IOWA ACADEMY OF SCIENCE 
119 
It by no means proves that the manure nourished the clover in the case under 
investigation directly by its soluble organic matter. 
But why should it seem strange that flowering plants should be able to 
assimilate soluble organic matter as such and use it directly? We are still 
slaves to Liebig’s mineral theory. Plants undoubtedly can be brought to per- 
fection on mineral food alone, but this is not saying that they could not use 
other food. In fact many plants do use organic food. The non-chlorophyl 
humus plants subsist on organic food, and if they can, why is it not possible 
for chlorophyl plants to do so? . In fact some of them do. The mistletoe uses 
elaborated food. The carniverous plants, as the sundew and the pitcher plant, 
growing on the northern bogs use insects as food. The sundew captures and 
digests insects on its leaves and these help it to grow better and produce more 
and better seeds. The pitcher plant also is helped by the insects that it en- 
traps and digests. 
This investigation of the plant food problem has brought out the following 
points: it has proved that manure has a beneflcial action other than that 
attributable to the soluble mineral plant food found in it, and this beneflcial 
principle is not lost by boiling the liquid manure. It has been shown by scien- 
tiflc investigators that plants under certain conditions can directly , assimilate 
many soluble nitrogen compounds, and also certain carbohydrates. 
The evidence is far from being conclusive that in the case of the clover in 
question the soluble organic compounds of the manure were used directly as 
food. It is not proved, but enough evidence has been submitted to keep the 
question open. 
THE ANTITOXIC ACTION OF THE MANURE. 
The Bureau of Soils has advanced a theory in regard to the cause of unpro- 
ductive or worn-out soils, which is well stated in the following extract.® “Infer- 
tility is often due to the presence of toxic organic bodies in the soil, either 
excreted by the previous crops or perhaps formed by the action of bacteria, 
molds, or ferments from the plant remains. These toxic bodies are organic. 
They may be fatty bodies, nitrogenous bodies, or non-nitrogenous. They appear 
to be quite unstable in the soil, changing rather easily by oxidation into harm- 
less or even into beneflcial bodies. They are not equally harmful to all plants.” 
There were many things in connection with this investigation that suggested 
a toxin in the soil as the cause of the poor growth of the clover, and for this 
reason the toxin theory was given as thorough an investigation as the circum- 
stances permitted. One of the occurrences that supported the theory was the 
fact that the soil when kept in the greenhouse gradually changed in its atti- 
tude towards the growth of clover. As evidence of this, the following pictures 
are submitted: 
Pots 44 and 39 were planted soon after the soil came to the greenhouse and 
the pictures were taken sixty-five days after the series was planted. Pot 71 
was filled from the same sample of soil that had been kept in the greenhouse 
and allowed to dry for nearly three months. The picture was taken sixty-nine 
days after planting, therefore it is only four days older than pot 44, but there 
is a remarkable difference in the growth. A change has undoubtedly taken 
place in the soil and the most probable explanation is that some harmful sub- 
stance that was hindering the growth of the clover in the first place, has dis- 
^Bulletin 55, Bureau of Soils, p. 64. 
