Irrigation scheduling is an important component of profitable cotton management. University of California researchers have shown a yield increase of 400 pounds of cotton lint per acre from some fine-tuning of the timing for the first irrigation. Yet, scheduling the first irrigation can be most difficult.
Dramatic fluctuations in air temperature and drying winds, the amount of rainfall since the preplant irrigation, and differences in rooting volume of small cotton plants, because of soil conditions and seedling diseases, add uncertainty to scheduling the first irrigation. Slow plant and root growth resulting from the cooler than normal temperatures would suggest delaying the first irrigation. However, limited root expansion may also prevent sufficient uptake of water if the temperature would quickly turn very hot. The plant is the best indicator of the total environment thus measurements on the plant are the most reliable for scheduling irrigations.
The use of pressure chamber measurements can be a very effectively tool in cotton irrigation scheduling. In a test comparing the time honored art of irrigation scheduling based on monitoring the reddish color on the mainstem with respect to the terminal bud versus pressure chamber measurements, use of the latter method produced 120 pounds more lint per acre because of better timing of the irrigations for managing plant stress.
The best results were obtained when mid-day leaf water potential declined to –16 bars just prior to the first irrigation and from –18 to –20 bars for subsequent irrigations. Research results for Pima cotton would add an additional –2 bars to the recommended thresholds. Measurements are made on the newest fully expanded leaf, usually the third or fourth node down from the terminal. Declines in leaf water potential are nearly linear for all soil types. It can take up to 2 weeks for cotton plant water potential to change from –15 to –18 bars on a clay loam soil, and as short as 6 days on a sandy loam soil.
Data taken from plants smaller than about 8 nodes are very erratic and not considered reliable for irrigation scheduling. The best tool for small cotton is a shovel. If the roots are in some moisture and secondary root formation is occurring, transpiration demand can be met and plant growth and root expansion will continue. A short, light first irrigation is appropriate. For most soils, 2 to 4 inches of water are sufficient. Alternate row irrigation should be considered for soils with high water intake.
Timing irrigations for optimum production is a no cost management decision. The use of a pressure chamber to monitor leaf water potential is an effective tool available to cotton growers.