58 HINTS ON SOILS AND FERTILIZERS 
a clover or an alfalfa plant. These nodules 
serve as small houses in which these bacteria 
that convert the nitrogen of the air into nitro¬ 
gen for the plant, live. When the soil is not 
too acid, these bacteria enter the roots of these 
plants, in great numbers, and have the peculiar 
ability of taking the nitrogen from the air— 
the air enclosed between the particles of the 
soil—and changing it so that the plant can use 
this nitrogen for growth. In return, the bac¬ 
teria suck a small part, of the plant juices from 
the plant, but not in a sufficient amount to 
cause any check in the growth of the plant. 
When the crop has been cut, or harvested, and 
the season is too far along to make any more 
real growth, these bacteria then leave the 
roots of these plants and return to the soil. 
In soils that are quite acid, these bacteria, 
for some reason that has not yet been deter¬ 
mined, cannot thrive, and we must add lime 
to make it possible for them to work. If we 
wish to get a crop of alfalfa started, and the 
particular kind of bacteria that work on alfalfa 
roots are not present, what then? Well, we 
could do one of two things. We could either 
go to a field that had already raised alfalfa on 
it, and take off a few bushels of the soil, and 
haul it over to the field that was to be sown 
to alfalfa. This soil from the field that already 
had raised alfalfa would contain a sufficient 
number of these bacteria to give the new alfalfa 
a good start. But this method is sometimes a 
very laborious job. A much simpler method is 
for the farmer to send to his Agricultural Col¬ 
lege, and ask them to send him a bottle of 
