1913] J- E- Greaves 15 



Zinc arsenite, lead arsenate and arsenic trisulfide stimulate the 

 ammonifying activities of a soil, and their toxicity is not very 

 marked until comparatively large quantities of arsenic are present, 

 The two former reduce the ammonifying and nitrifying activities 

 only one-half when 1,120 parts per million of arsenic are present, 

 while arsenic trisulfide exerts a stimulating influence upon the am- 

 monifying activities of the soil in the lower concentrations and does 

 not become very toxic even in the highest concentrations. 



Paris green exerts marked toxicity on the ammonifiers, even 

 "when present in small quantities. When present in large quantities 

 it practically stops ammonification in soil. 



All these Compounds stimulated nitrification, the Stimulation 

 being least for Paris green and greatest for lead arsenate. 



Arsenic trisulfide and Paris green, when present in large quan- 

 tities, nearly stopped nitrification. 



Arsenic stimulated ammonification and nitrification, when it 

 was present in soils in small quantities, but in very large quantities 

 it was toxic. It is improbable, however, that lead arsenate, zinc 

 arsenite, or arsenic trisulfide, will ever be applied to agricultural soil 

 in quantities sußicient 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 added 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 ammonification 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 re- 

 ported in the literature on the subject, this seems to be the sequence 

 of toxicity when tests are made on the higher plants. 



