Jan. 27. 1919 Influence of Salts on Nitric Nitrogen in Soil 123 



Sodium sulphate and potassium sulphate are the only sulphates 

 which fail to increase the nitric-nitrogen content of the soil, and in this 

 respect they are similar to the ammonifiers (j). The degree and concen- 

 tration at which the various salts stimulate vary with the compound, 

 being greatest with calcium sulphate and least with magnesium sulphate. 



Calcium sulphate is the most powerful soil stimulant known. This is 

 due not to a direct nutritive value but to the liberation of plant food 

 which may in a m.easure be due to the direct interchange between cal- 

 cium and potassium. However, these results clearly indicate that its 

 main influence is upon the bacterial activities of the soil, especially the 

 ammonifying and nitrifying organisms of the soil. In this manner the 

 available nitrogen of the soil is increased. Furthermore, in the metabolic 

 processes of these bacteria there are formed acids and other compounds 

 which act as solvents for the potassium and phosphorus of the soil. It 

 is well known that the addition of gypsum increases the potassium of the 

 crop, and some other experiments show that it increases the phosphorus, 

 for example, Boussingault (9, v. i, p. 327) found a greater amount of phos- 

 phorus in land manured with gypsum. The phosphorus in the clover from 

 the manured land was 10.57 kilos; that from the immanured 4.80 kilos. 

 The following year, although no more manure was applied, the phos- 

 phorus from the hay grown on the manured land was 6.93 kilos more than 

 from the unmanured. 



Although magnesium, manganous, and iron sulphate all increase 

 at some concentration the nitric nitrogen of the soil, they are not nearly 

 as active as is calcium sulphate. 



The toxicity of sodium and potassium sulphate is very marked even 

 at the lowest concentrations tested, and increase with increasing quantities 

 of the salt. Although the toxicity of potassium sulphate does not in- 

 crease as rapidly at the lower concentrations as does that of sodium 

 sulphate, yet at the highest concentration tested, 3X10""^ mole, the so- 

 dium-sulphate-treated soil contains 50.8 per cent normal of nitric nitro- 

 gen, whereas that of the potassium sulphate treated soil contains 32.4 

 per cent. 



Calcium sulphate is not toxic at any of the concentrations tested, 

 whereas magnesium sulphate probably first becomes toxic at a concen- 

 tration of 628 X io~^ mole, and at 3X io~^ mole the nitric nitrogen con- 

 tent of the soil had been reduced to about one-half normal. 



Manganous sulphate and iron sulphate are similar in that they are only 

 slightly toxic even at the higher concentrations. It is evident from 

 these results that the stimulating action is due mainly to the electro- 

 positive ion, whereas the electro-negative ion determines the toxicity of 

 the compound. 



