902 CONCENTRATION FACTORS CHAP. 27 



proportionate increase in crop. Experiments tend to confirm this con- 

 clusion. 



Among the earliest attempts at carbon dioxide fertilization were those of Demoussy 

 (1904) ; the first practical successes were achieved by Klein and Reinau (1914). Among 

 the more recent investigations, those of Lundegardh (1924), Rippel (1926), White 

 (1930), Harder, Keppler and Reuss (1931), Johnston (1935), Richter (1938) and Katun- 

 sky (1939) may be quoted. In numerous experiments, carbon dioxide-fertilized cultures 

 produced crops 50-100% larger than control cultures grown in rooms with the normal 

 concentration of this gas. Carbon dioxide fertilization has found some practical applica- 

 tion (c/., for example, Reinau 1927) in the suburban greenhouse cultivation of fruits and 

 vegetables, since greenhouses can be "fertilized" comparatively easily and inexpensively 

 by compressed carbon dioxide from cylinders. The possibility of a similar fertilization 

 on a large scale in open fields depends on the availability of cheap combustion gases 

 rich in carbon dioxide, but free of sulfur dioxide and other plant-damaging components 

 (cf. Katunsky 1939). 



Thomas and Hill (1949) made improved measurements of the rate of photosynthe- 

 sis of tomatoes, sugar beet and alfalfa under field conditions, and found continuous in- 

 crease in rate even at 0.3 or 0.4% CO2 — except in the case of a sulfur-deficient beet 

 culture, which apparently was unable to use an increased carbon dioxide supply. The 

 maximum fertilization effects observed in these experiments were rate increases by a 

 factor of about three. 



Possibility of fertilization by bicarbonates plays an important role in speculations 

 on large-scale culturing of unicellular algae as source of fuel or food, for man, animals, or 

 protein and fat-producing microorganisms, such as yeast. 



It was mentioned above (see page 901) that Chesnokov and Bazyrina (1932) 

 denied that external carbon dioxide concentration has a direct effect on the rate of photo- 

 synthesis at all. Bazyrina and Chesnokov (1930) sought a different explanation of the 

 phenomenon of CO2 fertilization, and thought they found it in the stimulating action of 

 carbon dioxide on plant growth. They denied that field crops are primarily determined 

 by the intensity of photosynthesis, and pointed out that accelerated photosynthesis 

 sometimes causes an unbalanced or premature development and thus diminishes rather 

 than increases the crop. Granted that the size of the crop depends on many factors, it 

 is certain that photosynthesis is one of them, if not the main one; and it is hardly 

 a coincidence that not only the possibility of crop increase by carbon dioxide fertiliza- 

 tion, but also its approximate maximum extent (50-100%) can be anticipated from the 

 shape of the carbon dioxide curves obtained under controlled laboratory conditions. 



The question of how strongly the actual concentration of carbon dioxide 

 surrounding vegetation deviates from the average (0.03%) has been much dis- 

 cussed, and widely divergent opinions have been expressed on this sub- 

 ject. Undoubtedly, carbon dioxide concentration near the ground in dense 

 vegetation can rise considerably above the average, particularly at the end 

 of night. However, extreme figures such as [CO2] > 1%, given by some 

 investigators, are unlikely. We will quote two examples of more reliable 

 determinations: Verduin and Loomis (1944) found that, in a maize field, 

 the concentration of carbon dioxide 100 cm. above ground, was 0.055- 

 0.080% at night, and rapidly declined to 0.045% in the morning. Fuller 

 (1948) found that CO2 concentration near the ground (0-1 cm.) reached 



