Evolution of CO^. from Soil Humus 153 



4. Methods in which the e\okitioii of CO2 from a measured area 

 in a given time is determined in the field, as by Lundegardh and by 

 Humfeld. 



Some of the results may be summarized as follows: 



When soil samples are taken in the field for extraction in the 

 laboratory, the greatest precautions are necessary to devise apparatus 

 for taking the sample without driving out the gases already present 

 or allowing diffusion between the sample and the atmosphere. 



z\ppleman devised a sampling tube for collecting the soil atmos- 

 phere. This consisted of two brass tubes, the smaller fitting tightly 

 inside the larger and extending about 1 inch at each end. One end 

 was groo\"ed for a rubber tube; the other was fitted with a point 

 the size of the larger tube, thus leaving a space between the point 

 and the large tube, in which twelve holes were drilled to drain the 

 soil gases from the free space. The outer tube was slipped down 

 o%er the holes, and the instrument was thrust into the soU to the 

 desired depth; then the smaller tube was pushed in far enough to 

 uncover the holes while the outer tube was stationary. Rubber 

 tubing was then attached and suction applied. The first 100 ml of 

 gas collected over mercury was discarded; then 250-ml samples were 

 taken for analysis. 



In the cooler part of the season, between May 1 and June 1, the 

 COo from the soil varied from 0.13 to 0.38 per cent. Later in the 

 season, July 16, Appleman found 6.91 per cent in the soil under al- 

 falfa; 2.97 per cent under cabbage leaves; 5.05 per cent in a check; 

 and only 1.4 per cent in the soil between the rows. The air 1 foot 

 above the soil contained 0.03 per cent COo. When the plots high in 

 CO2 were cultivated, the total COo dropped off 90 per cent in the first 

 day, showing that cultivation brings aeration and dissipates the 

 accumulated gas. 



The difficulty of thrusting an instrument into the soil without 

 opening channels, cutting across wormholes, insect burrows, or other 

 cracks which bring atmospheric air to the instrument in but slightly 

 changed condition, is readily seen. 



In the study of the capacity of a soil to evolve COo, the sample is 

 collected as representative of the area. It may be partly dried, at 

 least to a known water percentage. It is crumbled or ground, sifted, 

 and weighed, in lots of 50 or 100 gm, into flasks with or without the 

 admixture of specific fertilizing or organic manuring substance. 

 Water content is brought to a definite point, and the flask is con- 

 nected into the collecting system to be held for a definite time at con- 



