370 HOW CROPS FEED. 
Boussingault does not state the kind or concentration, 
or temperature of the acid employed to extract the soil » 
for the above analysis. These are by no means points of- 
indifference. Grouven (lter & 3ter Salemiinder Berichte) 
has extracted the same earth with hydrochloric acid, con- 
centrated and dilute, hot and cold, with greatly different 
results as was to be anticipated. In 1862, a sample from 
an experimental field at Salzmiinde was treated, after be- 
ing heated to redness, with boiling concentrated acid for 
3 hours. In 1867 a sample was taken from a field 1,000 
paces distant from the former, one portion of it was treat- 
ed with boiling dilute acid (1 of concentrated acid to 20 
of water) for 3 hours. Another portion was digested for 
three days with the same dilute acid, but without applica- 
tion of heat. In each case the same substances were ex- 
tracted, but the quantities taken up were less, as the acid 
was weaker, or acted at a lowertemperature. The follow- - 
ing statement shows the composition of each extract, cal- 
culated on 100 parts of the soil. 
EXTRACT OF SOIL OF SALZMUNDE. 
Hot strong acid. Hot dilute acid. Cold dilute acid. 
Potash, 635 116 -029 
Soda, .127 067 .020 
Lime, 1.677 1.046 1.098 
Magnesia, .687 .5389 237 
Oxide of iron and alumina, 7.931 3.180 AC Dae ; 
Oxide of manganese, .030 .086 O71 - 
Sulphuric acid, .059 .039 .020 
Phosphoric acid, 059 -091 057 
Silica, é 1.785 . 234 1% 
Total, 12.990 5.398 2.357 
The most interesting fact brought out by the above fig- _ 
ures, is that strong and weak acids do not act on all the 
ingredients with the same relative power. Comparing the 
quantities found in the extract by cold dilute acid with 
those which the hot dilute acid took up, we find that the 
latter dissolved 5 times as much of oxide of iron and 
alumina, 4 times as much potash, 3 times as much soda, 
