124 
INVESTIGATIONS ON KoTHAMSTED SOILS. 
in the second, and even in the third, 9 inches. The accumulation in 
the subsoil is most evident in the eases of the dunged plats and of the 
plat which, in addition to potassium salts, has received superphos- 
phate and mixed sodium and magnesium sulphates, without nitrogen. 
Both sodium and magnesium salts have exercised a distinct influence 
in increasing the proportion of citric-acid-soluble potash in all depths 
on the plats to which no potassium salts have been applied for forty 
years. These plats still maintain a higher yield of potash in their crops 
than does the plat manured with superphosphate and ammonium 
salts only, though the equivalent of the potash originally added has 
been more than exhausted in one case and nearly exhausted in the 
other. Furthermore, sodium and magnesium salts, used in conjunc- 
tion with potash salts, have caused a much larger retention of potash 
in a citric-acid-soluble condition than when potassium sulphate lias 
been used without them, although the potash taken up by the crops 
has been greater than in the latter case. 
It has usually been considered that potash is pretty firmly retained 
in the surface soil on land containing a fair proportion of cla}\ That 
this is the case, as compared with sodium salts, has often been shown, 
and, apart from earlier investigation, was clearly brought out in the 
drainage water analyses of the late Dr. Voelcker, just referred to; 
'but, as we have seen, even these analyses showed a considerable 
loss of potash in drainage in certain cases, and it is evident from the 
results of the analyses of the soils and subsoils that though, relatively 
to sodium salts, potassium salts readily become fixed in clay soils — 
often, probably, passing into a very stable insoluble form — they are 
nevertheless far more "migratory" than phosphoric acid. 
