360 HOW CROPS FEED. 
avidity (an insoluble double silicate being formed just as 
in the experiments of Way, p. 343). According to Liebig, 
a quantity of hydrated alumina equivalent to 2.696 grms. 
of anhydrous alumina, absorbed from a liter of solution 
of silicate of potash containing 1.185 grm. of potash and 
3.000 grm. of silica, fifteen per cent of the silicate. Doubt- 
less hydrated oxide of iron would behave in a similar 
manner. 
3. The organic acids of humus are usually the most 
effective agents in retaining the bases when the latter are 
in the free state, or exist as soluble carbonates or silic- 
ates. The properties of the humates have been detailed _ 
on page 230. It may be repeated here that they form 
with the alkalies* when the latter preponderate, soluble 
salts, but that these compounds unite readily to other 
earthy* and metallic* humates, forming insoluble com- 
pounds. Lime at once forms an insoluble humate, as 
do the metallic oxides. When, as naturally happens, the 
organic acids are in excess, their effect is in all cases to 
render the soluble free bases or their carbonates nearly 
insoluble. 
In some cases, ammonia, potash and soda are absorbed 
more largely from their carbonates than from their hy- 
drates. Thus, in some experiments made by the author, 
a sample of Peat from the New Haven Beaver Meadow 
was digested with diluted solution of ammonia for 48 — 
hours, and then the excess of ammonia was distilled off 
at a boiling heat. The peat retained 0.95°|, of this alkali. 
Another portion of the same peat was moistened with 
diluted solution of carbonate of ammonia and then dried 
at 212° until no ammoniacal smell was perceptible. This 
saniple was found to have retained 1.30°|, of ammonia. 
This difference was doubtless due to the fact that the 
*In the customary language of Chemistry, potash, soda, and ammonia are 
alkalies or alkali-bases. Lime, magnesia, and alumina are earths or earthy bases, 
and oxide of iron and oxide of manganese are metallic bases. ' 
