UTAH ACADEMY OF SCIENCES 129 
to become injurious to soil bacteria. Paris green may, 
but the quantity added would have to be large. 
The stimulating activity of the various compounds add- 
ed to the soil, upon ammonifying organisms and especially 
upon the nitrifying forms, is partly due to the anion and 
partly to the cation. Much of their action may be due to 
their influence upon injurious species. 
Water-soluble arsenic may exist as such in soils to the 
extent of 82 parts per million without entirely stopping 
ammonification and nitrification. Large quantities of 
ammonia and nitric nitrogen may be produced in a soil 
containing 50 parts per million of water-soluble arsenic, 
which is a greater quantity than any ever found in an 
agricultural soil. 
Measured in terms of their influence upon ammonifi- 
cation and nitrification as it takes place in soil, the toxicity 
of lead arsenate is the least. Next come zinc arsenite and 
arsenic trisulfide. The greatest toxicity is exerted by Paris 
green. From the results reported in the literature on the 
subject, this seems to be the sequence of toxicity when tests 
are made on the higher plants. 
There is nothing in these results to indicate that 
arsenic trisulfide, or zine arsenite, is as safe as lead arsen- 
ate for use as an insecticide. Arsenic trisulfide may be 
safer when first added to the soil, as is shown by its being 
almost insoluble when first applied and having practically 
no toxic influence upon ammonification; but, as bacterial 
action takes place in the soil, the arsenic of the arsenic 
trisulfide is much more soluble than that of lead arsenate, 
and becomes toxic to the nitrifying organisms when it is 
present in large quantities. 
