4o6 Journal of Agricultural Research voi. vi, no. n 



only 0.80 mgm. of organic phosphorus. This excess of organic phosphorus 

 could not have come from the water-soluble phosphorus, as there was a 

 difference of only 0.07 mgm. in the two soils; hence, it must be concluded 

 that the arsenic increases the solubility of the phosphorus. This, how- 

 ever, may be due either to a direct interchange between the insoluble 

 phosphorus of the soil and the arsenic or to its action upon bacteria, which 

 causes them to become more active in growth and formation of various 

 acids which act upon the insoluble phosphates of the soil, rendering them 

 soluble. 



GENEIL\L CONSIDERATIONS 



The data reported prove conclusively that the arsenical compounds, 

 with the single exception of Paris green, stimulate the nitrogen-fixing 

 organisms of the soil and that this influence varies qualitatively but not 

 quantitatively with the various soils. The results also bring out the fact 

 that both the anion and the cation of the compounds have a marked 

 influence upon the growth of the organisms. With some compounds 

 both the anion and cation act as stimulants, while with others one stimu- 

 lates and the other is markedly toxic. It is likely that little or no influ- 

 ence is exerted upon the nitrogen-gathering organisms by the sodium (Lip- 

 man and Sharp, 1912), and that the stimulating influence noted with 

 dilute solutions and the toxic influence exerted with more concentrated 

 solutions are due entirely to the arsenic. It is quite likely that the 

 stimulating influence which Riviere and Bailhache (19 13) have found 

 sodium arsenate to have upon wheat and oats is an indirect effect which is 

 exerted upon the bacterial flora of the soil and which in turn influences the 

 yield of the various grains. 



Both the anion and cation undoubtedly act as stimulants in the lead 

 arsenate. Stoklasa (191 3) has shown that lead when present in soil 

 stimulates the growth of higher plants. This he (191 1) ascribes to the 

 catalytic action of these elements on the chlorophyll. The results herein 

 reported, together with those previously published (Greaves, 1913a), 

 indicate that it is due to the influence of the compounds upon the biologi- 

 cal transformation of the nitrogen in the soil. The fact that the lead 

 plays no small part in the stimulating influence is borne out by the work 

 of Lipman and Burgess (1914), who found lead to stimulate nitrifying 

 organisms. 



Paris green is toxic to the nitrogen-fixing organisms in the lowest con- 

 centration tested. This is due to the copper and not to the arsenic, as it 

 is well known that the copper ion is a strong poison to many of the lower 

 plants. Brenchley (191 4) found it to be toxic to higher plants when 

 present in water to the extent of i part in 4,000,000,000. Although 

 Russell (191 2, p. 47) states that it is not as toxic in soil as in 

 water, Darbishire and Russell (1905) found it to be toxic in soils, and 

 they failed to get a stimulating influence with it. Montemartini (1911) 



