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



lation by manganese of the growth of barley so far as the first 

 crop on the soil in question is concerned. In the case of the 

 second crop, however, a depression in yield of considerable mag- 

 nitude is induced by MnSO^ and a stimulation produced by 

 MnClo in the higher concentrations of the salt, while the lower 

 ones depress the jdeld like MnSO^. In the third crop, as we 

 have already seen, there is a practical disappearance of all 

 toxic effects in both of the manganese series which we had under 

 observation, and taking the place of the former toxic effects we 

 find marked stimulating effects. The indication is therefore that 

 in general our results are in accord with those of the investi- 

 gators cited above who attributed to manganese stimulating 

 .effects for plants. 



ADDITIONAL INVESTIGATIONS 



Nitrification 



Earlier experiments by P. S. Burgess"' and the senior author 

 had demonstrated the stimulating effects of CuSO^, FeSO^, 

 ZnSO^, and PbSO^ on nitrification in soils. "We were therefore 

 led to wonder whether much, if not all, of the stimulation ex- 

 erted on the higher plants by most of the salts in the first crop 

 was due to the increase in the available supply of nitrogen there 

 through the effects of the salts. Accordingly, tests of the nitrify- 

 ing powers of the soils in a number of the pots in every series 

 were made by the usual laboratory methods emploj^ed for such 

 purposes. Dried-blood nitrogen was used as the nitrifiable 

 material at the rate of 1 per cent of the dry weight of the green- 

 house soil. Lack of space forbids the presentation here of the 

 large amount of data collected on the subject now under con- 

 sideration. We may, however, refer to the striking features 

 thereof, owing to their undoubted connection with the cause or 

 causes of the stimulating effects above noted. In the second crop 

 of the copper series in the greenhouse soil the nitrifying power 

 was from 10 per cent to 50 per cent greater in the "coppered" 

 than in the "uncoppered" soil. In the third crop, which, it 



115 Univ. Calif. Publ. Agr. Sci., 1, p. 127. 



