Potassium Deficiency

 “Soil fertility testing has a long history at the University of Missouri. We have records of soil samples from Sanborn 
 Field in Columbia tested in 1915” said Dr. Gene Stevens, cropping systems specialist at the University 
 of Missouri Fisher Delta Research Center at Portageville, Mo.

 Photo by John LaRose Jr.

Portable Meter Can Help Monitor Potassium In Cotton Plants

MidAmerica Farmer Grower

   Dr. Gene Stevens, cropping systems specialist at the University of Missouri Fisher Delta Research Center, discussed potassium fertilization at a recent meeting.
   “Soil fertility testing has a long history at the University of Missouri,” he said. “We have records of soil samples from Sanborn Field in Columbia tested in 1915. While that was many years ago, we’re still learning new things to improve soil test recommendations for crops. We have been fortunate to receive funding support for research from checkoff dollars by commodity groups. The best information has come from long-term experiments. In 2009, the Fisher Delta Center completed an eight-year fertility experiment with rice and soybeans funded by the Missouri Rice Council. In that study, we learned that we needed to increase the target potassium and phosphorus levels for rice so after we finished the test our recommendation program was modified. Today, when a farmer submits a soil sample to our soil lab at the Delta Research Center, the past field research is reflected in the fertilizer recommendation report.”
   The research center is currently participating in a long-term test with scientists from Mississippi, Arkansas, Texas, and Louisiana evaluating soybean rotations with corn, grain sorghum, and wheat. The project is funded by the Mid-South Soybean Board. Soil fertility is an important component of the study.
   “So far, we have found that rotating soybeans with corn increased yields compared to continuous soybean mono-cropping. Burning corn stalks in the fall saved on tillage trips, but yields averaged across all five states were lower in burned plots. The impact of crop rotations on cyst nematodes is also being monitored over time,” Stevens said.
   Dr. Stevens focused the rest of his talk on potassium (K) fertility research funded by Cotton Incorporated. From 1996 to 2005, Stevens worked with David Dunn, soil lab manager, to conduct calibration validation research on low K testing in farmer fields.
   “For the most part, we found that P and K recommendations from the lab were still current for cotton,” he said. “When farmers submit soil samples to a lab, they have the option to select how many years to divide buildup applications to bring low testing fields up above target levels. Our field trials showed yields were best in low testing fields when four-year buildups were selected rather than the eight-year default.”
   “Andrea Jones has been working with other cotton state specialists on a K test for several years,” Stevens said. “Most Missouri cotton farmers maintain fairly high fertility levels in their fields. I would like to know how yields will be impacted if a farmer reduces annual K fertilizer rates or skips applying K one year all together.
   “What’s interestingly about this field at the Delta Center is that it is close to the critical target level for potassium,” he added. “The soil tests showed that it was around 275 pounds of ammonium acetate extractable K per acre and for this soil type and soil texture this is about the point that you would expect to start seeing a yield response. The critical level is 220 plus 5 times CEC (Cation Exchange Capacity). Above that level you would not expect to see a response. This is the third year of the experiment and the yield data from the previous two years research showed no response from applying potassium compared to the untreated check. About 55 pounds of K20 per acre is being removed with the seed cotton each year. Eventually that will draw the soil down enough and we will see a yield response from K fertilizer; but we’re probably not at that point right now because I am not seeing K deficiency symptoms in the cotton leaves. I hope this experiment will continue enough years to see the tipping point.”
   Stevens also provided information to help clear up confusing parts of soil test recommendations reports.
   “When you submit your soil sample and get the results back from the lab, the measured levels in the soil are shown in pounds of K,” he explained. “But the recommendations at the bottom of the page are in pounds of K2O fertilizer which is not the same. Most soil labs in the Mid-South use ammonium acetate or Mehlich 3 solutions to extract the soil sample. These solutions don’t extract 100 percent of the K, just a portion of it. It takes roughly six pounds of K2O applied fertilizer to increase the soil test level one pound of K. If you’re low and trying to build it up it’s going to take six pounds of K for each unit going up; and since you’re removing it, in this case no additional fertilizer is added, then it draws it down at the same rate. But it can take years.”
   There are high fertility fields on the Delta Center-Lee Farm that contain over 450 pounds of extractable K per acre. One of the test fields has not received fertilizer other than nitrogen for 23 years as the nutrient levels were mined down. It took over 10 years before any deficiency symptoms at all started to appear. This shows that a farmer with extremely high soil test levels should consider saving money by cutting fertilizer rates. A maintenance application every year to balance crop removal will keep the field where it is now.
   “Potassium is a regulator in the plant, it controls the opening and closing of pores on the underside of leaves called stomata,” Stevens explained. “Open leaf pores allow water to evaporate cooling leaves and bringing in CO2 for photosynthesis. So it’s really an important nutrient. Cotton leaves in K deficient plants have a yellow area between the veins and a bronze or brown area around the edges. Cotton fields that are K deficient prematurely drop their leaves which cuts short the photosynthesis period and doesn’t give the cotton plants a chance to make larger bolls and open them.
   “We evaluated a small battery powered portable meter to monitor potassium in cotton plants. In the experiment, we applied different rates of potassium on plots for several years then used a potassium ion meter to measure the sap of the potassium on the petiole leaves,” Stevens said.   “Cotton is harder to squeeze out leaf sap than leaves like watermelons.  We found that the trick was putting the cotton petioles in a freezer overnight then thawing them the next morning. The freezing breaks down the cell walls and allowed us to easily squeeze the sap out with a garlic press. We were able to calibrate or correlate the petiole potassium from the normal lab to what we’re doing in the field, so a farmer can go out and test the potassium level in the field in the middle of the season and, if it’s low, make a potash application then if needed.”
   Dr. Stevens said they published the results of the potassium sap research on cotton in a scientific journal. It explains how they calibrated the instrument. If you would like a PDF preprint of the paper, send him an email at or call 573-379-5431. ∆
   BETTY VALLE GEGG-NAEGER: Senior Staff Writer, MidAmerica Farmer Grower
MidAmerica Farm Publications, Inc
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