44 TEXTBOOK OF PLANT PHYSIOLOGY 



content of the carbon dioxide of the air, namely, about 0.03 per 

 cent, is by no means the optimum but rather the minimum amount 

 for the plant, and that any increase of carbon dioxide appears 

 favorable. On the basis of the above statement, fertilization with 

 carbon dioxide has been applied during the past few years with a 

 view of obtaining higher yields. This has been done first in green- 

 houses and afterwards under the conditions of field experimenta- 

 tion, where carbon dioxide was conducted to the field by pipes. 

 This method, of course, is practical only in the neighborhood of 

 large factories, which throw into the air immense quantities of 

 carbon dioxide (the Krupp factories in Essen, for instance, produce 

 as much as 3,000,000 kg. carbon dioxide per day). Especially 

 favorable results have been obtained in greenhouses where, with 

 insufficient aeration, often not even traces of carbon dioxide can 

 be found in the air during the day. When this gas is introduced 

 artificially, frequently a 100 to 150 per cent increase in yield has 

 been secured. The rapid development of the plants in hotbeds 

 underlaid with manure seems to be due not only to the higher tem- 

 perature thus produced, but also to the abundant supply of carbon 

 dioxide. 



Under natural conditions plants obtain carbon dioxide not only 

 from the atmosphere, but also from the soil. Due to the processes 

 of decomposition of the organic substances in the soil by different 

 microorganisms, carbon dioxide is liberated. Diffusing from the 

 soil in the lower layers of the atmosphere, it is caught by the leaves 

 of the plants. According to the calculations of Lundegardh, a 

 sandy soil, poor in humus, liberates about 2 kg. of CO2 per hour 

 per hectare, while loam and clay soils containing a greater amount 

 of humus, eliminate about 4 kg., and forest soils, extremely rich in 

 humus, produce from 10 to 25 kg. Moderately fertilized soils 

 produce on an average 5 kg. of carbon dioxide per hectare per hour. 



When the consumption of carbon dioxide by the plant is con- 

 sidered with the above figures in mind, the following values are 

 obtained. A field of 1 hectare sown to oats consumes in the 

 process of photosynthesis about 15 kg. of carbon dioxide per hour. 

 Five of these are supplied by the soil; the other ten are obtained 

 from the atmosphere. In spite of the low percentage of carbon 

 dioxide contained in the atmosphere, an air layer 100 m. thick and 

 1 hectare in area contains about 550 kg. of carbon dioxide. If it is 

 assumed that in summer the most intense assimilation continues 



