4o8 



Journal of Agricultural Research 



Vol. VI, No. II 



have shown that it does stimulate the nitrifying organisms and that the 

 influence is shown by the yield obtained from such soils (Lipman and 

 Wilson, 1 91 3). The great variation in the results reported by the vari- 

 ous investigators for zinc, arsenic, and lead is probably due to the fact 

 that it modifies the bacterial flora of the soil, and when heated soil or 

 water cultures are used a different result is noted. This, however, is not 

 the only factor which enters, for these results show a marked difference 

 in soil and in water. The lead arsenate stimulates the nitrogen-fixing 

 organisms when placed in soils but becomes very toxic to the same 

 organisms when placed in nutritive solutions. 



The difference is due in part to the adsorption of the soil, but in this 

 case we would have to attribute it to the silica compounds of the soil, for 

 the nitrogen-fixing organisms are stimulated by arsenic in quartz sand 



zo 



1^ /o 



I 



si 



20 



^O 



GO &0 /OO /20 



/^o 



/<5-0 /&0 



Fig. 2. 



-Graph showing the effect of aeration on the nitrogen-fixing activity of soil containing compounds 



of arsenic. 



free from organic colloids. In this case the arsenic becomes concen- 

 trated at the surface, layers of the silica leaving the inner part of the 

 water film comparatively free from arsenic, in which the micro-organisms 

 multiply and carry on their metabolic processes. This being the case, 

 one should, and probably could, find a water solution weak enough to 

 stimulate bacteria. A great difference, however, between the solution 

 and the sand-culture method is the greater aeration in the latter than in 

 the former. That the aeration of a cultural medium does play a great 

 part in determining the activity of the nitrogen-fixing powers of a soil is 

 very strikingly brought out in figure 2. The graphs in this figure are 

 made from the data given in Tables IV, V, and X. 



It is remarkable how the aeration of the soil or the filtering of the soil 

 extract can prevent the great loss of nitrogen which is noted at first in 

 the unaerated soil. This can not be attributed directly to the denitrifying 

 organisms; otherwise it would not be removed by filtration. The graphs 



