Q2 THE VEGETATIVE FUNCTIONS OF PLANTS 
theory was advocated by the 7 great agricultural chemist, 
Liebig. Undoubtedly it is correct, as far as it goes, but 
a more thorough consideration of the question indicates 
that it is not adequate as a complete explanation. 
(b) Theory of Toxic Excreta. A second hypothesis is 
that advanced by certain investigators in the Bureau of 
Soils of the United States Department of Agriculture. 1 
This is based on the fact that the roots of plants are known 
to excrete substances which are poisonous or toxic to the 
species producing them. These toxic substances accumu- 
late in the soil during a succession of the same kind of 
crops, and thus gradually render it toxic to that kind of 
plants. By following with a succession of different kinds 
of crops the toxic excreta of the first are either removed 
by seepage of soil-water, or destroyed, either by being 
oxidized, or by the addition of other substances which 
render them harmless. By this theory the function of 
fertilizers is not so much to renew exhausted mineral 
"nutrients," as to render harmless the accumulated 
excreta of the previous crop. 
(c) Sanitary Theory. This theory has been carefully 
worked out by Professor Bolley, of the North Dakota 
Agricultural College. By thorough studies of the wheat 
crop he has been enabled to make the following positive 
statements: 2 Constant or rather constant culture of 
wheat on the same lands brings about wheat-sickness, or 
wheat-sick soil. Wheat does not thrive well in the 
presence of its own dead bodies, no matter how fertile 
the soil. Constant wheat cropping does not especially 
1 A similar hypothesis was advanced by A. P. DeCandolle in his 
Physiologic V6g6tale, Paris, 1832. 
2 The phraseology of Professor Bolley (North Dakota Agric. Coll. 
Bull. 107), is closely followed. 
