158 
PACIFIC SCIENCE, Vol. I, July, 1947 
phosphorus level on trivalent and penta- 
valent arsenic to determine whether the de¬ 
crease in toxicity was due to decreased 
absorption of the toxic element in the pres¬ 
ence of large amounts of phosphorus or 
whether it was due to some inhibitory effect 
which phosphorus might have on arsenic 
toxicity after absorption by the plant tissues. 
Bean, Sudan grass, and tomato plants 
were grown in culture solutions at three 
phosphorus levels—10 ppm, 60 ppm, and 
120 ppm—and subjected to treatment with 
sodium arsenate and sodium arsenite. 
In Tables 1, 2, and 3 are recorded the 
data for the tests with pentavalent arsenic. 
It can be seen that an increase in the phos¬ 
phorus level markedly reduced the amount 
of pentavalent arsenic absorbed, and re¬ 
sulted in better growth. For example, the 
tomato plants grown in low-phosphorus 
solutions containing 30 ppm of arsenic con¬ 
tained 106.3 ppm of arsenic in their tops, 
as compared to 49.7 ppm and 26.6 ppm in 
plants grown in medium- and high-phos¬ 
phorus solutions, respectively, containing 
similar amounts of arsenic. 
Similarly, increases in the phosphorus 
level reduced the absorption of arsenic by 
Sudan grass. The analyses of plant tops 
showed 88.8 ppm, 41.0 ppm, and 20.0 
ppm of arsenic from the low-, medium-, 
and high-phosphorus solutions containing 
15 ppm of arsenic. 
The relationship of the phosphorus level 
to arsenic absorption by the bean is not so 
decisive. The bean has a very small range 
of tolerance to arsenic and, as a result, the 
magnitude of the differences between the 
various cultures is correspondingly small. 
The beans showed, for example, 10.3 ppm, 
2.7 ppm, and 2.0 ppm of arsenic in their 
tops when grown in low-, medium-, and 
high-phosphorus solutions, respectively, 
containing 1.5 ppm of arsenic. 
As might be expected, a reduction in the 
absorption of the toxic element by the plant 
resulted in better plant growth. The con¬ 
trols showed no significant differences in the 
dry weights at the three phosphorus levels, 
indicating that 10 ppm of phosphorus were 
adequate for the three plants studied. Analy¬ 
sis of the plant tops showed no significant 
differences among plants grown in low-, 
medium-, and high-phosphorus levels. 
Furthermore, the dry weights seemed to cor¬ 
relate with the arsenic concentration in the 
plant, irrespective of the phosphorus level 
of the culture solution. These considerations 
would seem to indicate that the phosphorus 
in the culture solution is effective in inhibit¬ 
ing the absorption of pentavalent arsenic by 
the plant but not in reducing the toxicity of 
the element within the plant. 
Hurd-Karrer (1939) suggested that 
arsenates would be non-toxic if the P : As 
ratio were more than 4:1. Although a high 
phosphorus level greatly reduced the toxic 
effects of a given concentration of arsenic in 
the culture solution by^reducing its absorp¬ 
tion by the three plants studied here, it did 
not prevent injury. For example, tomatoes 
growing in solutions in which the P : As 
ratio was 10 : 1 (120 ppm : 12 ppm) had 
a dry weight of 23 grams as compared to 28 
grams in the control—a reduction of 18 per 
cent. Sudan grass in solutions containing 
120 ppm of phosphorus and 12 ppm 
of arsenic (P : As = 10 : 1) weighed 28.5 
grams as compared to, 36 grams in the con¬ 
trol—a reduction of 21 per cent. Bean 
plants growing in solutions containing 6 
ppm of arsenic and 120 ppm of phosphorus 
(P:As = 20:l) weighed 37 grams as com¬ 
pared to 57 grams in the control—a reduc¬ 
tion of 35 per cent. A high phosphorus level 
reduces but does not prevent the absorption 
of pentavalent arsenic. The degree of injury 
depends on the amount of the toxic element 
absorbed. 
The data for the studies made with tri¬ 
valent arsenic are recorded in Tables 4, 5, 
and 6. The results indicate that the action 
