Water and nitrogen are the main constraints to cotton production in the semi-arid western United States. Sub-surface drip irrigation areas in cotton land in West Texas are expanding at a rapid rate. The efficiency of water application to cotton in sub-surface drip is greater than 90 percent. This is because losses due to evaporation from center pivot irrigation and deep leaching losses of water associated with furrow irrigation are negligible in sub-surface drip.

The efficiency of nitrogen fertilizer utilization has the potential to be high as well, because fertilizer losses are often related to inefficient water applications. Furthermore, nitrogen fertilizer can be injected at small rates on a daily basis into sub-surface drip irrigation systems, for true “spoon feeding.” Our cotton research team in West Texas has found that nitrogen fertilizer use efficiency is 60, 40, and 20 percent recovery for sub-surface drip, center pivot, and furrow irrigation, respectively. We are researching nitrogen “fertigation” management practices that might increase the efficiency further.

There are three basic fertigation approaches that we have been evaluating for six years. The first is the source of nitrogen fertilizer. The conventional source of urea ammonium nitrate (32-0-0) is being compared with a blend of urea ammonium nitrate and ammonium thiosulfate (28-0-0-5S). Thiosulfate has been reported to stabilize ammonium forms of nitrogen in soil by inhibiting certain bacteria. The second technique we employ is to fine-tune the timing of nitrogen injection into sub-surface drip cotton.

The researchable issue on timing is not when to commence nitrogen injections (this should be at the same time irrigation starts, at first square in mid- to late-June), but when to terminate the injections for the season. We compared injecting nitrogen between first square and early bloom with first square to peak bloom. Finally, we have had a major research thrust on testing in-season sensors like the chlorophyll meter and the spectroradiometer to help us determine the amounts and timing of nitrogen fertigations.

The timing and source of nitrogen fertilizer studies were conducted for two years in Halfway and Lubbock, Texas. So far, we have not observed any advantages to using 28-0-0-5S instead of 32-0-0 or to terminating nitrogen injections at peak bloom as opposed to early- to mid-bloom. Quality of lint is likewise not sensitive to these management changes. We will continue testing these treatments in 2006. Currently, we now recommend that nitrogen fertilizer in sub-surface drip irrigated cotton be injected over a five-week time period.

The total amount of nitrogen fertilizer to inject should be based on a total nitrogen supply target of 150 pounds of N per acre. It is important that producers soil sample to two feet and subtract this amount of nitrate (N) in the top two feet from the 150 pounds per acre of total N supply. On loam and clay loam soils this amount of N is sufficient to produce 3 bales of lint per acre.

We think the chlorophyll meter technology is ready for adoption by producers and consultants working on sub-surface drip cotton. It is vital that a well-fertilized nitrogen plot or strip (i.e. 150 pounds of N/acre applied pre-plant) be established to serve as a reference for the bulk cotton area.

Chlorophyll meter readings of the bulk area should be 97 percent of the readings in the nitrogen-rich area. Our results indicate that in many cases, nitrogen fertilizer is saved using the chlorophyll meter (or spectroradiometer) in sub-surface drip cotton, without hurting yields. The chlorophyll meter should not replace spring soil test results (which should be done yearly), but has strong potential to supplement soil test data to further improve nitrogen fertilizer use efficiency in drip cotton and sustain producers’ profits in this time of high nitrogen fertilizer prices.

Contact Kevin Bronson at the Texas Ag Experiment Station in Lubbock, Texas, at
k-bronson@tamu.edu or (806) 746-6101.