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."
- Early Spring 2013