THE AMMONIA OF THE SOIL. 243 
proximate ingredient of soils that under ordinary cir- 
cumstances exerts a considerable surface attraction for 
ammonia gas is clay. Knop examined the deportment of 
ammonia in this respect towards sand, soluble silica, pure 
alumina, carbonate of lime, carbonate of magnesia, hy- 
drated sesquioxide of iron, sulphate of lime, and humus. 
To recapitulate, the soil contains carbonate of ammonia 
physically absorbed in its pores, 1. e., adhering to the sur- 
faces of its particles,—as Knop believes, to the particles 
of clay. The quantity of ammonia is variable and con- 
stantly varying, being increased by rain and dew, or ma- 
nuring, and diminished by evaporation of water. The 
actual quantity of physically absorbed ammonia is, in 
general, very small, and an accurate estimation of it is, 
perhaps, impracticable, save in a few exceptional cases. 
ce. Chemically combined Ammenia.—The reader wiil 
have noticed that in the experiments of Brustlein just 
quoted, a greater quantity of ammonia was absorbed by 
the soil than afterwards escaped, either when the soil was - 
subjected to a current of air or allowed to dry after moist- 
ening with water. This ammonia, it is therefore to be be- 
lieved, was in great part retained in the soil in chemical 
combination in the form of compounds that not only do 
not permit it readily to escape as gas, but also are not 
easily washed out by water. The bodies that may unite 
with ammonia to comparatively insoluble compounds are, 
Ist, the organic acids of the humus group*—the humus 
acids, as we may designate them collectively. The salts 
of these acids have been already noticed. Their com- 
* Mulder asserts that the affinity of ulmic, humic, and apocrenic acids for 
ammonia is so strong that they can only be freed from it by evaporation of their 
sulutions to dryness with caustic potash. Boiling with carbouate of potash or 
carbonate of soda will not suffice to decompose their ammonia-salts. We ho!d 
it more likely that the ammonia which requires an alkali for its expulsion is 
generated by the decomposition of the organic acid itself, or, if that be desti- 
tute of nitrogen, of some nitrogenous substance admixed. According to Bous- 
singault, ammonia is completely removed from humus by boiling wicm water and 
caustic magnesia, 
