SOIL AND SUBSOIL. 147 



vent nitrification altogether, while smaller amounts retard it 

 proportionally. According to Dumont and Crochetelle, potas- 

 sium chlorid acts favorably up to .3 per cent, but at .8 per 

 cent, suppresses nitrification. Earthy and alkaline sul fates, on 

 the contrary, seem to act favorably throughout, at least up to 

 .5 per cent, this is especially true of gypsum, which, according 

 to Pichard, accelerates the process more than any other sub- 

 stance known. Taking the effect of gypsum as the maximum, 

 he found that, other things being equal, the amounts of nitrates 

 formed were as shown in the table below, the effect of gypsum 

 being taken as 100: 



Gypsum i oo 



Sodic Sulfate 4 7 .9 



Potassic Sulfate 35.8 



Calcic Carbonate 1 3.3 



Magnesic Carbonate 12.5 



The above estimates are markedly confirmed by the observations of 

 the writer in the alkali soils of California. In these, nitrates exist most 

 abundantly when the salts contained in the soil are mainly sulfates ; 

 while wherever common salt or sodic carbonate are present in con- 

 siderable amounts, the amounts of nitrate found are notably less. In 

 saline seashore lands nitrates are usually present in traces only. Wollny 

 has moreover shown that the nitrates themselves exert a repressive 

 influence on nitrification. 



Effects of .-I era t ion and Reduction. "While the fostering 

 effect of sulfates upon nitrification is very energetic in well 

 aerated soils, they become injurious whenever by a reductive 

 process in ill-drained lands, the sulfates are reduced to sulfids. 

 Under such conditions the process will in any case be much im- 

 paired. On the other hand, the favoring effect of abundant 

 aeration was strikingly shown in the experiment made by 

 Dcherain. in which a cubic meter of soil was left unmoved for 

 several months, while a similar mass was thoroughly agitated 

 once a week during the same time. The proportion of nitrates 

 formed in the latter case was as 70 to I formed in the quiescent 

 soil mass. It follows that the intensity of nitrification is essen- 

 tially dependent upon the porosity of the soil; and that it is 

 thus greatly favored in the pervious soil-strata of the arid re- 



