52 
HOUSEHOLD BACTERIOLOGY 
Nitrogen 
Traps 
Enrich 
the Soil 
Inoculating 
the Soil 
It fertilizes # the ground in which it has been sown as 
well as any manure.” The lupine and vetch are also 
mentioned in ancient writings as enriching the soil and 
supplying the place of fertilizers. 
On the roots of these leguminous plants, clovers, 
alfalfas, peas, beans, etc., are seen little nodules which 
have been found to be filled with bacteria. Fig. 29. 
If these “nitrogen traps” are absent or are removed 
the plants are less vigorous. Fig. 30 shows the com¬ 
parative size of two plants of a variety of pea; (a) 
grown on soil containing the proper kind of bacteria; 
(b) grown in the same conditions and soil, but with¬ 
out the bacteria. The nitrogen is stored up in the 
knots, swellings, or nodules on the roots. 
Not only do these nitrifying bacteria thus feed the 
plants which carry them, but also when the plants de¬ 
cay they enrich the soil in which the plants grow. Soils 
“run out,” as the farmers say, that is, there is not plant 
food enough to sustain luxuriant vegetable life. Here 
is a place for the legumes to supply with their tiny 
balls of bacteria the nitrogen which has been with¬ 
drawn. In some way, not understood, the clover or 
similar plant in company with the bacteria stores up 
nitrogen from the air, which is finally returned to the 
soil when the decomposition bacteria have accom¬ 
plished their work, thus making the soil richer in 
nitrogen. 
When the proper kind of nitrifying bacteria are not 
present, the scientist comes to the aid of the farmer 
