Water Savvy

 Dr. Brian Leib, University of Tennessee irrigation specialist in the Biosystems Engineering and Soil Science 
 Department at the University of Tennessee, discusses nine years of cotton research.

 Photo by John LaRose, Jr.

Matching Irrigation To Soils Produces Favorable Results

MidAmerica Farmer Grower

   Nine years of cotton research was presented by Dr. Brian Leib, University of Tennessee irrigation specialist in the Biosystems Engineering and Soil Science Department at the University  of Tennessee. As state irrigation specialist he covers row crops and vegetable crops, tobacco and many others.
   His presentation focused on nine years of cotton research, looking at overall production during that time rather than the year to year results. Others recognized for work on this project were: Owen Gwathmey, retired UT Plant Physiologist, Chris Main, UT Cotton Specialist – private industry; David Verbree, UT Plant Physiologist – private industry; Adam Duncan, UT MS Graduate Student now graduated; Amir Haghverdi, UT PhD Graduate Student, about to graduate; and Tim Grant – UT MS Graduate Student, about to graduate. The project was funded by Cotton, Inc. and a USDA NRCS Conservation Innovation Grant.
   “We recapped experiments from 2006 when we picked out some very good soils to raise cotton,” he said. “By good soils we mean deep silt loams that have water holding capacity of over two inches for every foot of soil. We use drip irrigation, not because we’re promoting drip irrigation, in fact we design the experiment to mimic center pivot irrigation that predominates in Tennessee; but we really need to make a lot of small plots, so there’s a lot of treatments and a lot of replications to make this work. So we used drip on the surface, but were able to make 10 different water treatments.”
   Some treatments started at square, some were initiated at bloom and two weeks post bloom. When those treatments were initiated, irrigation rates used were one and a half, one inch and a half inch of water applied per week. These amounts were only applied if there was no rainfall.
   “I was really expecting deficit irrigation to be important in cotton because we’re in an area where rainfall is very highly variable,” Leib said. “You don’t really know what to expect from year to year. 
   “If you fully irrigate and keep your soil moisture at field capacity, replace all the water that is used by the plant almost on a daily basis, you leave no room for rainfall to be captured; so you end up irrigating more. The other thing that you don’t know that could happen is you could have a  2 1/2- to 3-inch rainfall event on top of having irrigated a lot, and then you have areas that are too wet in your field. So you run the risk of losing yield from anaerobic conditions.”
Cotton is a perennial. Though it’s harvested as an annual, it is really a perennial and would keep growing if that were allowed. Many studies on things like apples and grapes have shown that if you hold back water at strategic growth periods you cut down on vegetative growth but promote reproductive growth, so it seems the same thing might happen with cotton. That really did turn out to be true.
   “So, we did verify that you should hold back the water, especially in good soils,” Leib said. “When we applied high rates of water at square, we reduced our irrigated yield very consistently through time. Most of our yield was gained by waiting all the way to post bloom, and most times we got away with only an inch per week.
   “But there were a few years where the drought was intense enough where we had to start earlier or bump that up to an inch and a half per week to get our optimum yield. Still, deficit irrigation was very important and irrigating early at high rates was something to be avoided at least in good soils for most years.”
   These experiments produced some really good information, however it didn’t cover every situation and all the needs under every condition. There are many people who have poor soils that don’t hold as much water and they have both types of soils mixed together in the same field. A recommendation for that situation also was needed.
   “So, we found a location near Jackson where the soil is filled with sand deposits from an ancient ocean and where the glaciers contributed to silt being blown on top of the sand,” he explained. “In this particular location there is a silt over sand at different depths, so where it’s sandier there’s less water holding capacity, where there’s deep silt loam there’s more. We were able to distinguish those areas by using ground penetrating radar and then soil cores and we put our same irrigation plots over those different soils. Just like we thought would happen, the sandy soils needed more water earlier to maximize yield, and it was in conflict with the loam soils. If you irrigated for the sandy soil you’d be hurting the yield of the silt loam soil. If you irrigated for the silt loam soil you’d be hurting for the sandy soil. So we were able to give some more recommendations based on variable water holding capacity.”
   Next it was decided to go off-station to see if these same relationships are common in producer’s fields. The last two years, intense soil sampling was done and Varis EC readings were performed in a producer’s field. The irrigation water was varied over those soils using the speed control of a center pivot system and the yield data was analyzed.
   “We need to go a little farther with that, but it looks like some of those same trends we saw at the research farm are showing up on producers’ fields,” Leib said. “We’re going to recommend zoning for one producer’s field this year, so we’re making quite a bit of progress.”
   Some studies still need to be done. While cracked boil has been the recommendation for terminating irrigation, that idea needs to be reconsidered.
   “I’m thinking in some dry years, irrigation past that point might be really beneficial,” he said. “Another thing is we have some major soil types that we really haven’t delved into yet with cotton, while we have with some other crops. We have a lot of ground here that’s a mixture of hilltops, side slopes, and bottoms and those water conditions are really different, so we’re trying to branch out in that direction also.
   “I thought cotton would be the trickiest crop we had to irrigate because it’s a perennial and has idiosyncracies in terms of shedding boles for different reasons; it turns out that is so.
   “So, my take home message is we can do a good job and it will make a difference in the bottom line if we pay attention to our soil types and how they should be irrigated; if we use some tools like soil moisture sensors or water balanced programs, I think we can help ourselves and ensure optimum yields year in and year out,” Leib summed. ∆
   BETTY VALLE GEGG-NAEGER: Senior Staff Writer, MidAmerica Farmer Grower
MidAmerica Farm Publications, Inc
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