EFFECT OF THALLIUM ON PLANT GROWTH a 
LABORATORY INVESTIGATIONS 
Experiments on a laboratory scale were conducted on loam soil in 
trays and in pots at Denver, Colo., over a period of a year. In series 
1, the seed of peas, sweet corn, field corn, clover, and timothy were 
planted in trays 6 by 6 by 3 inches. 
In control trays these were allowed to germinate over a period 
of 2 months, which established the rate of normal growth. On the 
sixty-first day a solution contaiming 49 mg of thallium (Gn the form 
of T1,SO,) was applied once to each tray. This represents the average 
quantity of thallium that might be applied in an ordinary bait spot 
of thalgrain placed in ground-squirrel control. No detrimental effects 
on plant growth were noted in these trays. 
Seeds in other trays were handled similarly, except that 49 mg 
of thallium per tray were mixed with the soil before the seeds were 
planted. Germination was delayed and reduced in all these trays, 
and after slow growth all plants died. 
- Seeds in other trays were allowed to germinate and grow nor- 
mally for 12 days and then were irrigated with a solution containing 
100 mg of thallium per liter Gn the form of TI,SO,) on the twelfth, 
twentieth, twenty-seventh, and thirty-third days. Each irrigation 
contained 100 mg of thallium, so that a total of 400 mg of thallium 
was applied to each tray over a period of about a month. This com- 
pletely inhibited vegetative growth. 
These laboratory results suggest that thallium is toxic to the 
germinating plant but not to the growing plant, except in strong 
concentrations. 
In series 2, in a large mixer, 0, 5, 10, 100, 1,000, and 10,000 mg of 
thallium per kilogram (as Tl,50,) were thoroughly mixed with loam 
soil and three lots of each were placed in 1-gallon jars. The surface 
soil was then prepared for planting, and 12 corn seeds, 1 g of timothy, 
or 1.5 g of clover were planted in each jar. The 18 jars were then 
watered from time to time. With the soils containing 5 and 10 mg 
of thallium per kilogram the growth of corn was definitely stimulated; 
the clover was somewhat less stimulated; and the timothy showed 
some slight stimulation in later growth. Im soils containing 100 mg 
per kilogram or more, toxic effects were noted with all seedlings. A 
concentration of 1,000 mg per kilogram stopped the germination of 
timothy, and 10,000 mg per kilogram stopped the germination of 
corn and clover. These results would suggest that thallium is benefi- 
cial to plant growth when present to the extent of 5 to 10 mg per kilo- 
gram of soil; that the toxic effect begins to develop with concentrations 
of 100 to 1,000 mg per kilogram; and that 10,000 mg per kilogram of 
soil are necessary to prevent germination. 
FIELD INVESTIGATIONS 
Field studies were made on typical areas of plant growth in Cali- 
tornia, the thallium being applied in the form of thallous sulphate 
directly to the soil, either as a dry salt, in irrigation water, or as 
thalerain. The regular thalgrain for ground-squirrel control was used 
in these experiments, and only the thalgrain results are incorporated 
in this report. Every attempt was made to duplicate the conditions 
under which it is commonly employed in rodent-control operations, 
to learn what damage (if any) might follow routine rodent-control 
