276 SCIENCE AND FRUIT GROWING 



fluctuations of temperature and humidity than in other cases 



(in. 40). 



Other experiments with trees grown in covered pots, with the 

 air excluded as far as was possible, yielded equally negative 

 results (III, 32). 



Though some air is undoubtedly necessary for the welfare of 

 plant-roots, and for the proper functioning of the soil, the minimum 

 which is essential is probably very small, as is evidenced by the 

 flourishing of trees in cities when their roots are nearly entirely 

 covered by flagstones. An interesting instance in point is pre- 

 sented by some trees in the old burying-ground at St. Pancras, 

 which in 1900-1 were covered up with a layer of rubble and 

 slag thirteen feet in depth, long since thoroughly consolidated, 

 leaving little more than the heads of the trees above ground. 

 Yet these trees are in a fairly flourishing condition at the present 

 day (Fig. 36). 



Intimately connected with the question of aeration is that of 

 the accumulation of carbon dioxide in the soil surrounding the 

 tree-roots. This was examined in a series of determinations 

 made between the end of May and the middle of October, 1902, 

 in which the ^air from a depth of six inches below the surface, 

 and twelve inches distant from apple trees, was drawn, in one 

 case where the ground was kept tilled, and in the other from 

 ground which was grass-grown. On every occasion, with only one 

 exception, it was found that there was less x carbon dioxide in 

 the soil under the grass than in the tilled ground, the averages 

 being 



Grassed ground . . 13*8 vols. CO 2 per 10,000 

 Tilled ground . . 21-4 



though in both cases there was a much larger proportion of that 

 gas than exists normally in air 3 to 4 volumes in 10,000. As 

 might have been expected, the air immediately above the soil 

 one inch above the surface was found to contain an excess of 

 carbon dioxide derived from the overladen air in the soil, the 

 average values being 6*2 volumes for the air above the grassed 

 ground, and 8-0 for that above the tilled ground. 



1 Wollny (Forsch. Geb. Agrik.-Physik., 3, i) also found this to be so 

 during the summer months, but not so in winter. The composition of 

 soil-gases has been the subject of much investigation, the results of which, 

 up to date, will be found summarised in a paper by Bizzell and Lyon 

 (Journ. Amer. Soc. of Agronomy, 10, March, 1918). The composition is 

 evidently affected by numerous factors : the nature of the soil, and its 

 bacterial contents ; the nature of the crop ; the temperature, humidity, 

 and the time of year. 



