E. W. Hilgard—Objecis and Interpretation of Soil Analyses. 195 
depth may fall below 0°100 consistently with good productive- 
ess and durability ; the former depending upon the amounts 
of lime and phosphoric acid with which it is associated. Virgin 
soils falling below 0:060 in their potash-percentage seem, in all 
cases that have come under my observation, to be deficient in 
available potash, its application to such soils being followed by 
an immediate great increase of production. 
Since but few soils fall below this minimum, my general 
inference has been that potash manures are not among the first 
to be sought for after the soils have become “ tired ” by exhaus- 
tive culture. The universal preference given to phosphatic and 
nitrogenous fertilizers in the west and south, is in accord with 
this inference. In the older portions of the United States, 
‘‘kainite” is becoming more important, while in tie alkali 
lands of California, soluble potash salts often impregnate the 
soil water. 
In all soils not specially impregnated with sea or other 
salts, the amount of soda extracted by the acid is considerably 
below that of potash in the same soil, varying mostly from one- 
eighth to one-third of the percentage of the latter. When much 
more is found in such soils, a repetition of the determination 
will usually show that the separation from magnesia was imper- 
fectly made. I can trace no connection between the soda per- 
