ABSORPTION BY SOILS. 279 



THE AIR OF SOILS. 



The Empty Space in Soils. In dry soils the empty space, 

 usually amounting to from 35 to 50 per cent of its volume, is 

 filled with air; 1 in moist or wet soils the space unoccupied by 

 water is similarly filled. Hence when soils are in their best 

 condition for the support of vegetation (chap, n, p. 202), 

 about one half of their interstices is filled with water, the other 

 half with air. Actual measurements of the amount of air 

 contained in well-cultivated garden soil have been shown by 

 Boussingault and Levy to range between 10,000 and 12,000 

 cubic feet per acre, substantially agreeing, therefore, with the 

 above statement. In uncultivated forest soil, on the contrary, 

 they found only from somewhat less than 4000 to 6000 cubic 

 feet of air per acre. Extended observations since carried out 

 by Wollny, Ebermayer, and others have in general confirmed 

 the earlier observations, while adding greatly to their signifi- 

 cance in respect to their relations to plant growth, and to the 

 process of humification and soil-formation. 



As a matter of course, when water evaporates from the soil 

 in drying, its place is taken by air so far as it is not filled by 

 capillary water drawn from below. 



Functions of Air in Soils. That roots require for the per- 

 formance 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 



1 The normal composition of atmospheric air is given on p. 16, chap. 2. 



