Nitrogen removed in milk 147 Economics of the nitrogen question 147 Loss of nitrogen-compounds in the arts 148 Loss due to use of gunpowder 148 Loss due to sewage disposal 149 Our artificial nitrogen supply 150 Nitrate of soda and sulphate of ammonia 150 Peruvian guano 151 Bones 151 Other nitrogenous manures 152 Oil-seeds and oilcakes 153 Other imported sources of nitrogen 153 Conclusion 153 APPENDIX TO CHAPTER III. NOTE I. Determination of the quantity of nitrogen supplied by rain, as ammonia and nitric acid, to an acre of land during one year 155 II. Nitrogen in soils at various depths 156 III. Nitrogen as nitrates in cropped soils receiving no nitrogenous manures, in lb. per acre (Rothamsted soils) 157 IV. Nitrogen as nitrates in Rothamsted soils 157 V. Examples of increase of nitrogen in Rothamsted soils laid down in pasture 158 VI. Loss by drainage of nitrates 158 VII. Examples of decrease of nitrogen in Rothamsted soils 159 VIII. Amount of drainage and nitrogen as nitrates in drainage-water from unmanured bare soil, 20 and 60 inches deep 160 CHAPTER IV.--NITRIFICATION.