_Functions of Air in Soils._—That roots require for the performance of their vegetative functions the presence of oxygen, has already been discussed; but there can be no question that the higher productiveness of well-cultivated soils is largely due to the greater and readier access of air to the roots. Apart from this direct function, however, the presence of oxygen in the soil serves other important purposes, and among these doubtless the most dominant is the promotion of the oxidation of the organic matter of the soil through the agency of micro-organisms; and more particularly that of nitrification, which chiefly governs the supply of nitrogen to non-leguminous plants. In the case of leguminous plants, the presence of air as a furnisher of nitrogen as well as oxygen is absolutely essential. The injurious effects of insufficient aeration of the soil have been repeatedly referred to already (pp. 45, 76). In water-logged soils reductive fermentations are soon set up, and the nitrates of the soils are reduced partly with the evolution of nitrogen gas, partly to ammonia; while their oxygen is consumed to supply the demands of the roots. Ferric oxid is reduced to ferrous carbonate, sulfates to sulfids; thus deranging the whole process of plant-nutrition and absorption of plant-food. If continued for any length of time these conditions end in the death of the plant. Too much importance cannot therefore be attached to the proper aeration of the soil and subsoil. _Excessive Aeration; Compacting the Soil._—On the other hand, excessive aeration of the soil may be injurious in causing a serious waste of moisture; especially in arid climates, where the hot, dry winds may readily destroy the germinating power of the swollen seed when the seed-bed is too loose and open, and later may injure or destroy the feeding roots. The abundant growth of grain often seen in the tracks of a wagon carrying the centrifugal sower, when the stand in the general surface is very scanty, is usually due to the consolidation of the seed-bed, and suggests at once the well-known efficacy of light rolling to insure quicker germination and a better stand. Similarly, the rolling of grain fields in spring is often the saving clause for a crop in dry years. But such needful consolidation must not, of course, be carried to the extent of creating a surface crust which would subsequently serve to waste the subsoil moisture. Hence, the soil-surface should be rather dry when rolling is resorted to. The pressing of the earth around transplanted plants, similarly, is a needful precaution, not only with respect to the drying-out of the soil, but also to insure close contact between the roots and the soil.