724 PRINCIPLES OF SOIL MICROBIOLOGY 



position, (4) soil aeration, (5) moisture content, (6) physical condition 

 of the soil, (7) chemical composition (altered by fertilization), (8) soil 

 reaction and (9) kinds of plants grown. The C0 2 thus determined is a 

 measure of the rapidity with which the soil organic matter itself de- 

 composes under the influence of the sum total of microbiological 

 complexes. It can be determined in three different ways: 



1. One-kilogram portions of fresh soil from a composite sample taken to a 

 depth of 65 inches and put through a 3 mm. sieve are placed in pots. Enough 

 water is then added to bring the moisture content of the soil to the optimum. 

 The pots of soil are then placed in the respirator and the amount of C0 2 evolved 

 in fourteen days is determined at various intervals. 



2. One kilogram portions of air-dried, sieved soil, taken to a definite depth, 

 are placed in proper containers; the necessary amount of water is added and 

 the CO2 evolved in 24 hours is determined. By this method, Stoklasa 29 found 

 that an infertile soil, poor in organic matter, produced 8 to 14 mgm. CO2, 

 while a good beet soil produced 56 to 68 mgm. 



3. One hundred-gram portions of fresh soil, prepared as in the first method, 

 are placed in 300-cc. flasks with long necks (A in figure 60). Cotton plugs are 

 placed in the necks of the flasks and in the glass connections. After the proper 

 amount of water is added (50 per cent of total moisture holding capacity), the 

 flasks are sterilized for 1 to l- 2 - hours, on two consecutive days, at 15 pounds pres- 

 sure. The soils are then inoculated with a culture of Trichoderma which was 

 found to be one of the most active groups of soil fungi decomposing celluloses, 

 proteins, pectins and other complex organic substances; a suspension of fresh 

 cow manure may also be used for inoculation. The flasks are then connected 

 with the Ba(OH) 2 tubes in the respirator and the amount of C0 2 evolved is 

 determined for 12 to 14 days. By this method, two soils, one, fertile and rich 

 in organic matter, and another, infertile and poor in organic matter, were found 

 to produce 124.08 and 37.40 mgm. of C0 2 respectively, in eight days. 



The decomposition of fresh organic matter added to the soil can be 

 determined by a group of methods, which differ chiefly in the nature 

 of the organic matter used. A few substances have been commonly 

 employed : 



1. Glucose. This substance is very readily decomposed in the soil; an ex- 

 cess of the material, as well as a long period of incubation, may obliterate finer 

 differences in the activities of the microorganisms in the different soils. It is 

 best to add 500 mgm. of glucose to 100 grams of soil and determine the C0 2 

 evolved every 6 or 12 hours for a period of 48 to 72 hours. The resulting curves 

 bring out distinctly the differences in the microbiological activities of the dif- 

 ferent soils. Since glucose is used very readily as a source of energy not only 



29 Stoklasa, J. Le role de l'acide carbonique degage par les microorganismes 

 dans l'amelioration des terres culturales arables pour obtenir le meilleur rende- 

 ment. Compt. Rend. Agr. France, 8: 594-596. 1922. 



