170 
PACIFIC SCIENCE, Vol. I, July, 1947 
ture solutions show trivalent arsenic to be ap¬ 
proximately 10 times as toxic to Sudan grass 
and tomato plants as the pentavalent form 
and approximately four times as toxic to 
bean plants as the pentavalent form. The 
trivalent form acts more quickly and vio¬ 
lently on plant tissues. 
2. Studies on the relationship of the phos¬ 
phorus level to the toxicity of pentavalent 
arsenic show that an increase in the phos¬ 
phorus level materially reduces the absorp¬ 
tion of arsenic by bean, Sudan grass, and 
tomato plants. The phosphorus was found 
to have little or no effect on the toxicity of 
the element after it has been taken into the 
plant. 
The phosphorus had little, if any, effect 
on the absorption of trivalent arsenic from 
culture solution by bean, Sudan grass, and 
tomato plants. 
3. Results are presented for several crops 
of Sudan grass, tomato, and bean plants in a 
re-cropping experiment with red and black 
soils treated with increments of sodium 
arsenite. It was found that as time elapsed, 
more and more of the arsenic was fixed by 
the soil, a fact indicated by a reduction in the 
amount of arsenic found in the plant tops. 
Growth curtailment, however, was observed 
to take place each time at the same levels of 
soil arsenic, irrespective of the levels of 
arsenic absorbed. 
It was found that the plant species varied 
in the ability to withdraw arsenic from the 
soil medium, tomato and bean being low 
and Sudan grass high in ability to withdraw 
the element. 
4. Sudan grass and tomato plants were 
grown in a black alluvial soil treated with 
sodium arsenite, and the results were com¬ 
pared to those in the red soil experiment. 
Marked differences were found in the re¬ 
sponse of the plants to a certain concentra¬ 
tion of arsenic in the two soils. 
5. Whereas in culture solution Sudan 
grass was as tolerant to arsenic as the tomato 
and much more so than the bean, in soil, 
Sudan grass was less tolerant to arsenic than 
either. 
6. The removal of soil arsenic by crops 
which are tolerant to arsenic will at best be 
a very slow process. 
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