244 HOW CROPS FEED. 
pounds with ammonia are freely soluble in water; hence 
strong solution of ammonia dissolves them from the soil. 
But when ammonia salts of these acids are put in contact 
with lime, magnesia, oxide of iron, oxide of manganese, 
and alumina, the latter being in preponderating quantity, 
there are formed double compounds which are insoluble 
or slightly soluble. Since the humic, ulmic, crenic, and 
apocrenic acids always exist in soils which contain organic 
remains, there can be no question that these double salts are 
a chemical cause of the retention of ammonia in the soil. 
2d. Certain phosphates and silicates hereafter to be no- 
ticed have the power of forming difficultly soluble com- 
pounds with ammonia. 
Reserving for a subsequent chapter a further discussion 
of the causes of the chemical retention of ammonia in the 
soil, we may now appropriately recount the observations 
that have been made regarding the condition of the am- 
monia of the soil as regards its volatility, solubility, ete. 
Volatility of the Ammonia of the Soil. — We have 
seen that ammonia may escape from the soil as gaseous 
carbonate. The fact is not only true of this substance as 
physically absorbed, but also under certain conditions of 
that chemically combined. When we mingle together 
equal bulks of sulphate of lime (gypsum) and carbonate 
of ammonia, both in the state of fine powder, the mixture 
begins and continues to smell strongly of ammonia, owing 
to the volatility of the carbonate. If now the mixture be 
drenched with water, the odor of ammonia at once ceases 
to be perceptible, and if, after some time, the mixture be 
thrown on a filter and washed with water, we shall find 
that what remains undissolved contains a large proportion 
of carbonate of lime, as may be shown by its dissolving 
in an acid with effervescence; while the liquid that has 
passed the filter contains sulphate of ammonia, as may be 
learned by the appropriate chemical tests or by evaporat- 
ing to dryness, when it will remain as a colorless, odorless, 
