RESPIRATION AND OXYGEN. 43 



ter also showed that soil-air is richest in C0 2 in the summer, less so 

 in autumn, still less in spring, and least in winter. For the growing- 

 season, Jodidi and Wells (1911) found the smallest amount of carbon 

 dioxid in May and the largest in August. In India, the minimum 

 occurred in May and the maximum in September, the ratio being 

 1 : 7 (Howard and Howard, 1920). 



As would be expected, sand contains the least carbon dioxid and 

 the most oxygen. This was early shown by Boussingault and Lewy 

 (1853), who found 0.11 to 0.19 per cent of C0 2 and over 20 per cent 

 of oxygen in sand, while Fleck showed that the soil-air of sand con- 

 tained about one-fourth as much carbon dioxid at 2 meters as gar- 

 den soil, and about one-tenth as much at 6 meters. Moller (1878) 

 obtained considerably more C0 2 from calcareous soil than from clay, 

 and several times as much from both as from sand, while Ebermeyer's 

 results showed quartz sand poorest in C0 2 , calcareous sand and clay 

 twice as rich, and dry moor-soil far richer. Lau likewise determined 

 that sand was poorest, clay intermediate, and moor-soil richest in 

 carbon dioxid. 



The general effect of organic matter and manure is to greatly 

 increase the amount of carbon dioxid and to decrease the oxygen. 

 The most C0 2 and the least oxygen were found by Boussingault and 

 Lewy in soils containing abundant humus, and in those recently 

 manured. The latter yielded 10 times as much CO 2 as soils not 

 fertilized for a year. Smolensk! (1877) obtained from 5 to 100 

 times as much carbon dioxid in contaminated as in ordinary soil, 

 and Salger confirmed his general results. Wollny (1880) determined 

 that the amount of carbon dioxid in the soil rose and fell in general 

 with that of organic material, but the amounts were proportional 

 only when the organic matter was not too abundant. Lau found 

 that organic material increased the production of C0 2 , while Jodidi 

 and Wells found little relation to previous differences of treatment. 

 Harrison and Aiyer (1913) obtained as much as 14 to 21 per cent of 

 carbon dioxid from manured plots, while Leather observed a maxi- 

 mum of 18 per cent in fallow land after green-manuring. 



Cultivation and plant-growth generally augment the carbon dioxid 

 of the soil-air and diminish the oxygen in proportion. Moller 

 showed that the soil of cultivated fields regularly contained 5 to 6 

 times as much carbon dioxid as that of fallow fields, while Fleck 

 observed that in a soil covered with plants, carbon dioxid was most 

 abundant in the upper layers. On the other hand, Ebermeyer 

 concluded that a living cover reduced decomposition and the conse- 

 quent production of C0 2 . He found forest-soil much poorer in car- 

 bon dioxid than unforested, and 5 to 6 times poorer than cultivated 

 soil. The soil-air of a deep beech forest contained but half as much 

 carbon dioxid as that of a pine forest. Wollny's results as to the 

 effect of a living cover appear somewhat contradictory, but his final 



