The Rothamsted experiments and the nitrogen question 115 Different forms in which nitrogen exists in nature 116 Relation of "free" nitrogen to the plant 117 Combined nitrogen in the air 118 Amount of combined nitrogen falling in the rain 119 Nitrogen in the soil 120 Nitrogen in the subsoil 121 Nitrogen of surface-soil 121 Amount of nitrogen in the soil 123 Soils richest in nitrogen 123 Nature of the nitrogen in the soil 124 Organic nitrogen in the soil 125 Differences of surface and subsoil nitrogen 126 Nitrogen as ammonia in soils 127 Amount of ammonia in soils 127 Nitrogen present as nitrates in the soil 128 Position of nitric nitrogen in soil 128 Amount of nitrates in the soil 129 Amount of nitrates in fallow soils 129 Amount of nitrates in cropped soils 130 Amount of nitrates in manured wheat-soils 131 The sources of soil-nitrogen 131 Accumulation of soil-nitrogen under natural conditions 133 Accumulation of nitrogen in pastures 134 Gain of nitrogen with leguminous crops 135 The fixation of "free" nitrogen 136 Influence of manures in increasing soil-nitrogen 136 Sources of loss of nitrogen 137 Loss of nitrates by drainage 137 Prevention of loss of nitrogen by permanent pasture and "catch-cropping" 138 Other conditions diminishing loss of nitrates 139 Amount of loss of nitrogen by drainage 140 Loss of nitrogen in form of "free" nitrogen 141 Total amount of loss of nitrogen 142 Loss of nitrogen by retrogression 142 Artificial sources of loss of nitrogen 144 Amount of nitrogen removed in crops 144 Losses of nitrogen incurred on the farm 146 Loss in treatment of farmyard manure 146