ABSTRACT OF PROCEEDINGS. xili. 
putrefactive moulds and bacteria converting it first into 
humus with evolution of carbonic acid and metabolism of 
the nitrogenous matter into simpler forms, such as amides 
and ammonium carbonate. The further nitrification of the 
ammonium salts is the work of other specialized organisms, 
some of which convert the ammonium compounds into 
nitrites and others complete the oxidation to nitrates. 
Conditions favourable to nitrification are (1) presence of 
suitable food for the development of the organisms, namely 
lime, potash, sulphates and phosphates and free carbonic 
acid. (2) Suitable temperature, the optimum being about 
36° OC. (3.) Presence of a base, such as carbonate of lime, 
to combine with the iree nitrous and nitric acid produced. 
(4.) Presence of a suitable amount of moisture. (5) Absence 
of too strong light. (6.) Presence ofa sufficiency of oxygen, 
the process being essentially an oxidising one. 
All the nitrogen is ultimately derived from the atmo- 
sphere, which is converted into organic material in several 
ways. Free-living nitrogen-fixing organisms which are 
present in all soils have the power of fixing atmospheric 
nitrogen and thereby enriching the soil in nitrogenous 
organic material. These thrive best in soils rich in organic 
matter but poor in nitrogen, the fixation being due to the 
oxidation of carbohydrates which supplies the energy. In 
the decayed leaves of forest trees these organisms are very 
abundant, the fallen foliage of beech-trees having been 
found to accumulate 19 tbs. nitrogen per acre. 
In the case of leguminous plants, nitrogen-fixing bacteria 
are found in nodules formed on the roots, which bacteria 
are either parasitic or symbiotic with the host plant. This 
explains the enormous gain in nitrogen resulting from the 
growth of such crops as cow-peas, even when the crop is 
not ploughed under. Attempts have been made to prepare 
pure cultures of these root organisms by different means for 
