38 AERATION AND AIR-CONTENT. 



the finer the particles. Carbon dioxid increased with the depth of 

 the soil layer. A soil shaded by living plants contained during the 

 warm season a smaller amount of CO2 than a fallow soil, and this 

 again less than one covered with a layer of dead plant material. 

 In the last case the amount of CO2 increased with the thickness of 

 the layer. It was also found (1889 : 385) that the soil in bright, 

 warm, and dry weather is the richer and in cloudy, cool, and moist 

 weather the poorer in CO 2 under conditions otherwise similar, the 

 darker the color of the surface. 



Wollny (1890) summarized his results upon the influence of organic 

 matter as follows: The amount of CO 2 in the soil-air is proportional 

 to the amount of organic material present only when this is small. 

 The production of CO 2 with a high content of the soil in organic 

 material increases in smaller degree than the amount of organic 

 material, or remains the same, because at a higher C02-content the 

 activity of the organisms of decomposition is Hmited, and with the 

 increase of the organic material above a certain limit the properties 

 of the soil most important for decomposition are changed in a fashion 

 unfavorable for the intensity of the process. The amount of CO 2 

 present in the soil gives neither a measure of the intensity of or- 

 ganic processes nor of the amount of humus material present. In a 

 later study (1896 : 151) it was shown that soil covered with plants 

 possesses a higher content of CO2 than bare soil under conditions 

 otherwise similar. The reverse is true when the bare land is ma- 

 nured. The soil-air in soil covered with grass or with birches is 

 richer in CO2 than one covered with pine. The soil under the pines 

 without a covering of straw contains larger amounts of CO 2 than one 

 with a straw cover. 



Mangin (1895 : 1065) analyzed the soil-air for the purpose of de- 

 termining the cause of retardation in the leafing of AUanthus and 

 Ulmus at Paris. In the case of AUanthus with open buds, he found 

 the C02-content of the soil to range from 0.35 to 3.43, with the aver- 

 age at about 1.5 per cent, while in AUanthus, with buds still closed, 

 it varied from 3.89 to 24.84, the average being about 10 per cent. 

 The oxygen in the first case ranged from 17.86 to 20.3 per cent, with 

 an average of about 19 per cent, and in the second from 31.6 to 15.92 

 per cent, the average about 10 per cent. In the case of elms with 

 normal leafing, the carbon dioxid varied from 0.67 to 2.12 per cent, 

 the average being about 1 per cent, while those with leafing retarded 

 15 to 20 days showed a range of 1.71 to 5.81 per cent, with the aver- 

 age about 3 per cent. Similarly, the oxygen ranged from 17.86 to 

 20.05 per cent, and from 6.26 to 17.72 per cent, the average being 

 about 14 per cent. 



Later researches. — Letts and Blake (1900 : 213) have made a com- 

 prehensive summary of the results of Pettenkoffer, Fodor, Wollny, 

 and others with respect to soil-air. This results from atmospheric 



