Jan. 27, 1919 Influence of Salts on Nitric Nitrogen in Soil 119 



had been applied to the soil, the sulphate at 2.9 p. p. m., and the car- 

 bonate at 744.6 p. p. m. The chlorid is a much more powerful stimulant 

 than are any of the other salts, and in this respect there is a marked sim- 

 ilarity between the ammonifying and nitrifying organisms. 



The results offer a very likely explanation of why there is an increased 

 yield obtained when iron compounds are applied to the soil, as the stim- 

 ulation of the soil organisms would greatly increase the available plant 

 food. There would be not only more available nitrogen but the increased 

 bacterial activity would render soluble more potassium and especially 

 more phosphorus; the results reported by Griffiths (5) indicate that the 

 plants growing on soil manured with iron sulphate contain more phos- 

 phorus than those growing on unmanured soil. We would have to as- 

 sume either that the application of iron to the soil stimulates a plant so 

 that it requires more phosphorus or else that the iron compounds in- 

 crease the availability of the phosphorus, and, hence, the plant takes up 

 more. This latter explanation seems the more reasonable, but here we 

 have to look for an indirect effect, for the iron directly depresses the 

 solubility of phosphorus {2). 



The ferric nitrate becomes toxic to the nitrifying organisms at a much 

 lower concentration than any of the other iron salts. Furthermore, its 

 toxicity increases much more rapidly than that of any of the other com- 

 pounds. The chlorid does not become toxic until 372.3 p. p. m. of the 

 iron has been added to the soil, whereas the sulphate is toxic at 5.8 

 p. p. m. The carbonate was toxic at none of the concentrations tested. 



The highest concentration used 1,116.9 P- P- rn- o^ ^''on in the form of 

 chlorid, reduces the nitric nitrogen content of the soil to 10.9 per cent, 

 the sulphate to 87.9, the nitrate to 7.9, whereas the carbonate containing 

 soil contains 104.5 per cent of normal. 



INFI^UENCE OF CHLORIDS 



So far in this discussion we have been comparing the action of com- 

 pounds having the same electro-positive but a varying electro-negative 

 ion. Hence, the results considered have given us an insight into the 

 influence of the anions CI, SO4, NO3, and CO3 upon the nitrifying effi- 

 ciency of the soil. It is therefore important that the compounds be 

 compared where the anion is a constant and the cation a variable. This 

 is done in Table X. In this series we have the chlorids of sodium, 

 potassium, magnesium, calcium, manganese, and iron. The experiment 

 was so arranged that equivalent quantities of chlorin in the various forms 

 were applied in 100 gm. of soil. Each reported results is the average 

 of four or more closely agreeing determinations. 



