— 82 — A brief reference was made to these facts when considering the enrichment of the soil by green manures, but no explanation was afforded of the processes whereby the insoluble organic material becomes capable of assimilation by the plants of the crop.

The question is purely bacterial and involves the co-operation of no less than four different groups of micro-organisms.

Considering only the formation of nitrate, the first step is the fixation of nitrogen from the atmosphere by the nodule-forming organisms, whereby complex nitrogenous bodies are built up within the substance of the nodule.

These by reason of their insolubility cannot be used as food by plants, but owing to the activities of a certain group of bacteria they are quickly oxidised, or hydrolysed, into simpler bodies and ammonia.

The ammonia is converted into ammonium carbonate by interaction with either the carbonate present in the soil or the carbon dioxide of the soil atmosphere.

This is transformed by the nitrosomonas group into nitrite, and finally the nitrite is changed to nitrate through the agency of the nitrobacter group.

Bio-chemical investigations conducted with farm soils seem to show that they are deficient in nitrite and nitrate-forming organisms, or at any rate conditions are not favourable for their successful development.

Synthetic media inoculated with soil invariably show that while the ammonification of peptone and the denitrification of potassium nitrate is very rapid, the formation of nitrite from ammonia and its subsequent oxidation to nitrate is feeble, and that if any nitrate is formed it is rapidly decomposed. A typical series of sixteen samples gave the following results : Formation of NHg from Peptone ... ... Positive after seven days.

Formation of Nitribes from (NH) SO ... Negative after one month.

Formation of Nitrates from Na No ... Negative after one month.

Denitrification of KNOg Positive after seven days and complete in three weeks.

It has been stated that the thermal death point of the nitrosomonas group is 45 deg. C, and of the nitrobacter group from 50 deg. C. to 55 deg. C. If this were the case with the Sudan type, the top layers of soil would be quickly sterilised whenever fallowing occurred.

In the writer's opinion it is the poor aeration of the heavy clay soils, and not deficiency of bacteria which is responsible for the poverty in nitrogen, for in lighter soils quite different results were obtained.