Is Your Soybean Crop Getting Enough Potassium?

Managing plant nutrition in soybeans can sometimes be a little confusing. What nutrients will your crop get naturally from the soil? When do you need to provide additional nutrients to ensure a healthy soybean crop? 

Some farmers will fertilize corn for two successive crops, leaving soybeans to scavenge for the nutrients that remain. But to achieve optimal soybean yields, you may need a more focused plan for nutrient management. 

Potassium is a vital part of a robust nutrient plan 

Potassium (K) plays an important role in the plant, influencing photosynthesis and metabolism. More specifically, it helps regulate the opening and closing of the stomata, which facilitate the exchange of water and gas vapor in and out of the plant. 

In soybeans, K needs are especially high: Around 1.4 lbs potash (K2O) is removed per bushel. By means of comparison, a 200-bushel corn crop will remove around 50 lbs of K2O per acre, while a 70-bushel soybean crop will remove 84 lbs per acre. 

So why is K overlooked in many nutrient management plans? Because most soil tests will come back with sufficient K levels. The issue is that K is mostly immobile in the soil—meaning it doesn’t move easily to the plant in the same way nitrogen does.

K occurs in the soil in 3 different forms:

  1. A readily available form, sometimes called exchangeable K. This form is water soluble and held on the exchange sites of clay particles. This type represents about 1-2 percent of K in soils and is what you will find measured in soil tests. 
  2. A slowly available form that is trapped in clay mineral layers. The amount of this that can become available will depend on several factors, mainly the dominant type of clay. 
  3. An insoluble mineral form that plants cannot use. This form represents 90-98 percent of K found in soils and can become available over time as the minerals weather.

Since many farmers are using minimum or no-till systems, the available K in soils has become even more concentrated in the top few inches of soil—away from the root zone. 

There are other environmental factors that will affect K availability. Because it moves by diffusion, drought conditions will worsen the uptake of K by plants. Drought will also trap K between clay particles as part of a shrink/swell phenomenon. Also, compaction will limit root growth and the ability to explore soil surfaces with available K.

While K is immobile in the soil, it is mobile in the plant. This means deficiencies will show up first on the lower, older leaves and then move up the plant (an exception to this rule is in fast-maturing crops like wheat and cotton, which can show deficiencies in new leaves). In soybeans, K deficiency will present as yellowing along leaf margins, followed by scorching and dieback. 

Addressing a possible K deficiency

The first step toward addressing potential K deficiencies would be to utilize a soil test to see what your potassium levels are. Even if your results are above the critical level, you may still want to consider a K application to ensure the concentration is high enough in the root zone for the amount the crop will need. 

The ideal situation would be to fertilize before each crop, accounting for nutrient removal rates. If you have an actively growing soybean crop, tissue sampling can help to determine if deficiencies are present. This can also help you to form a game plan for nutrient management in future crops.

It’s important to note your soil type. Pay particular attention to the following

  • Sandy soils with low CEC (cation exchange capacity); these soils can struggle to hold potassium, so keep a close eye on K levels.
  • Coarse soils with low CEC as well as organic soils, you should avoid fall-applied K in these types of soils.

Timing and placing your application

One of the best options for providing adequate potassium to your soybean crop is to broadcast and incorporate K prior to planting. 

Surface applications of K can also be effective in no-till managed acres.

You may deep band K, but research has shown that the best benefit for soybeans would be if soybeans are planted directly over deep-banded rows. Because soybeans are extremely sensitive to salt injury, a banded application of K at planting is not recommended. 

PRO-TIP: Foliar applications of K can serve as a bandage when deficiencies are severe or on sandy soil types, but remember the amount of potassium a plant will uptake in a growing season is quite large and the plant simply doesn’t have the ability to uptake that large amount of nutrient need via a foliar application.

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