The nitrogenous organic matter contained in soils is for the most part an insoluble substance, a fact of the greatest importance for the maintenance of the fertility of the soil. While in this condition it is of little use to the higher order of plants among which our ordinary crops are included. To become available as plant food, it must be oxidized and rendered soluble, but as soon as this step is effected it becomes liable to be lost by drainage. Not many years ago we would have been satisfied in explaining the oxidation which occurs in soil as due to a simple contact with oxygen. We now take a different view of these changes. We know that the organic matter of a soil is split up and oxidized by means of living agents. A fertile soil is, in fact, teeming with life of many kinds. Many of these living agents are quite invisible to our eyes, and yet are performing changes on a great scale, upon the accomplishment of which the growth of our food crops depends. The living agents which attack the organic matter of soil may be classed as (1) animal life—worms and insects; (2) fungi; (3) bacteria. The worms, beetles, larvae, etc., in a surface soil feed on the recently dead vegetable matter left by the crop or weeds which previously had possession of the soil. The carbon of this vegetable food is oxidized in their bodies and exhaled as carbonic acid, while the nitrogen is excreted in simple forms of combination. The fungi also feed on the nitrogenous organic matter of soil; carbon is oxidized in their cells and exhaled as carbonic acid, while their dead nitrogenous tissues restore to the soil a great part of the nitrogen which they had assimilated. The conditions which favor the complete oxidation effected by bacteria are aeration of the soil by tillage, the presence of a suitable amount of water and of calcium carbonate, and a high temperature. It has been mentioned above of the natural limits to the accumulation of nitrogen in the soil. Prof. Warington thinks he can now perceive some of the causes of such limits.