It is safe to say that among the elements which are essential for plant life, nitrogen plays the most prominent part. While nitrogen is considered to be one of the most useful constituents of soil organic matter or humus, it is the available nitrogen which is of especial value. The data given in this bulletin as well as in previous publications, show, in agreement with numerous investigations by others, that the amount of ammonia in soils is quite small, ranging from a few thousandths to a few hundredths of one per cent.
Jodidi, S. L.
"The chemical nature of the organic nitrogen in the soil,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 1
, Article 1.
Available at: http://lib.dr.iastate.edu/researchbulletin/vol1/iss1/1