Aside from the carbonates, some neutral salts favor nitrification very markedly; while others tend to depress it. Deherain found that .5 per cent of common salt suffices to prevent nitrification altogether, while smaller amounts retard it proportionally. According to Dumont and Crochetelle, potassium chlorid acts favorably up to .3 per cent, but at .8 per cent suppresses nitrification. Earthy and alkaline sulfates, 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 substance 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 100 Sodic Sulfate 47.9 Potassic Sulfate 35.8 Calcic Carbonate 13.3 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 considerable 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 Aeration 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 impaired. On the other hand, the favoring effect of abundant aeration was strikingly shown in the experiment made by Deherain, 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 1 formed in the quiescent soil mass. It follows that the intensity of nitrification is essentially dependent upon the porosity of the soil; and that it is thus greatly favored in the pervious soil-strata of the arid regions. It also follows that thorough and frequent tillage and fallowing greatly favor nitrification; thus explaining one of the beneficial results of these operations. At the same time, it is true that we may thus in a short time seriously diminish the reserve stock of nitrogen contained in the soil in the form of humus-amids; and since nitrates are exceedingly liable to be lost from the soil in several ways, such excessive nitrification is to be avoided.