PLANT FOOD 15 
The work of Schloesing and Muntz attracted a great 
deal of attention, and soon there were a number of 
workers in the field of soil bacteriology. Most striking 
results were obtained in relation to the nitrogen cycle. 
In 1886 Hellriegel and Wilfarth demonstrated the 
presence of nitrogen-fixing bacteria in the root-nodules 
of leguminous plants, and thus solved the problem of 
the nitrogen nutrition of these plants. Another most 
important discovery was the isolation by Beyerinck in 
gor of a special group of nitrogen-fixing bacteria, 
known as Azotobacter, which live free in the soil. 
From the standpoint of crop production these organisms 
possess great significance, for they are constantly adding 
to the stock of available nitrogen in the soil by convert- 
ing the atmospheric nitrogen into nitrogenous organic 
matter. 
The science of soil bacteriology has grown so rapidly 
that the most recent textbook on the subject contains 
goo pages. Over 300 species of the bacillus group, 
and some 200 species of the bacterium group have been 
isolated from soil. 
In some quarters it is now held that the real makers 
of plant food in the soil are bacteria, and they are 
essential to the growth of all plants. At any rate, the 
close connection between bacterial activity and the 
nutrition of plants has been amply demonstrated by” 
numerous experiments, and forms the basis of our 
modern conception of the soil as a producer of crops. 
How different now is our conception of soil from 
what it was even thirty years ago. The term “ living 
soil’’ is literally true. No longer is soil regarded as 
a heterogeneous mixture of rock particles, clay and 
