Soil/Moisture Balance






 Matt Rhine, Senior Research Associate on the cropping systems project at the Fisher Delta                  
 Research Center recently presented the University of Missouri’s Crop Water Use application
 for cotton irrigation scheduling.

 Photo by John LaRose, Jr.












Water Application Tool Provides Better Way To Schedule Irrigation

BETTY VALLE GEGG-NAEGER
MidAmerica Farmer Grower

PORTAGEVILLE, MO.
   Cotton irrigation scheduling with the University of Missouri’s Crop Water Use application was presented recently by Matt Rhine, Senior Research Associate on the cropping systems project at the Fisher Delta Research Center.
   “We developed this app that gives us a more efficient way of irrigating our crops across the Bootheel, whether it be corn, soybeans, cotton, or furrow irrigated rice,” he said.
   A number of different inputs need to be put into the crop water use app, including field location, soil type, and crop specific information such as planting date and effective rooting depth. While many of these inputs are straightforward, the effective rooting depths of each crop must be determined with field research to be sure we are irrigating efficiently.
   “We wanted to validate our most effective rooting depth to use as an irrigation trigger to schedule irrigation,” Rhine continued. “We know historically that cotton has a rooting depth up to 40 inches because we’ve seen that in the field. But the deeper you get into the soil profile, the fewer roots you will find. We can’t always irrigate based on that lowest root zone because the small number of roots found there do not hydrate the plant efficiently. So we tested a number of different cotton rooting depth triggers with the app in order to validate our best irrigation trigger.”
   Tests were done on rooting depths of six-inch increments, all the way down to 30 inches in the root zone. Irrigation was based on those depths. The crop water use app is a soil-water balance model and the rooting depth can essentially be considered the size of your checkbook. The deeper your roots go, the larger your balance, and the more potential your crop has to utilize soil water. 
Irrigating a six inch rooting zone requires more frequent irrigations than with a 30-inch because there’s less soil to pull moisture from in a shallow rooting depth. Research in 2016 showed the six inch rooting zone required irrigation six times even though it was an unusually wet summer. The 12-inch needed irrigation three times and the 18-, 24- and 30-inch depths only required one irrigation.
   “We found that by irrigating the 6 inch rooting zones six times we actually lost 95 pounds of lint yield compared to our non-irrigated check. We know cotton doesn’t like wet feet, and even our 12-inch rooting depth, which was irrigated three times, lost yield compared to not irrigating at all.”
   The bottom three rooting triggers, the 18-, 24- and the 30-inch were all irrigated one time, but at different times.
   “The 18 triggered just a little bit sooner than the 24 and the 24 a little bit sooner than the 30. However, the 18-inch rooting depth trigger provided the highest yield over the non-irrigated check. It yielded 98 pounds more lint than not irrigating at all and only required that one irrigation. So irrigating as much as six times wasted not only energy and pumping costs but also robbed us of lint yield. Based on 2016 data we’re looking at 18 inches as our target rooting depth trigger for cotton in this area.”
   These trials were repeated again in 2017, which was a little bit drier than 2016. In 2017, irrigation was needed nine times on the 6-inch depth cotton, six times on the 12-inch depth, then twice on the 18,- 24- and 30-inch depth.
   “We don’t have those yields yet, but based on the status of the crop we’re still looking at the 18-inch rooting depth as our best irrigation trigger, and that’s our take home message,” he said. “When you’re using the MU Crop Water Use app for cotton irrigation scheduling, we’re targeting that 18-inch rooting depth as our irrigation trigger.
   “When you go into the app, set your field up, put in cotton as your desired crop, and state your planting date, the app will ask you for a rooting depth. We’re recommending an 18-inch depth for that right now. That helps us most efficiently irrigate cotton by getting the water on at the right time. Again, even though cotton roots are down as low as 40 inches, the effective rooting depth for the app is right about that 18-inch depth. That’s where we have the most roots that can most efficiently grow that cotton crop,” Rhine stated. Cotton has a tap root that can be as deep as 40 inches, but these lower roots can’t pull enough water to maintain yield.”
   The cropping systems project is working on rooting depth validation for a number of crops. “We’re also doing a validation in furrow irrigated rice, and rice generally has about 75 percent of its roots in that eight-inch soil depth,” Rhine explained. “You can find probably about 25 percent of them as low as 12 to 15 inches, but irrigating based on those deeper roots may not optimize rice growth and yield.”
   Current recommendations for rooting depth to trigger irrigation for corn and soybeans is about 20 inches. “Those rooting depths have been established in other studies, so we usually try to stick around that 20 inch zone,” Rhine said.
   On the cotton trials, the six-inch rooting trigger check provided a loss of 95 pounds of lint compared to the non-irrigated check. The 18-inch provided a gain of 98 pounds of lint compared to the non-irrigated.
   “Again, with the six-inch you’re spending more money on irrigation costs and losing cotton yield,” he summed. “In the past, whether you turn the pumps on once a week or just when the crops shows visual stress, that’s not an efficient way to irrigate. If we can base irrigation on these soil-water balances, then we can more efficiently use our water while maintaining yield.” ∆
   BETTY VALLE GEGG-NAEGER: Senior Staff Writer, MidAmerica Farmer Grower
MidAmerica Farm Publications, Inc
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