IOWA ACADEMY OF SCIENCE 
129 
The first trial shows that the bur extract was poisonous to clover, even the 
one-fourth strength entirely killing the clover. In the second trial the results 
are somewhat erratic. The N — 4 strength killed only one of the plants, and 
the N— -8 and N — 16 dilutions were actually stimulating to two of the plants. 
Still even in the weakest dilution one of the plants was killed, showing toxicity. 
The clover is not quite as susceptible to the toxin in the burs as the corn. 
From the trials with the com and the clover the general conclusion can be 
drawn that there is a substance in the cocklebur which can be extracted with 
hot water, which is poisonous to them both. Calculations show that the ex- 
tract from 15-16 of a bur was found in each 5 c. c. of the N — 4 extract, and 
that 9 per cent of the bur had been extracted. Therefore there was enough 
poison in one bur to seriously poison the corn or clover radicle in one day. 
Work had to stop at this point, but the following facts were proven: There 
is a toxin in the burs, and a sure, easy, and delicate method was found for 
detecting this toxin. This toxin is not only poisonous to corn and clover 
when the radicles of these plants is immersed in a water solution of the toxin, 
but it is also poisonous to germinating clover when growing in the soil. The 
pot cultures also give strong indications that the presence of the burs in the 
soil is detrimental to the growing clover. 
SUMMAEY. 
The Southern Iowa loess from a certain field was studied in the laboratory 
to ascertain the reason that stable manure caused the remarkable increase in 
productiveness which it did when applied to the soil. 
It was found that the soil was not acid. 
The physical effect of the manure on the soil was not responsible for the 
better growth of the crop. 
Bacteria were in no way responsible for t|ie beneficial action of the manure 
on the growth of the crop. 
The investigation of the plant food brought by the manure proved that the 
manure had a beneficial action other than that attributable to the soluble 
mineral plant food found in it. Evidence was brought to support the theory 
that the clover used the soluble organic matter directly as food, but this point 
was not proven. 
The study of the antitoxic action of the manure showed that this was the 
only explanation for the action of the manure on the soil. There was a toxin 
in the soil which was neutralized by the manure. The soil pots treated with 
mineral fertilizers had the toxin corrected by the combined action of the 
clover roots and the chemicals, but this was a slower process than the action 
of the manure. 
A study of the cockleburs which grew abundantly on this field, showed that 
there was a toxin in the burs which was fatal to both corn and clover. The 
inference is natural that the burs caused the toxicity of the soil. 
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