228 PLANT SOCIOLOGY 



we are in a position to measure two additional features. By saturating 

 a known volume of soil with water, the air and water capacities are 

 determined. In the water-saturated soil we have the equation: 



Total volume of soil = H2O content + air content + 



volume of solid particles. 



The air content of the saturated soil (air capacity according to 

 Kopecky, 1914) is that pore volume which, after saturation of the soil 

 up to its absolute capillary capacity with water, still remains filled with 

 air. 



The air capacity thus indicates the air content still available even 

 in the water-saturated soil. This is the air content which is available 

 to plant roots under all circumstances. In short, these are hollow 

 spaces, which due to their size have no capillary effect. In all regions 

 with abundant, continued precipitation, as well as in swamps, the 

 available air content is occasionally reduced to a minimum and then 

 becomes decisive. Since, as we have seen, the lack of O2 in poorly 

 ventilated soils is less detrimental than the accumulation of CO2, the 

 determination of air capacity, which runs more or less parallel to the 

 CO2 measurement, is sufficient in most cases. 



Burger (1922, p. 54), who examined air capacity from the forester's 

 point of view, always found that the greater air capacity indicates a 

 higher value in forestry. Siegrist (1930) also found that Picea excelsa 

 thrived on a soil with an air capacity of 10 per cent producing needles 

 10 to 12 mm. long, while on a neighboring area with soil with an air 

 capacity of 2 per cent it grew feebly with yellowish needles only 4 to 6 

 mm. long. When in spring with alternating rain and snow the soil 

 almost reaches its saturation point, root respiration in soils of low air 

 capacity is stopped or at least restricted. This is especially true of 

 deep-rooting trees. As with pore volume, so air capacity decreases 

 with depth, first rapidly, then more gradually. Burger found the 

 highest air capacity (25.2 per cent) in the uppermost layer (0.10 cm.) of 

 a mixed fir and spruce forest with scattered beeches. Dry meadow 

 communities mostly show a low air capacity in the uppermost layer. 

 The proportions of solid particles, soil water, and air at various depths 

 in the soil of a mixed deciduous forest near Zofingen (beech-oak forest) 

 and of a dry meadow (probably Mesobrometum) are clearly shown in 

 Fig. 116. 



Aichinger and Siegrist (1930) studied the air capacity of the soil in 

 relation to the successions on the gravels of the Drava in Carinthia. 

 They showed that the appearance of the assembly of plants of the 

 mixed forest floor, including such species as Convallaria majalis, 



