21 
turn to another side of the subject. It has been found by experience 
that a continual growth of the same crop in a field year after year 
ensures a gradual failure of the crop, of course assuming that the 
crop is cut and carried every year. This arises from the fact that 
the particular foods required for that plant are not formed in the 
earth sufficiently quickly to supply a full amount year after year. 
But if different crops are sown in rotation, as they do not all require 
exactly the same proportion of the same compounds, much better 
crops can be maintained. But in addition to this, certain necessary 
compounds can be added to the soil as manures, to supply afresh 
the deficient ingredients. Several manures are used that are 
specially noted for the amount of N they supply to the soil, either 
as nitrates or ammonia. These manures are, many of them, very 
expensive. Some scientific men, working at the chemistry of 
agriculture, found that when certain leguminous plants were grown 
in soil, the soil ultimately became much richer in nitrates. Others 
working at the farmer’s side of the question, found that a crop of 
leguminous plants in the rotation of crops added very much two the 
success of the latter crops. Many botanists had long known that 
the Leguminose, or plants of the pea and bean tribe, usually had 
little swellings or nodules like little tubercules on their roots, and 
these nodules were found to be highly nitrogenous. Recently all 
these isolated facts, the special property of the chemist, the farmer 
and the botanist, have been welded together in a most interesting 
manner by the bacteriologist who makes a special study of bacteria 
and all microscopic organisms. It is now known that these nodules 
are little exuberant growths of the roots containing colonies of 
bacilli. The irritation of the bacilli causes the extra growth in the 
root thus forming a nodule. It has been proved by experiment 
that if the plants are grown in sterilised sand or any medium con- 
taining no bacilli, the nodules do not form, but if the bacilli are 
sown in the soil the nodules form, showing that the nodules are a 
consequence of the presence of the bacilli. The bacilli exist in all 
ordinary soils, but if there is a large proportion of nitrates already 
in the soil, then the tubercules are very poorly developed, as the 
plant does not need them; if there is a great want of nitrates then © 
the tubercles are developed to their utmost degree. Itis not known 
yet what exactly happens to change free nitrogen into a form which 
the plant can make use of, but it is certain now that the bacilli in 
- some way make the free N of the air which exists in the soil avail- 
able for the plant, which no other agency is at present known to be 
capable of doing. It seems to have been proved by scme good 
observers that different bacilli are found in different plants, and 
that the same bacilli will not grow in the roots of different plants. 
But [ should think that this only applies to different genera or to 
species of very different habit. It does not seem probable that 
