132 University of California Publications in Agricultural Sciences [Vol. 1 



30 per cent, zinc by 20 per cent, lead by about 15 per cent, and 

 iron by only about 12 per cent of that of the normal yield. 

 Despite this fact, however, copper begins to manifest its toxicity 

 most slowly of the four metals, since it shows little if anv toxicity 

 at a concentration of .005 per cent, whereas all the other metals 

 show clearly a slight toxic effect even at that concentration. 

 Just why this peculiar relationship between the metals tested 

 and the ammonifying flora should obtain is by no means easy 

 of explanation. 



Speaking of the data in Table 1. by and large, it seems justi- 

 fiable to venture the assertion that all the metals become markedly 

 more toxic at concentrations of 0.1 per cent of the dry weight 

 of the soil or in concentrations in excess of that amount. Other 

 details worthy of mention here are the fact that at times the 

 depressing effect of a certain concentration is followed by a 

 stimulating effect at an increased concentration of the same 

 metallic salt. Also, the very striking fact is noted of the rela- 

 tively slight increases of toxicity which accompany relatively 

 large increases in concentration of the salt. The latter is par- 

 ticularly marked in the case of FeS04. Whether or not this 

 latter result is to be explained by the differences in solubility 

 which obtain between the different metallic sulfates employed, as 

 well as by the different adsorptive power exercised by the soil 

 towards every one of them, still remains to be shown. 



In general, it is worthy of special remark here that though 

 these so-called very toxic metals manifest undoubted toxicity 

 towards the ammonifying flora, and no stimulation in any con- 

 centration of them, the toxic effect is relatively small and in 

 some cases even absolutely slight. 



The Nitrification Experiments 



To 100-gram portions of sandy soil in tumblers were added 

 two grams of dried blood (13.29% N). enough water to make 

 optimum moisture conditions, and the necessary amounts of the 

 salts to be tested. The mixture was prepared for incubation as 

 above described for the ammonification cultures and incubated 



