“After the organic nitrogen of decomposing substances has been converted into ammonia, and to a small extent into free nitrogen, the latter can at once be utilised by the root-nodule organisms and other bacteria in the soil, but the ammonia must undergo two further changes and combine with a base to form a nitric salt before it is available for plant life. These two changes are brought about by bacteria, which convert the ammonia first into nitrous and then into nitric acid; this process has been called ‘nitrification.’”
It will be clear from the foregoing remarks that the process of mineralisation is a very complicated one, which under favourable conditions, for instance in the pores of an open soil, may come to an end fairly quickly, but which under very unfavourable conditions—such as the interior of large heaps of refuse—may last many years.
[Sidenote: Chemical purification on sewage farms.]
Concerning the chemical purification effected on sewage farms, i.e. the purification of the sewage as revealed by chemical analysis, it has been put on record over and over again, and is now fully and universally understood, that suitable land well managed is capable of changing even the foulest sewage to a perfectly clear water devoid of smell and danger, so that this point need not be laboured here. For instance, on the Berlin sewage farms the degree of purification attained has averaged for a period of 20 years 97 per cent., and on the farm at Gennevilliers—one of the Paris sewage farms—the effluent is so sparkling, bright and clear that the inhabitants drink it in preference to other available water.
[Sidenote: Micro-organic purity of effluent from sewage farms.]
But in reference to the purity of the effluent as to the products of micro-organic activity and pathogenic micro-organisms, it will be necessary to make a few observations with a view to remove misconceptions that have from time to time been put forward.
[Sidenote: Ptomaines have not been found in effluents from well managed sewage farms.]
The question whether the specific products of putrefaction, i.e. the putrefactive alkaloids “ptomaines and toxines,” are capable of doing further mischief by escaping with the effluent into the stream, may be answered as follows. These substances are fortunately very unstable, and the experiments conducted by Falk and others seem further to indicate that soil is capable of retaining them and of rendering them harmless. At any rate there is no well authenticated case on record of these bodies having wrought mischief on sewage farms. (See here also the remarks made on pages 51 and 52 under the heading “The Absorbing Powers of Soil.”)