A Fresh Look at Potassium for a Balanced Fertility Plan

When it comes to crop nutrients, the three that get most of the attention are nitrogen, phosphorus and potassium. All are used by crops in significant quantities, but for various reasons, the first two garner a lot of attention. Still, there are significant benefits to yield, water use and nutrient balance afforded by focusing on potassium needs.

Expertise provided by Calvin Murphy and Jessica Pupo; written by Rachel Sim.

The Third Macronutrient

A good deal of attention is paid to the first two nutrients of the NPK triad, nitrogen and phosphorus, and for good reason. 

A lot of that is because nitrogen and phosphorus are so prone to loss,” says Calvin Murphy, Senior Agronomist at Sound. Typically, only between 40 and 50% of the nitrogen that’s put on a field is actually captured by the crop. And with phosphorus, nearly everything in the soil is trying to tie it up so it’s not plant-available.” 

In the soil, phosphorus forms strong bonds with other minerals like calcium, aluminum and iron and plants alone are unable to break these bonds to access the phosphorus they need in season. Nitrogen poses efficiency problems as well — depending on the form used, it’s prone to loss through volatilization, leaching and denitrification. Access to potassium, on the other hand, seems relatively simple; it’s largely related to the swelling and drying of soils, something growers don’t always have much control over. 

Nitrogen and phosphorus are pretty well understood,” says Jessica Pupo, Senior Agronomist at Sound. In the industry, there’s a good understanding of how plants access these nutrients, what happens to them in the soil, how they move through the field and the role microbes play in plant-availability. Researchers and growers are still developing an understanding of how potassium is accessed and interacts with the soil microbiome, however. 

We’re only now starting to understand more about potassium solubilizing microbes, but there is still a lot we don’t know,” says Jessica. We’re still just on the cusp of understanding the soil microbiome in general and for a long time the focus has been on phosphorus and nitrogen.” 

Excessive potassium application also isn’t associated with the same severe environmental impacts as the other macronutrients. When washed off the field and into nearby water bodies, phosphorus and nitrogen contribute to eutrophication — an overgrowth of algae and other microorganisms that can deplete or eliminate oxygen in the water. Nitrogen fertilizer is also prone to volatilization and denitrification depending on the type of fertilizer used, the method of application and application timing. The resulting gasses can have negative impacts on both human and environmental health. 

Potassium, on the other hand, isn’t prone to loss in the same way, even if it’s still unavailable to the plants. 

The Role of Potassium

Potassium is an important nutrient for many plant functions; consider the adage up, down, all around” to describe the roles of N, P and K in plant health. While limiting nitrogen to upward growth and phosphorus to root development is a deeply simplified explanation, potassium really does play a part in many processes throughout the plant. It activates up to 60 enzymatic and hormonal reactions within the plant, supports seed and fruit development, and is a key part of protein synthesis and photosynthesis. Perhaps most importantly, it helps with water use and regulation; potassium plays a fundamental role in regulating the opening and closing of stomata, managing the exchange of carbon dioxide, oxygen and water. 

When we think about water, we usually think of what’s coming into the field from rain and irrigation and not necessarily how to utilize the water in our fields through chemistry and soil microbes,” says Jessica. But potassium plays a big chemical role in how water moves into the plant.” 

Unlike deficiencies in nitrogen, phosphorus and even many micronutrients, potassium deficiency can be hard to spot, especially early in the season. 

With potassium deficiency, we typically don’t see or identify it until the end of the season,” says Jessica. It has a big impact on seed development, particularly quality and fill, so with a potassium deficiency, your seeds won’t fill properly. In corn, if that top layer is not filling in at the end of the season, that’s typically why,” she explains. 

Potassium in the Soil

Like phosphorus, potassium exists in the soil in pools that are more or less available to the plant. There are typically four potassium pools:

  1. Crystalline structures: Most potassium is in this form as part of the soil parent material in rocks and other minerals in the soil and is unavailable to plants. These minerals do weather and break down over time, moving into other, more available pools, but the process is very slow.
  2. Non-exchangeable: This form of potassium is held within layers of clay in the soil. While it is not readily available to plants, potassium can move between this pool and more accessible pools.
  3. Exchangeable: Like non-exchangeable potassium, this form is also held by clay within the soil but it can be released to the available pool when those layers swell.
  4. Available: Potassium in this pool is water-soluble and readily available for plants to use when they need it. When the concentration of available potassium in the soil drops, more is released from the less accessible pools.

Potassium moves between the available, exchangeable and non-exchangeable pools based on a variety of factors including soil moisture, temperature and whether potassium fertilizer is applied. Plants are able to take up most nutrients when they’re dissolved in the water within the soil, known as the soil solution.

Like phosphorus, potassium moves from soil solution back into these non-exchangeable formats,” explains Jessica. Depending on your soil structure, you may have more potassium being held onto by clay, and if you’re putting more potassium in the soil solution with fertilizer, there’s more opportunity for that to move into non-exchangeable forms in your soil.” 

Potassium is abundant in most soils, but largely in non-exchangeable forms that plants can neither use nor access.

Anywhere from 90 to 95% of the potassium in the soil is plant-unavailable,” says Calvin. Between 2 and 10% is somewhat available, and between 1 and 2% if it is actually available to plants.”

Like magnesium, calcium, sodium and hydrogen, potassium is a positively charged cation, and it competes with these other minerals for space in the soil. A look at base saturation can determine how much of a grower’s CEC is occupied by one of these cations, and more of one cation means less of the others. 

You want to have a balance of those nutrients, and while there’s some discussion on what the right amounts are, there is typically a lower percent of potassium,” says Jessica. 

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Potassium deficiencies are visible in corn when examining grain fill, clearly seen at the top of these ears of corn.

Managing Potassium

Things like pH and soil structure impact both CEC and the balance of cations in the soil. Sandy soils have a lower capacity for cations, while soils with more clay have more. Soils with higher pH are more likely to have high calcium and magnesium content, leaving less space for potassium; an overabundance of one mineral leaves less room for others. 

In situations where you have really high magnesium levels, potash fights for the same position on the soil colloid,” says Calvin. Colloids are very small soil particles made up of clay and other organic materials, and their negative charge holds the positively charged cations in the soil. 

In Mississippi and other areas of the mid-South where Calvin works, soil magnesium levels are particularly high. In our base saturation, our magnesium levels are often 20% and above,” he says. In situations like that, it’s really hard to get potassium into the plant, because it’s fighting for the same space in the soil as magnesium.”

To address this, growers can band potassium close to the plants, in either a liquid or dry form. By applying an easily available form of the nutrient near the plant, growers can usually use less since it’s in an area and a form that the crop can easily access. 

Anywhere from 90 to 95% of the potassium in the soil is plant-unavailable

Jessica, who works with growers in the Pacific Northwest, says soils in her area have a different makeup that presents its own challenge.

Some of our soils have higher levels of sodium, and sodium and potassium compete with each other for plant uptake because plants identify them as the same nutrient,” says Jessica. If a plant is searching for potassium and finds sodium first, it’ll take the sodium.” In cases like these, she says growers have to make sure their soil includes more potassium compared to sodium while still considering how to balance calcium and magnesium.

A lot of this management has to do with making sure you don’t have one nutrient more dominant than the others, decreasing the ability for a different nutrient to occupy your soil,” she explains.

With any nutrient, anytime that you over-apply, you get imbalances and that means you’re typically tying something else up,” adds Calvin. If nitrogen is extremely high, you can tie calcium and potassium. If phosphorus is really high, you may be tying up some micronutrients like zinc and copper and iron.”

In the soil, macro- and micronutrients interact with each other in complex and varied ways to impact plant availability. Tools like the Mulder’s Chart help show the relationships between nutrients and their tendency to antagonize each other in large amounts. For example, excess potassium can hinder plants’ ability to take up magnesium, calcium, phosphorus, nitrogen and boron. This antagonism goes both ways: too much calcium or magnesium makes it harder for plants to get enough potassium. In appropriate amounts, however, nutrients can also support each other, with adequate potassium supporting iron and manganese use.

There are other challenges beyond these nutrient interactions, too. When there are very high amounts of potassium available to the crop, plants will simply keep consuming the nutrient, even beyond the amount they need. This is called luxury feeding,” and, in addition to providing no yield boost to growers, it can actually have negative impacts. Research shows that excess potassium can result in reduced biomass, root activity, photosynthetic rate and carbon- and nitrogen-metabolizing enzymatic activity.

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Potassium interacts with a number of different nutrients in the soil - using Mulder's Chart, you can see that the nutrient levels of phosphorus, nitrogen, calcium and boron all interact with potassium.

Understanding Potassium Needs

To maintain ROI, growers aim to provide crops with appropriate amounts of nutrients like potassium when the plants need it. By knowing their soils and crops, growers can avoid both excessive and insufficient nutrient applications, both of which can impact the bottom line. 

It’s about understanding what a balanced nutrient profile looks like and not adding inputs that would cause that balance to fluctuate beyond where it should be,” says Jessica. 

As an example, consider a wheat grower with high potassium soils who continues to apply potash, or potassium chloride (KCl), believing it’s benefitting his plants. 

The reason that grower would be seeing a positive impact in the short term is likely a result of the chloride part of potash, not the potassium, because wheat needs a lot of chloride,” explains Jessica. But by putting more potassium on the fields when there is already a high level of potassium, calcium and magnesium are likely being displaced as potassium is added into the base saturation. That will eventually cause other impacts, because plants need calcium and magnesium as well.”

How much of a grower’s focus is on potassium not only depends on location, but also on what crops they’re growing. Because potassium is key for fruit and seed development, it’s a key nutrient for a lot of crops destined for human consumption, including potatoes, legumes, and various fruiting vegetables.

Soil and tissue samples are important tools in understanding a field or crop’s nutrition needs, and comparing the two can shed light on whether plants are actually taking up those nutrients. Growers should make sure they’re going beyond just basic soil sampling, which may show only soil pH and nitrogen, phosphorus and potassium levels. A full panel soil sample can give growers a better picture of their cation makeup, which is key for potassium.

The most basic soil samples rarely show base saturation, and if we’re not looking at base saturation, we don’t know whether we’re creating an imbalance in our soils,” says Jessica. 

Tissue sampling can reveal imbalances that may be affecting the crop and help growers assess whether their practices are having the desired results. Crops that have low potassium despite sufficient levels in the soil could indicate an imbalance or tie up that simply applying more potassium won’t solve. 

An in-season tissue sample can shine light on whether what a grower is doing is working, not just for potassium but for all nutrients,” says Calvin. You see what nutrients your soil has, build a plan and then check that plan during the season with tissue tests. If you’re not getting the results you need, go back to the drawing board and figure out how to address the issue.” 

Both Calvin and Jessica recommend growers do regular soil sampling every 1 to 3 years and in-season tissue sampling as needed, depending on a grower’s location, crop and management practices. 

To make the most of in-season testing, tissue samples are also recommended, says Jessica, and can be taken based on when in the plants’ life cycle the target nutrient is needed. This can help growers achieve more precise nutrient management. 

We know potassium demand is especially high at the end of the season for fruit development,” explains Jessica. Having tissue tests done before that reproductive stage can help growers better manage potassium at the end of the season.” 

Sources of Potassium

When growers do apply potassium, it matters where it comes from. Potassium chloride, or potash, is the most widely used source of potassium across the agriculture industry. When it enters the soil solution, KCl separates into potassium and chloride. While chloride is a necessary nutrient, it’s needed in much smaller quantities than potassium, nitrogen or phosphorus, and in large quantities it can actually become problematic. 

When chloride is in the soil, it can bind to other nutrients like calcium and forms calcium chloride,” says Calvin. If we’re thinking about soil health, it may not be the right choice.” 

Calcium chloride is a salt that attracts moisture, making it an excellent dust suppressant and de-icing agent for use on winter roads. Small amounts of calcium chloride are unlikely to harm plants, but in larger quantities, excess salts can be extremely harmful to crops and the soil microbiome, especially under drier conditions that prevent calcium chloride and other nutrients from diluting throughout the soil. Understanding how much chloride is in the soil can be hard, though. 

Even if you get an advanced soil test, it likely won’t show your soil chloride levels,” says Jessica. But in many situations, you don’t want more chloride in your soil.” 

Tools like the Fertilizer Salt Index can help growers make sure any inputs are supporting their goals rather than hindering them. At 116, potash has the highest salt index of any fertilizer, followed by ammonium nitrate and ammonium thiosulfate at 104 and 90, respectively. For growers concerned about salt levels in their soil, robust soil testing to understand existing nutrient deficiencies and excesses and the Index can help them choose appropriate fertilizers that meet their needs. 

It’s becoming more common to talk about the salt level of fertilizers, but it’s also important to think about the reactions those fertilizers have in the soil,” says Jessica. 

Potassium and the Microbiome

Although the most well known beneficial soil microbes are those that solubilize phosphorus and fix nitrogen in plant available forms, there are also those that can help move potassium into the exchangeable and available pools.

Potassium solubilizing microbes can move a lot of potassium out of an unavailable form and into much more readily available forms,” says Calvin, and when so much of the potassium in the soil is generally unavailable, that can be a big help to growers.

While potassium solubilizing microbes aren’t talked about as often as other beneficial microbes, but they do play a role in potassium availability, as do AMF,” says Jessica.

AMF, or arbuscular mycorrhizal fungi, acts as an extension of a plant’s roots to increase access to water and nutrients. They carry sugars from the plant to microbes like potassium solubilizing microbes and take back plant-available phosphorus to the crop.

Good soil health is key to maintaining that relationship between plants and beneficial microbes. SOURCEⓇ, Sound Agriculture’s microbiome activator, mimics the plant-to-microbe signal to attract microbes already present in the soil and increases crops’ access to a variety of soil nutrients.

When conducting tissue tests on SOURCE-treated and untreated fields, Jessica says she’s seen an increase in potassium levels in treated tissue samples and an impact on soil structure.

Soil structure relates to the ability to hold and release water more efficiently, which is related really directly to the ability to access potassium availability,” she explains. To release some of the non-exchangeable potassium in the soil, clay layers need to swell, which often only happens under the right environmental conditions such as rain or irrigation.

Not only is SOURCE increasing the availability of potassium through the activation of potassium solubilizing microbes, it’s also improving soil structure, which allows greater root access,” says Jessica.

The biggest impact of SOURCE, however, is a more balanced nutrient profile in the plants.

When we pull samples from a control area, we typically see a nutrient or two that is out of bounds, either too low or too high, but with SOURCE, the nutrient ratios seem to be better,” says Calvin. A more balanced tissue sample means that the plant is more efficient, and if you’re more efficient, you need less nutrients to make the same bushel.”