ENERGY EFFICIENCY IN PHOTOSYNTHESIS 277 



the efficiency obtained in ordinary agricultural practice. In the United 

 States a square foot of land receives on the average about 1 kcal of solar 

 radiation for about 500 min each day. Multiplying this total of 500 kcal/ 

 day by 43,560 (the number of square feet in an acre), we find that about 

 21,000,000 kcal of heat falls on an acre of land each day. About half of 

 this radiation is in the infrared and, since it is not absorbed by chloro- 

 phyll, cannot take part in photosynthesis. Inasmuch as the efficiency 

 of the light that is absorbed is about 35 per cent, or }i, it may be con- 

 cluded that only ^ of Ys of the 21,000,000 kcal can be stored in grow- 

 ing plants under optimum conditions. This calculation gives 3,500,000 

 kcal/day/acre. 



This theoretical value is high for several reasons. The average corn 

 crop of the United States in 1946 was about 35 bu/acre, which is an 

 equivalent of about 2 tons of organic material — 1 ton for the corn kernels 

 and another ton for the cobs, leaves, and stalks. Hybrid corn on rich, 

 fertilized land with good growing conditions can give 100 bu/acre, with 

 a weight of 5 or 6 tons of dry organic material. A good silage crop in 

 Wisconsin gives about 21.^ tons/acre/year of dry organic material. 

 Wheat and hay give of the order of 1 ton/acre/year. An aspen forest 

 in northern Wisconsin could give 2 tons of organic material per year if it 

 were scientifically forested and if all organic material were collected. 

 Algae in some Wisconsin lakes produce organic material at the rate of 

 about 2 tons/acre/year. Sugar cane growing in Hawaii the year round 

 can give up to 40 tons of dry organic material per year. 



When organic material such as sugar or wood is burned in air, it gives 

 off about 3,500,000 kcal/ton. We have just seen that 3,500,000 kcal 

 (corresponding to 1 ton of organic material) would be a reasonable value 

 for the storage of an acre of sunlight through photosynthesis for 1 day, 

 as calculated on the basis of algae grown under optimum laboratory con- 

 ditions. It must be emphasized that this high value is calculated on the 

 basis of laboratory measurements of photosynthesis over short time inter- 

 vals. On this theoretical basis, then, the equivalent of a ton of dry plant 

 material could be produced on an acre of land in a day, and this ton 

 would release 3,500,000 kcal when burned in the air. There are several 

 reasons, of course, why this extraordinary efficiency cannot even be 

 approached in agriculture. These optimum yields imply an ample sup- 

 ply of water, fertihzers, and all inorganic and organic material necessary 

 for full nutrition. The temperature must be kept at an optimum; the 

 carbon dioxide concentration must be about 3 per cent instead of the 

 0.03 per cent found in ordinary air. The young plants do not cover 

 the whole area of the ground, and only late in the growing season does 

 all sunlight of the acre fall on the growing plants. Most important of 

 all, the high efficiencies of photosynthesis are obtained only in light of 

 low intensity, less than 50,000 ergs/sec/cm^, whereas the energy of bright 



