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IOWA ACADEMY OF SCIENCE 
The conclusion, reached as a result of this work was that bacteria are in no 
way responsible for the beneficial action of the manure on the growth of the 
clover, for in the first place it was shown that the sterile manure was as bene- 
ficial as the unsterilized, and in the second place it was shown that when the 
whole pot was sterilized, the crop did not suffer. 
THE PLANT FOOD IN THE MANURE. 
How do plants feed and where does their food come from? The old humus 
theory as advocated by Thaer^ was to the effect that “the fertility of the soil is 
really determined by its humus, for excepting the water, it is the humus alone 
that supplies nourishment to the plant in the soil.” Liebig proposed a 
“mineral” theory which was directly opposed to this, and such was his great 
reputation and prestige that he carried the scientific world with him. Dr. A. 
Voelcker^ said this: “The humus theory has retarded rather than promoted 
agricultural improvements. Happily it may be regarded at present as fully 
exploded.” The transition from this extreme doctrine to the theories held 
today has been gradual. Undoubtedly plants can grow and thrive without 
humus or organic matter of any kind if mineral plant food is supplied in the 
proper proportion. Still, partially decayed organic matter is a help to the 
growth of plants. Liebig and his associates explained this by ascribing it to 
the physical effects of the organic matter. Later researches have developed 
the doctrine that this organic matter through processes of decay becomes 
broken down into simpler forms, and then can be used by plants, but that 
organic matter as such is not plant food. The nitrogen must appear as nitrates 
and the potash and phosphorus as soluble salts. The plant obtains its carbon 
from the carbon dioxide of the air, and never from the carbohydrates in the 
manure or the soil. For this reason it is customary to value manure in all 
writings of the present day according to the amount of the three elements, 
nitrogen, potassium and phosphorus found in it. It is also recognized that the 
organic acids produced by the manure or humus help to make plant food avail- 
able which is not otherwise obtainable by plants. Growing crops exhaust the 
soil of a portion of the plant food contained in it, and this must be replaced 
or the soil will in time become exhausted. The enormous trade in commercial 
fertilizers is an outgrowth of this doctrine. Although this is the generally 
accepted view, there has lately been a question raised as to some of these 
points, especially the action of manurial salts in the soil. 
The most natural assumption in studying the action of manure on this soil, 
is that manure helped the growth of the clover because it supplied plant food. 
If this is the case, then these elements supplied in forms as available as that 
in the manure, should give as good returns. This is the whole doctrine of 
commercial fertilizers in a nutshell. This point was thoroughly tested. The 
nitrogen was supplied as sodium nitrate, as a nitrate is the form in which 
nitrogen is used by the higher plants, and is more available than the nitrogen’ 
of the manure which is largely protein. The phosphorus in the first experi- 
ment was a solution of calcium phosphate. In later experiments sodium phos- 
phate was used. Potash was supplied as potassium sulfate. 
-See Heiden Lehrbuch der Dungerlehi'. Vol. 1. 
3 Journal Royal Agl. Society, 18, p. 345. 
