In the process of nitrification, that is in the formation of nitrates, there is required, not only the presence of calcium, or some other alkaline element, in suitable form, but also a good supply of the element oxygen; for calcium nitrate, Ca(NO₃)₂, contains one atom of calcium (Ca), two atoms of nitrogen (N)₂, and six atoms of oxygen (O₃)₂, in each molecule as indicated in the formula, Ca(NO₃)₂. Magnesium nitrate, Mg(NO₃)₂, potassium nitrate, KNO₃ (K is from the Latin word _Kalium_, which means potassium), and all other nitrates, also, contain oxygen. The supply of oxygen for the formation of nitrates in the soil comes from the air, which consists of about twenty percent oxygen, seventy-eight percent nitrogen, and two percent of other elements and compounds, as argon, carbon dioxid, CO₂, water vapor, H₂O, etc. One of the important effects of cultivation, or tillage, is that it permits the air more freely to enter the soil, and thus promotes nitrification. THE NITROGEN-GATHERING BACTERIA. As stated above, the nitrogen naturally in the soil is contained almost entirely in the organic matter. Any process which tends to decompose or destroy this organic matter, such as nitrification or other forms of oxidation, will also tend to reduce the total stock of nitrogen in the soil. Because of this fact the matter of restoring nitrogen to the soil becomes of very great importance. Of course a part of the nitrogen removed in crops may be returned in the manure produced on the farm; and nitrogen may also be bought in the markets in such forms as sodium nitrate (containing 15 to 16 percent of nitrogen), ammonium sulfate (containing 20 to 21 percent of nitrogen), and dried blood (containing 12 to 15 percent nitrogen); but, when we bear in mind that such commercial nitrogen costs about 15 cents a pound, and that one bushel of corn contains about one pound of nitrogen, it will be seen at once that the purchase of nitrogen cannot be considered practicable in general farming, although in market gardening, and in some other kinds of intensive agriculture, commercial nitrogen can often he used with very marked profit. Nitrogen is removed from the soil not only in the crops grown, but also, and frequently in larger amounts per annum, in the drainage waters, and in some other ways, as by denitrification and by the blowing and washing of the surface soil. Professor Snyder, of the Minnesota Experiment Station, has shown that during a series of years the total loss of nitrogen from some Minnesota soils in some cases amounts to several times the amount actually used in the crops produced.