With respect to its response to carbon dioxide enrichment cotton is one of the most responsive crops grown. The average of all species studied is that doubling the CO₂ concentration of the atmosphere (to 660 ppm) increases yield by about 30%. Similar CO₂ enrichment for a cotton crop has shown the yield increase to be 70 to 90%. The question is, can this response of cotton be used in an agronomic way? Can the atmosphere of cotton fields be enriched with CO₂ and the yield increased enough to justify the cost?

To test this idea CO₂ was released at a constant rate through drip irrigation tubing placed on the soil surface in each row of test plots in Phoenix, AZ. Growth, flowering, boll setting and yield were monitored and compared to plots without CO₂ fertilization and to nearby plots in which the CO₂ was controlled for 24 hours each day to a level of 650 ppm.

In the first year the CO₂ was released for the duration of daylight hours. The yield increase for that treatment was about 42%, which was about two-thirds of the increase observed in the plots controlled to 650 ppm. However, the economic return for this fertilization was only 3%. That is, the cost of the CO₂, was about thirty times the value of the yield increase.

In the second year the time of the CO₂ release was reduced to 4 hours each day, 10A to 2P. This timing of release was chosen by using the observation that the concentration of leaf starch is influenced most by exposure of the plants to CO₂-enrichment during early afternoon hours. It has often been observed in cotton and other species that the increase in yield associated with CO₂ enrichment is correlated with an increase in concentration of starch in the leaves. The yield increase due to this treatment was about 27% which was also about two-thirds of the yield increase observed in the plots controlled at 650 ppm.

We interpret these data to be an increase in the efficiency of the CO₂ fertilization. The economic return in the second year was about 10% of the cost of the CO₂. That, of course, is a long way from justifying this practice agronomically. However, it shows that timing of the CO₂ release is important to the economic return. In future experiments we will limit the released time to fewer hours daily and to the days of heavy boll-loading that the leaf starch assay tells us is the time when the crop is under the most stress for photosynthate. At the present time, fertilizing the atmosphere of a cotton crop with CO₂ is not practical, but with additional refinements, it may become feasible.