine years respectively gave, as compared with 

 the same seven non-potash plots, hydrochloric acid soluble potash ratios 

 of 1*27 : 1 and 1-23 : 1, while the citric acid soluble potash ratios were 

 10-67 : 1 and 9\L7 : 1. 



Probably when a soil in the surface depth contains as much as 0*01 

 per cent, of citric acid soluble potash, the special application of potas- 

 sium salts is not needed. 



The largest accumulation of unused manurial potash, whether applied 

 as dung or as potassium salts, is in the surface soil ; but a large pro- 

 portion is also found by citric acid in the second and even in the third 

 9 inches. The subsoil accumulation is most evident in the dunged 

 plots, and on the plot which, in addition to potassium salts, has 

 received superphosphate with sodium and magnesium sulphates, but 

 without nitrogen (abundant supply and small utilisation). Both 

 sodium and magnesium salts, in presence of phosphates and nitrogen, 

 have exercised a distinct influence in increasing the proportion of citric 

 acid soluble potash in all depths on the plots on which no potash has 

 been applied for fifty years, and which still maintain a higher yield of 

 potash in their crops than that given by the plot with superphosphate 

 and ammonium salts alone, though the equivalent of the potash added 

 originally has been practically exhausted. Furthermore, sodium and 

 magnesium salts, used in conjunction with potassium salts, have caused 

 & larger retention of potash in a citric acid soluble condition than when 

 potash has been applied without them, although the potash yielded in 

 the crops has been greater under the influence of the other alkalies 

 .alluded to. 



, It is usually supposed that potash is pretty fairly retained by the 

 surface soil of land containing, like the Kothamsted land, a fair pro- 



