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Recent research suggests inadequate potassium (K) in the soil profile can result in increased anthracnose and increased winter damage in Poa annua. Not welcome news for golf course superintendents already stressed by tight budgets, high player expectations, unpredictable weather and persistent disease pressure.
To make matters worse, potassium deficiencies are not easy to detect, says Dr. Larry Stowell, a managing director at PACE Turf, a membership organization that provides information services to turf grass managers.
“Grass plants do not become chlorotic or turn purple when potassium is not present in adequate levels,” he says. “The plants simply do not grow and they will eventually die if potassium is not applied. It is therefore difficult to diagnose a potassium deficit based upon plant appearance.”
Testing plant tissue levels will help determine if potassium has not been applied at sufficiently high rates or the soil does not store sufficient potassium, Stowell says. The target for many turf species is above 2 percent tissue potassium with 2.5 to 3.5 percent potassium being common for healthy grass.
Potassium is the second most abundant element in plant tissues, while nitrogen (N) is the most abundant element, Stowell says. Potassium will be used by the plant in a ratio of about one-part potassium for every two-parts nitrogen. Because potassium is removed from the soil by the plant at about half the rate that nitrogen is removed from the soil, it is most important to routinely supply potassium when grass is grown on high-sand-content root zones where there is little cation exchange capacity. When the exchange capacity is higher, as in many soil areas of a golf course, potassium sometimes stored in the soil in sufficient quantities so that application of potassium fertilizer is not needed. Stowell says 37 parts per million Mehlich III potassium is the critical level that soils should be maintained above. There is likely some nutritional stress due to potassium deficiency when soil levels drop below this value.
Dr. Micah Woods, chief scientist at the Asian Turfgrass Center and an assistant adjunct professor at the University of Tennessee, recommends testing soil for potassium once a year.
“If one is really interested in this type of data, then test twice. I suggest a Mehlich III or a normal ammonium acetate extraction method for soil potassium,” Woods says. “Those are standard methods at soil testing laboratories in the U.S. and it is easy to interpret the test results.”
Dr. Woods’ personal preference for most regions is to test in autumn. “I like to know soil test results at the end of the growing season, so that I can plan what to apply in the next year,” he says. “But the best time of year is actually to test, for a given facility, at about the same date year to year. That makes the results more easily comparable.”
Superintendents should take plenty of subsamples from an area to make sure the sample submitted is representative of the area being tested. Also, take samples to a consistent depth. Woods suggests four inches (10 cm) for turfgrass. Greens should be tested because they are the most highly maintained areas on the course. But, the largest quantities of fertilizer are applied to the fairway and rough areas, so a superintendent needs to also know how much potassium is required in those areas to apply fertilizer efficiently.
Dr. Grady Miller, professor and extension turf specialist in the Crop Science Department at North Carolina State University, offers this analogy: “My childhood dentist once told me when I asked him if I had to floss between all my teeth. His quick response was,`Only those you want to keep.’ So, why not test all the areas of the golf course at least once? When I say all the areas, get a feel for your greens, tees, and fairway soils and what their inherent nutrient status. Once you have that -- and that may take testing a few areas over several years, you may determine that you may want to test all your greens or certain greens annually and the other areas every three to five years. Soil tests are cheap, so why not get the benefit of that information.”
Grady adds if the values are “good/high,” then skip that area next time and check other areas that you have not tested. “If they are “not so good/low,” then try to boost their nutrient status and a regular check on the status of that area.
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Soil testing should be most frequently conducted on high-profile, high-performance, and high-traffic areas, Stowell says. Greens and tees are frequently tested. Fairways are also important to test on a regular basis. Roughs are less of a concern and periodic testing can be included with the more routine testing of other more critical areas. Problem areas should be tested separately to determine whether or not poor turf performance is due to chemical, physical (shade, drainage, compaction, etc.), or biological (insects or disease) stress.
Analytical test results are only as good as the sample that has been provided to the laboratory for analysis.
“For tissue samples, washing sand and other debris out of the tissues is critical,” Stowell says. “Collect at least three large hands full of tissue and place the tissue into a clean bucket with water. Agitate the tissues to dislodge any sand or fertilizer from the tissue. Squeeze the water out of the tissue and spread the tissue onto clean newspaper to dry before shipping the tissues to the lab.” He advises that for soil samples, superintendents should collect 10 soil 1-inch diameter cores from the sample area to a depth of four inches. Either remove or leave the thatch and mat on the sample, but be consistent whenever you sample. Use an analytical laboratory that participates in regional sample exchange programs or a laboratory recommended by your state university extension service.
If it is determined potassium is less than one percent in the tissue of the plant it is deficient in potassium with greater disease susceptibility and potential turf thinning, says Dr. John Cisar, a retired University of Florida turf researcher.
“The nitrogen/potassium rate should be near a two-to-one ratio,” Cisar says. “Apply potassium when applying nitrogen at that ratio and apply when the grass is growing. Don't over apply, as potassium is easily leached in low CEC sand soils.”
If a course has sandy soils or greens, Dr. Doug Soldat, associate professor in the University of Wisconsin-Madison Department of Soil Science, recommends spoon feeding potassium along with your nitrogen.
“For finer textured soils, one to two pounds of potassium (KO2) per 1,000 square feet will meet the annual needs of your turf, and the applications can be made in one pound per 1,000 square feet increments or more if a coated source of potassium is used,” Soldat says.
However, Soldat cautions to avoid applying large doses of potassium to sandy soils. “These soils don't have the cation exchange capacity to retain large amounts of potassium, so you'd be wasting your money,” he adds. “Avoid a balancing approach to soil testing interpretation.”
John Torsiello is a golf and agronomics writer based in Torrington, Conn., and is a frequent GCI contributor.
