278 RADIATION BIOLOGY 



sunlight is of the order of 500,000 ergs/sec/cm-. In bright Hght the 

 secondary thermal reaction cannot keep up with the primary photo- 

 synthetic process, and there is photooxidation of the partially reduced 

 organic intermediates that are first formed by the photoreduction of 

 carbon dioxide. Many more than 8 photons of light are required for 

 the complete reduction of a molecule of carbon dioxide in intense light, 

 because some of the photons undo the photosynthetic work that is done 

 by the other photons. It is partly for this reason that the long summer 

 days in the subarctic regions produce luxuriant plant growth even though 

 the intensity of sunlight per hour is much less than that in the tropical 

 areas. It is clear that 1 ton of dry plant material per acre per day, as 

 calculated on the basis of optimum laboratory conditions, cannot begin 

 to be realized in ordinary agriculture. 



Instead of 1 ton of organic material per day, we have seen in an earlier 

 paragraph that a good agricultural yield is only about 2 tons/acre/year. 

 In connection with this apparently "low" yield of 2 tons/acre/year, it 

 must be remembered that the growing season is only about 100 days long, 

 whereas the calculations for the annual solar energy involve 365 days, the 

 majority of which are not suitable for agriculture in the United States 

 because of the winter temperatures. The agricultural crop of 2 tons/acre 

 per growing-year of 100 days is of the order of 2 per cent of what the 

 growth might be (1 ton/day for 100 days) if the conditions were as favor- 

 able as they are in the laboratory experiments with algae. 



This 2 per cent efficiency represents agricultural efficiency. If the 

 efficiency of energy storage is calculated on the basis of total annual 

 radiation, including infrared as well as visible light and the winter season 

 as well as the summer season, the agricultural crop of 2 tons/year corre- 

 sponds to a storage of only 



2 X 3.5 X 10" kcal 



365 X 21 X 10« kcal 



or about 0.001. The 2 tons of organic material produced in a year would 

 thus give back through combustion, at 3.5 X 10^ kcal/ton, only 0.1 per 

 cent of the total annual solar radiation. 



Although improvements in agriculture have greatly increased the utili- 

 zation of solar energy, it will be difficult to make improvements great 

 enough to approach the theoretical limit calculated here. In the last 

 few years considerable interest has developed in the possibihty of grow- 

 ing algae in tank farms, which might achieve greater production of organic 

 material per acre than is now possible under ordinary methods of agri- 

 culture. The capital investment for metal or concrete tanks or for glass 

 enclosures to contain the algae and provide means for increasing the car- 

 bon dioxide content of the air would seem to be prohibitive from an eco- 

 nomical standpoint. However, some attention has been given to the use 



