44 AERATION AND AIR-CONTENT. 



conclusion (1896) was that soil covered with plants possesses a higher 

 content of CO2 than bare soil under conditions otherwise similar. 

 The composition of the soil-air varies with the plant cover concerned, 

 as would be expected. Lau stated that the soil-air contains more 

 carbon dioxid at the root-level in a cultivated field than in a bare one, 

 and that there is more in the root-layer than beneath it. The car- 

 bon dioxid increases with the greater development of plants, and 

 also varies with the kind of plant. The effect of plants upon the 

 composition of the soil-air is most graphically shown by the results 

 of Leather, who found minimum amounts of 2 to 6 per cent of oxy- 

 gen and maximum amounts of 12 to 21 per cent of CO 2 about the 

 roots of crop plants in India. The results obtained by Howard and 

 Howard indicate that the amount of carbon dioxid is usually 2 to 

 4 times as great in grass plots as in those with the surface cultivated. 



While investigators have differed much as to the importance of the 

 various factors in modifying the composition of the soil-air, it seems 

 evident that this depends in the first place upon the intensity of 

 respiration and oxidation in the soil and secondly upon the possibility 

 of diffusion through it. The soil having the largest amount of 

 living organisms in it will consume the most oxygen and produce the 

 most carbon dioxid. Since many of the most active organisms are 

 microscopic, cursory observation is insufficient to determine the 

 actual importance of a plant cover or the presence of organic matter. 

 In addition to the flowering plants, ferns, mosses, and larger fungi 

 which may constitute the visible cover are a host of microscopic 

 fungi, molds, bacteria, and algae, together with soil animals from 

 amoebae to rodents. All of these have a larger or smaller part in 

 increasing the CO 2 and decreasing the oxygen of the soil, but the 

 algse may reverse this process under conditions permitting photosyn- 

 thesis. Any factor that promotes respiration or oxidation, such as 

 higher temperature or water-content, will increase the carbon dioxid 

 at the expense of the oxygen, and those that retard respiration will 

 have the opposite effect. The amount of organic material in the 

 form of humus, manure, or green manure is naturally of great im- 

 portance as a source of energy for molds, bacteria, protozoa, etc. 

 The porosity of the soil is directly important in determining the rate 

 at which oxygen can enter and carbon dioxid escape and indirectly 

 in that it affects the water-content, temperature, organic matter, and 

 number of organisms. 



Since water contains but 6 to 7 c.c. of oxygen per hter when 

 saturated during the growing season, the water-content and air- 

 content of the soil stand in inverse relation to each other, in so far as 

 respiration is concerned at least. The greater the water-content, 

 the smaller the air-content and the amount of available oxygen, and 

 the reverse. Dry soils contain relatively large amounts of air, and 

 wet soils httle air, regardless of their fineness. Under the same rain- 



