=150. Ammoniacal Nitrogen.=—In ammonia compounds, nitrogen is used chiefly for fertilizing purposes as sulfate. The ideal nitrogenous fertilizer would be a combination of the ammoniacal and nitric nitrogen found in ammonium nitrate. The high cost of this substance excludes its use except for experimental purposes. =151. Nitrogen in Guanos.=—The nitrogen in guanos may be found partly as organic, partly as ammoniacal, and partly as nitric nitrogen. The high manurial value of guanos and bat deposits in caves, is due not only to their phosphoric acid, but also to the fact that part of the nitrogen is immediately available, while a part becomes assimilable by nitrification during the growing season. The content of nitrogen in guanos is extremely variable, and depends largely on the climatic conditions to which the deposit has been subjected. The state in which it exists is also a variable one, but with a constant tendency to assume finally the nitric condition. The well-known habits of birds in congregating in rookeries during the nights, and at certain seasons of the year, tend to bring into a common receptacle the nitrogenous matters which they have gathered and which are deposited in their excrement and in the decay of their bodies. The feathers of birds are particularly rich in nitrogen, and the nitrogenous content of the flesh of fowls is also high. The decay therefore, of remains of birds, especially if it take place largely excluded from the leaching of water, tends to accumulate vast deposits of nitrogenous matter. If the conditions in such deposits be favorable to the processes of nitrification, the whole of the nitrogen, or at least the larger part of it, which has been collected in this débris, becomes finally converted into nitric acid, and is found combined with appropriate bases as deposits of nitrates. The nitrates of the guano deposits, and of the deposits in caves, arise in this way. If these deposits be subject to moderate leaching, the nitrate may become infiltered into the surrounding soil, making it very rich in this form of nitrogen. The beds and surrounding soils of caves are often found highly impregnated with nitrates. While for our purpose, deposits of nitrates only are to be considered which are of sufficient value to bear transportation, yet much interest attaches to the formation of nitrates in the soil even when they are not of commercial importance.