THE AMMONIA OF THE SOIL. 243 



proximate ingredient of soils that under ordinary cir- 

 cunistances 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, i. 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. 



c. Chemically combined Ammonia. — The reader will 

 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, 

 1st, 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 iihiiic, luimic, and apocrenic acids for 

 ammonia is so strong tliat they can only be freed from it liy evaporation of their 

 solutions to dryness with caustic potash. Boiling with carbonate of potash or 

 carbonate of soda will not suffice to decompose their ammonia-salts. We hold 

 it more likely that the ammonia which requires an alkali for its expulsion is 

 generated by ihe decomposition of the organic acid itself, or, if that be desti- 

 hite of nitrogen, of some nitrogenous substance admixed. According to Bous- 

 eingault, ammonia is completely removed from humus by boiling vn.iu water au4 

 caustic magnesia, 



