119 
between 12 and 30 per cent. of the total in periods ranging 
between six months and one year. 
Results are also quoted which show that under certain con- 
ditions and in the presence of an ample supply of organic 
matter the nitrogen content of soils may snow increases of 
appreciable magnitude as the result of the action of bacteria 
of the Azotobacter type, the existence of which has been 
demonstrated in soils throughout the Colony. 
The importance of bacteria of the putrefactive type is 
alluded to; these, by the continuous liberation of carbon 
dioxide, assist in rendering available the mineral plant food of 
the soil; energy for these and other bacterial processes is 
supplied by organic matter. There is also the well-known 
mechanical effect of organic matter on the character of soils. 
The results of a large number of manurial experiments on 
various crops have conclusively demonstrated the necessity for 
frequent applications of organic manures; these results have 
been substantiated in other Colonies. 
The continued fertility of soils in the tropics seems very 
largely bound up in the maintenance of the supply of organic 
matter; more attention is necessary to this point in the tropics 
than under temperate conditions, as owing to more favourable 
circumstances it is used up much more rapidly by biological 
processes. 
The question of the biological relationships of tropical soils 
is of much interest and importance; the natural conditions are 
particularly favourable to such development, and afford special 
facilities for study; it is to be hoped that in the near future this 
side of the problem will receive increased attention. 
FERTILITY SURVEY OF THE EGYPTIAN DELTA. 
By B. F. E. Keetine, M.A., 
Director of Physical Services, Survey Department, Ministry 
of Finance, Egypt. 
[ ABSTRACT. ] 
Very extensive operations are now being carried out by the 
Egyptian Government for the improvement of the drainage of 
the low-lying parts of the Delta. At the suggestion of Lord 
Kitchener it was decided to make a detailed survey of the 
present condition of the land so that in years to come the 
improvement effected by the works now in progress can be 
assessed. 
The fertility’ map shown with the paper is that of the first 
