The No. 1 challenge with potassium is to manage the nutrient so that it’s ready to work when it’s needed
Potassium’s behavior in the soil sets it apart from other nutrients such as nitrogen and phosphorus. That means you must manage it differently, in terms of timing your applications. The ideal timing varies in different types of soil, so different fields, or parts of fields, might require different strategies.
Before planning your potassium strategy, you need to know whether your plants are deficient. "Changing your management program will have no effect if potassium is not the limiting factor," explains Farm Journal Field Agronomist Ken Ferrie.
On soils that are testing at optimum levels for potassium, with no fixation or leaching issues, applications can be made every two years, in the fall or spring. In problematic soils, farmers should try to apply potassium close to or during the uptake period, such as spring broadcast, starter fertilizer, sidedressing and fertigation.
Know it when you see it. One clue to potassium deficiency will jump out at you before you even plant your 2013 crop: If a field has a history of standability problems every time you plant it to corn, the culprit probably is a potassium deficiency.
To confirm that, plan to take tissue tests in that field, early-, mid- and late-season. You need to find out when the potassium problem shows up because plants need larger amounts of potassium as the season goes along.
In your other fields, potassium strategy starts with scouting, beginning right after emergence.
"It’s very important to know the symptoms of potassium deficiency and to monitor your crop from the early vegetative stage through the reproductive stages," Ferrie emphasizes. "If you suspect plants are deficient in potassium, follow up your scouting with tissue testing to see what’s going on inside the plants."
If you confirm a potassium shortage in your plants, test your soil to make sure there’s an adequate supply of potassium, Ferrie advises. Most soils contain a lot of potassium, but only a small amount—the soluble form of potassium, stored in water held in soil macropores—is available to plants. As plants take up the soluble potassium, the supply is replenished from unavailable forms.
If soil test levels are optimum, plant deficiencies could be caused by drought because dry soil reduces plants’ ability to take up potassium. In dry soil, potassium ions become fixed between the lattices of clay particles.
"If it’s dry early, you’ll see potassium deficiencies earlier in the season," Ferrie says. "You could see deficiencies even with optimal soil test levels. This was common during the 2012 drought.
Deficiencies will show up first in fields where soil is low in potassium."
Most potassium deficiencies result from the plants’ inability to take up the nutrient. "If plants fall behind on uptake, it’s almost impossible for them to catch up later," Ferrie says.
Besides drought, other causes of poor uptake include soil compaction and high levels of nitrate-nitrogen. When soil is compacted, potassium ions become trapped inside compressed soil particles and unavailable to plant roots. When high levels of nitrate-nitrogen are present, plants take up the nitrogen rather than potassium.
"Both soil compaction and high nitrate-nitrogen levels may be man-made issues," Ferrie says. "So consider the possible impact of your tillage and nitrogen management programs on potassium availability. If you’re starting out with low potassium levels in the soil, the effect of compaction or high nitrate levels will be magnified during a drought."
If the availability problem is caused by drought, soil compaction or high nitrate levels, changing the method or timing of your potassium application won’t help as much as irrigating (if possible), removing compacted layers or changing your nitrogen program. But if the problem results from the soil type itself, you can modify your potassium management to help keep the nutrient flowing well into the reproductive stage of corn or soybeans.
Manage by soil type. Potassium management is pretty easy in a field with optimum soil test levels and no problems with fixation of potassium or leaching by water. "In that environment, you probably can apply potassium every other year," Ferrie says. "Base the rate on soil test results to maintain optimum levels."
In low-testing soils, with no fixation or leaching issues, try to raise the soil test level to fortify the soil for the next dry growing season. "As your finances permit, apply amounts above the removal rate on a yearly basis, until your soil test climbs into the optimum range," Ferrie says. "In this situation, and the previous one, with no leaching issues, you can apply potassium either in the fall or in the spring."
If you farm sandy soil, with low nutrient-holding ability (expressed as cation exchange capacity), you must take leaching into account. Potassium, like nitrogen, can be carried away by water.
"Here, you must prevent the loss of potassium through the system," Ferrie says. "Never build soil test levels higher than optimum—just maintain that levelthrough the growing season."
"In these soils, apply potassium at least on a yearly basis in the spring, close to the time of plant uptake because a fall application could be leached away," Ferrie says. "You may want to broadcast part of your potash and apply the rest in starter fertilizer and at sidedressing. If you irrigate, you also can apply some potassium through your center-pivot system."
Soils that are high in magnesium can present a different type of challenge. "Mostly, plants pick and choose which nutrients to take up," Ferrie says. "But potassium and magnesium are exceptions—with them, you can get luxury feeding. If the soil is relatively high in either potassium or magnesium, plants consume more of the nutrient that is in greater supply and less of the other nutrient.
High-magnesium soils. "In high-magnesium soils, apply potassium every year, in the spring, when the crop’s root system is present to use it," Ferrie advises. "Band potassium in the strip if you strip-till or on the surface if you do tillage. You also can apply potassium in your starter fertilizer at planting or sidedress it. Try to keep the potassium soil test reading at the upper end of the optimum level."
Some soils require special management to make sure plants are able to take up potassium through the growing season. They include sandy soils, soils high in magnesium and soils with high clay content.
For a longer-term solution, try to learn why magnesium levels are high. "If they are caused by applying dolomitic limestone, try to switch to a low- magnesium or calcitic source of lime, so you don’t keep pushing the magnesium levels higher," Ferrie says.
In heavy, high-clay content soils, with a high cation exchange capacity, potassium can become fixed to the organic matter in clay particles, making it unavailable to plants.
"In this kind of soil, it becomes difficult, and often prohibitively expensive, to apply enough potassium to change soil test levels and get adequate uptake," Ferrie says.
"The solution is to make an annual application in the spring," he continues. "That way, when the potassium you apply goes into the soil solution, plant roots can take it up before it becomes fixed to clay particles and unavailable. Band as much of your potassium as possible. If you do tillage, strip-apply potassium to the surface before you till."
To sum it up: Once you have a documented deficiency, the key to successful potassium management—as with many other things—is good timing.
Potassium in Organic Soils
High-organic matter soils—mucks and peats—require potassium management similar to soils with high-fixation and leaching issues. "In these soils, don’t try to build soil-test levels of potassium—just feed the crop," advises Farm Journal Field Agronomist Ken Ferrie.
"Apply potassium every year, in the spring," Ferrie continues. "Consider applying some of your potassium as starter at planting time, as a sidedress treatment and, if you irrigate, through your center-pivot system. You also can band potassium when you strip-till."
Try a Banding Approach to Potassium
Applying potassium fertilizer in bands, or strips, can be a valuable management tool. "But farmers often worry that, if they apply a band of potassium on the surface and then chisel the field, they’ll dilute and spread out the fertilizer," says Farm Journal Field Agronomist Ken Ferrie.
"That needn’t be a concern," he says. "The dilution effect of a chisel plow on a potassium band is pretty small. While you may move the location of the band, it has little effect on the concentration of the potassium."
To drive this point home, Ferrie uses the following illustration: If you apply a preplant grass herbicide in bands and then try to use a chisel plow to disperse the herbicide outward to get a uniform kill, you will not be successful. The result will be a weedy field, with streaks of herbicide damage where plants came into contact with the concentrated rate of herbicide.
"The purpose of a concentrated band of potassium is to create a high concentration of the nutrient in a small area of the soil," Ferrie says.
"Some of the potassium in the concentrated band will get fixed to clay particles, but the balance will be left available in the soil solution for plants to take up.
"Banding potassium beneath the surface as you strip-till also works well," Ferrie says. So does banding potassium with the planter or with a sidedress applicator. Making multiple applications of banded potassium is the ideal method.
"Farmers are finding that the more years they band the nutrient, the fewer potassium problems they see," Ferrie concludes. "If plants don’t use all the potassium in a band that year, the unused potassium remains to feed crops in future years."
Learn and Profit from Nutrient Navigator
The Nutrient Navigator series focuses on efficient, environmentally sound management of nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher.
You can e-mail Darrell Smith at email@example.com.