PART 11. 
to be premature. Field observations which have been substantiated 
in some cases by trials, have indicated that there is a wide spread 
condition of nitrogen deficiency. Whether there is an absolute 
deficiency of nitrogen or a deficiency of available nitrogen only it is 
at present impossible to say definitely; laboratory investigation into 
the matter has not yet been possible; the latter explanation 
seems to be the more probable; 1t seems unlikely that most 
of the great accumulation of nitrogen in the virgin soils could 
have been removed in the time during which the soils have been 
under cultivation. It is probable that there is some factor interfering 
with the processes of nitrification. At the same time it has been found 
that the residues of leguminous crops increase the yield of cereals. 
For the time being the nitrogen requirements of the main crops can 
most economically be met by the use of leguminous green manures. 
If laboratory investigation elucidates this problem it will be possible 
for the accumulated store of nitrogen to be drawn upon. 
In connection with the cultivation of legumes some most unusual 
observations have been made. It has been mentioned above that 
ordinary sweet clover is a failure as a green manure crop. Sweet 
clover seed was inoculated with a watery suspension obtained from 
lucerne nodules. It has been shown elsewhere that cultures of the 
bacillus obtained from lucerne would strongly infect sweet clover and 
vice versa. The plants from the inoculated seed, here, were quite 
free from nodules and were only dwarf plants. Recently it has been 
chserved that volunteer plants of sweet clover on the plots previously 
sown to sweet clover were carrying a few small nodules. Tt appears 
that the microbe is adjusting itself to the new host plant. Similar 
cbservations have been made in connection with other plants. These 
observations open up a new line of work upon the characteristics of 
the nodule organism. Is the variety, that is beginning to infect the 
sweet clover, the same as that infecting lucerne, or is it a variety, 
2ormally infecting some of the native legumes, adjusting itself to a 
new host plant supplying conditions not far removed from its original 
host? Or is there, on the other hand, some soil condition that, until 
the plant has adjusted itself, bestows increased immunity to infection 
by the requisite strain of Pseudomonas radicicola ? 
As soil work progresses in Kenya innumerable problems of this 
type, problems of high scientific interest having a direct bearing upon 
farming practice, will arise. Many of these are of fundamental 
importance and will have a bearing not only upon the type of soil but 
upon all the soils of that class. 
In addition to problems directly affecting the farmer or the 
farming practices there are awaiting study many soil problems, the 
connection of which to farming practice may not be evident but which 
will throw light upon a large number of partially understood 
phenomena. Into this category falls the relationship between the 
soll and the vegetation. This can only be studied properly while the 
true virgin vegetation still exists; changes due to cultivation would 
provide a corollary study. Of the various type soils only those derived 
from the Laikipian lavas carry true forest; the others carry grass or 
mixed bush and grass. The deciding factor for forest growth is not 
rainfall, since strips of soil derived from other formations, invading 
forest covered Laikipian soil, do not carry forest growth. There is 
some condition in the Laikipian soils that is favourable to forest 
orowth. It is not always present in the ILaikipian soils as is shown 
36