=143. Kinds of Nitrogen in Fertilizers.=—Nitrogen is the most costly of the essential plant foods. It has been shown in the first volume, paragraph =23=, that the popular notion regarding the relatively great abundance of nitrogen is erroneous. It forms only 0.02 per cent of the matter forming and pertaining to the earth’s crust. The great mass of nitrogen forming the bulk of the atmosphere is inert and useless in respect of its adaptation to plant food. It is not until it becomes oxidized by combustion, electrical discharges, or the action of certain microorganisms that it assumes an agricultural value. Having already, in the first volume, described the relation of nitrogen to the soil it remains the sole province of the present part to study it as aggregated in a form suited to plant fertilization. In this function nitrogen may claim the attention of the analyst in the following forms: 1. In organic combination in animal or vegetable substances, forming a large class of bodies, of which protein may be taken as the type. Dried blood or cottonseed-meal illustrates this form of combination. 2. In the form of ammonia or combinations thereof, especially as ammonium sulfate, or as amid nitrogen. 3. In a more highly oxidized form as nitrous or nitric acid usually united with a base of which Chile saltpeter may be taken as a type. The analyst has often to deal with single forms of nitrogenous compounds, but in many instances may also find all the typical forms in a single sample. Among the possible cases which may arise the following are types: _a._ The sample under examination may contain nitrogen in all three forms mentioned above. _b._ There may be present nitrogen in the organic form mixed with nitric nitrogen. _c._ Ammoniacal nitrogen may replace the nitric in the above combination. _d._ The sample may contain no organic but only nitric and ammoniacal nitrogen. _e._ Only nitric or ammoniacal nitrogen may be present. =144. Determination of the State of Combination.=—Some of the sample is mixed with a little powdered soda-lime. If ammoniacal nitrogen be present free ammonia is evolved even in the cold and may be detected either by its odor or by testing the escaping gas with litmus or turmeric paper. A glass rod moistened with strong hydrochloric acid will produce white fumes of ammonium chlorid when brought near the escaping ammonia.