483 
Even this theory is, however, now abandoned by various well-known 
investigators who believe that the actual amount of plant food in the 
soil has little if any influence on soil fertility,— but, on the other hand, 
there are many equally brilliant investigators who think that the 
abandonment of this theory is not logical. One has particularly to 
bear in mind in studying such a complex material as the soil and its 
fertility the question of limiting factors, i. e., if one particular in- 
gredient is entirely absent, the plant starves, however liberally it may 
be supplied with the others. 
A more recent theory expounded by several investigators, parti- 
cularly in America, and receiving support from others in various 
parts of the world, is that fertility is determined by plant excretions, 
that is, that plants excrete a poison which is injurious to themselves- - 
hence the necessity of crop rotation. 
One investigator in fact claims to have actually isolated com- 
pounds from such soils, injurious to the same crops which produce 
them, but the results are far from conclusive. 
Having failed to account for soil fertility by the amounts of plant 
food actually found in various soils by analysis, we are compelled to 
search for other causes, and although we have again to return to 
older theories— as has frequently happened in the advance of science 
in general, it was not till comparatively recently that such theories- 
then only advanced as theories— have been found to explain certain 
facts. 
Recent investigations have shown the presence in soils of bacteria 
now known as “ nitrifying bacteria ” which convert nitrogenous mat- 
ter into nitrites and subsequently nitrates by oxidation — and thus 
supply plants with nitrogen in an assimilable form — since with the ex- 
ception of a few plants, viz., those of the leguminous order— plants 
can only assimilate nitrogen after conversion to an oxidized form. 
Such bacteria are consequently regarded as a great factor in soil 
fertility. 
The proof of the existence of such oxidizing bacteria explained to 
a great extent the value of tilth in surface soil, the lack of value of 
the subsoil in which such bacteria do not exist, and the value of 
shallow ploughing contrasted with deep subsoil ploughing. 
These discoveries led to others, in which two brilliant investiga- 
tors, Hellreigel and Wilgarth and subsequently others made the in- 
teresting discovery that certain plants of the leguminous order 
possessed bacteria growing in symbiosis on the roots, which possessed 
the power of assimilating nitrogen directly from the atmosphere and 
converting it into a form suitable for the use of the plant. 
Long before this discovery, however, the value of leguminous 
plants had been realized by the farmer and a system of rotation of 
crops based on it. Since the discovery, other bacteria possessing 
similar properties, but not associated with any plant, have been found 
in the soil. 
