Soil Changes Produced By Micro-organisms. 31 
relatively large numbers, in these areas. If it is true that algae 
and organic matter are equally valuable as sources of energy, then 
there should be no question about energy for Colorado Azotobacter. 
However, in regard to algae and organic matter being the sole 
sources of energy for these organisms, we are open to conviction. 
Symbiotic Fixation of Atmospheric Nitrogen, 
The liberal use of leguminous crops such as red clover and 
alfalfa in the role of green manures has come to be recognized 
today as one of the most important steps toward placing our agri¬ 
culture upon a modern, scientific basis. There is not a practical 
farmer but who acknowledges the inestimable value of plowing 
under a heavy alfalfa sod for potato land. If he is a successful 
man, he does not, or at least he should not, follow wheat with wheat, 
but seeds his field to alfalfa or some other leguminous plant which 
he has found by experience will make up in some mysterious way 
for the deficiency in soil fertility caused by the preceding crop. 
This is the principle upon which the rotation of crops is based. 
If the Leguminosae possess such restorative properties, there 
must be some peculiarity in their nature and habits to account for 
this remarkable power. From the earliest days of agriculture, it 
has been recognized that this family of plants has a decidedly 
beneficial effect upon the soil. Almost nineteen hundred years ago, 
it was observed and noted by such men as Pliny the Elder that, 
“The bean ranks first among the legumes. It fertilizes the ground 
in which it has been growing as well as any manure. The lupine 
enriches the soil of the field or vineyard as well as the very best 
manure. The vetch, too, enriches the soil and requires no atten¬ 
tion in its culture.” Varro, in De Re Rustica, 1, 23, writes, 
“Legumes should be sown in light soils; indeed they are planted 
not so much for their own crop as for the following crop, since 
when they are cut and kept upon the ground they make the soil 
better. Thus the lupine is wont to serve as a manure where the 
soil is rather thin and poor.” The early writings on agriculture 
are full of just such references to the importance and necessity of 
including some leguminous crop in the regular rotation. Naturally, 
the explanations for this conservation of fertility were many and 
various, but the one which received most favor was the belief that 
the root system of these plants was much more extensive than that 
of grains and root crops, and in consequence brought up plant food 
from greater depths, which was not only available for the legume, 
but also served for subsequent crops. Thaer in 1809 ventured the 
