18 FODDER AND PASTURE PLANTS. 
up the superficial roots and the overground parts of the plants, 
which parts, when ploughed down, added this material to the surface 
soil. There is no doubt that plant food is removed from the subsoil 
and stored in the upper parts of the plants and that the above ex- 
planation should be considered. But the soil-enriching faculty of 
leguminous plants is connected with phenomena that render this 
explanation insufficient. Generally Alfalfa will not thrive on soil 
where it has never been grown before. The plants soon stop growth, 
‘turn yellow and finally die. If, however, some soil from an old 
Alfalfa field is sown on the land, a crop will be produced without 
any trouble. If the plants are examined, it will be found that the 
roots of those grown on old Alfalfa soil are provided with numerous 
nodules, whereas the roots of weak plants on virgin soil are destitute 
of them. Only quite recently have the origin and significance of 
these tubercles been understood. It has been proved that they are 
a kind of gall produced by certain bacteria. These bacteria live in 
the ground, attack the root hairs, break through their thin walls, 
and make their way to the interior of the root branches. There 
they propagate rapidly, forming masses within the nodules. Later 
on, most of the bacteria decompose and are used by the plants, 
which thus obtain additional food. As the bacteria are very rich in 
nitrogenous substances, the source of which is the air contained in 
the porous soil, leguminous plants are able to secure, indirectly through 
the bacteria, their nitrogen from the air. They are therefore able to 
accumulate nitrogen without robbing the soil and, when dying, to 
leave a supply of nitrogenous substances for succeeding crops. 
When soil from land where Alfalfa, for instance, has been suc- 
cessfully grown is put ona field, that field is supplied with the bacteria 
necessary for the development of Alfalfa. The amount needed ‘s 
not large, two hundred pounds being sufficient for an acre. Instead 
of soil from old fields, artificial cultures of bacteria are now available 
at many botanical laboratories. These cultures, with directions for 
their use, are on sale in bottles at a low price. 
Nodule-forming bacteria are necessary for the proper development 
of all kinds of leguminous plants. But this does not mean that 
bacteria which will serve for a certain plant will satisfy another kind. 
On the contrary, there are different species and races of nodule- 
forming bacteria, and each species or race is able to produce nodules 
only on a certain kind of leguminous plant. Thus the bacteria 
which work on the rcots of Red Clover are different from those 
which produce nodules on the roots of Alfalfa and are quite unable 
to benefit the latter plant. In using artificial cultures of nodule- 
bacteria, therefore, care should be taken to procure the right kind. 
