154 Decomposition of Soil Organic Matter 



trolled temperature. Marsh forced a stream of C02-free air through 

 the mass of soil, whereas Waksman and Starkey placed the soil in a 

 layer 2-3 cm deep in the bottom of an Erlenmeyer flask, through the 

 stopper of which a tube brought C02-free air into the flask, while a 

 second tube, reaching nearly to the surface of the soil, removed the 

 C02-laden stratum of air as fast as it was formed. The CO2 was 

 absorbed by alkali and determined by titration. 



The main weakness of these procedures is that conditions are arbi- 

 trarily chosen; they may not reproduce in any definite way the 

 demands made upon a soil in the field. They do not measure any- 

 thing which is definitely found in nature. 



Lundegardh devised a zinc bell— a pyramidal cover with straight 

 margins to be forced down into the soil, giving a collecting space of 

 a known volume. After the collecting bell remains in position for a 

 specified time, gas samples are pumped into containers and taken to 

 the laboratory for analysis. By standardizing the operation of his 

 collector, Lundegardh found it possible to move from plot to plot 

 across the field and take a sample every 20 minutes, accumulating 

 the receiving tubes and taking them all back to the laboratory for 

 microanalysis. 



Lundegardh's apparatus has the advantage of placing a known 

 container over undisturbed soil for a brief period and collecting for 

 analysis a part of the air and CO2 mixture resulting from interruption 

 of the difl:usion of the COo evolved during the period. The sample 

 taken can be calculated to milligrams of COo evolved per square 

 meter of surface per hour or to any other desired unit. Whether 

 confinement of the CO2 evolved during that period results in delay- 

 ing evolution of COo may be questioned. If so, the results may be 

 low. The work of Lundegardh does, however, give a comparison, 

 of actual COo evolved, between different plots of ground in the field. 



In the studies reported by Humfeld, the collecting apparatus was 

 modified in appearance from Lundegardh's bell, into a box 3 by 8 

 inches and 3 inches high, with the open side pressed into the soil 

 about 1 inch. At one end a collecting tube about ^ inch in diameter 

 was attached, and at the other a vent to which usually a tube was 

 attached with its open end carried 2-3 feet above the soil to give a 

 supply of atmospheric air to the collector. In this apparatus, the air 

 passing over the soil carried CO2; hence the extra tension of COo- 

 free air was eliminated, but the percentage of the COo present in 

 the atmosphere was determined regularly and deducted from the 

 totals found in the collection apparatus. 



