98 Decomposition of Plant and Animal Residues 



Abundance and Chemical Composition of Plant Residues 



Sachs calculated that the leaves of an ordinary sunflower plant, 

 having a surface area of 1.5 square meters, absorb two-thirds of a 

 liter or 1.3 gm of COo per hour. If the growing day is taken to be 

 10 hours, the plant will absorb 400 gm of COo a month. On the 

 basis of a milHon plants per square kilometer of land and a 3-month 

 growing period, the sunflower will consume annually 1,200,000 kg 

 of CO2. In view of the fact that the COo content of the atmosphere 

 is very small, only 0.03 per cent, the available supply of this essential 

 plant nutrient would soon become exhausted if it were not for the 

 continuous liberation of the COo from the soil by the action of micro- 

 organisms upon the plant and animal residues and upon the soil 

 organic matter. 



According to Lundegardh, the amount of CO2 produced in the 

 soil by microorganisms approaches that which is required by the 

 plants for the photosynthesis of organic matter. If it is assumed that 

 the average content of the organic matter in the upper 15 cm of 

 soil is 2-4 per cent, an acre of soil will contain 20,000-40,000 kg of 

 organic matter. Since the carbon content of the latter is 58 per cent, 

 it is possible to conclude that the average amount of organic carbon 

 in an acre of soil is 10,000-22,000 kg. In some soils, like prairie and 

 peat soils, the organic matter content may be considerably higher 

 (10 per cent or more), whereas, in poor sandy sofls, it may be 1 per 

 cent or less. 



A study by Waksman and Starkey of the evolution of COo from 

 soil revealed that, under favorable moisture and temperature condi- 

 tions, 1 kg of soil may give off, in 24 hours, 5-30 mg of carbon as COo. 

 Taking an average of only 10 mg of carbon and a period of active 

 annual decomposition of 4 months, we find that an acre of soil con- 

 taining 10,000 kg of carbon will give off during the wann months 

 1,000 kg of carbon in the form of COo. Under these conditions of 

 decomposition, the soil organic matter would become exhausted 

 within 10 years. If it were not for the constant addition of plant, 

 animal, and microbial residues to the soil, the amount of available 

 COo from the above source would soon also become a limiting factor 

 in plant growth. 



Ebermeyer calculated that the vegetation on 1 hectare of field soil 

 consumed annually 2,000 kg of carbon and on 1 hectare of forest 

 soil 3,000 kg, corresponding to 7,300-11,000 kg COo. The plant 



