Why Neglecting Phosphorus Could be Hurting Your Corn Crop

When it comes to corn, nitrogen gets a lot of attention, and deservedly so. It’s often the most yield-limiting nutrient for corn which, unlike soy and other legumes, cannot fix its own nitrogen. But phosphorus deficiencies in corn are more common than growers might think and they could be hurting yield.

With expertise provided by Nate Couillard, Agronomist, and Anne Kakouridis, Research Scientist with Sound.

Although phosphorus often gets less focus when it comes to corn, Nate Couillard, Agronomist with Sound Agriculture, says it’s actually a more common nutrient deficiency to show up in the crop than either nitrogen and potassium deficiency. 

Compared to phosphorus, nitrogen is a relatively easy to understand nutrient, says Nate. ​“You put nitrogen in the field and whatever the plants don’t take up is gone the next year when you do it all over again,” he says. 

Phosphorus’ behavior in the soil and plant is much more complicated; because phosphorus is so prone to becoming locked in the soil, it’s not uncommon for the crop itself to be deficient even when phosphorus levels in the soil are high. But with the right strategies and knowledge, growers can unlock free access to the phosphorus in their soil when their plants need it. 

Phosphorus plays an important role in photosynthesis, seed formation and root growth and is a key ingredient in the energy molecule ATP and both DNA and RNA. Because phosphorus is one of the building blocks for these critical plant components, without sufficient phosphorus, growers can see reduced yield and delayed crop maturity. 

Still, getting this nutrient into the crop can be challenging. In the soil, phosphorus often binds tightly to elements like aluminum, calcium and iron, forming compounds that plants cannot use. Different soil factors like soil texture or type and soil pH will affect exactly how much phosphorus is locked in the soil and inaccessible to plants. Soil tie-up is why many growers, especially in the mid-Atlantic region, may have high levels of the nutrient in their soil but still end up with phosphorus deficient crops.

“Only about one third of applied phosphorus is available to the crop; the rest becomes bound up in the soil,” says Nate. ​“That can be alarming, especially in areas with phosphorus management plans and concerns about polluting watersheds. Most of the acres in the mid-Atlantic have a good deal of bound-up phosphorus in the soil that isn’t always able to be converted into a plant-available form.” 

These high levels of soil phosphorus are one of the primary reasons for the development of Maryland’s Phosphorus Management Tool, which imposes limits to fertilizer application on fields where nutrient levels are already high. When phosphorus fertilizer leaves the field, it can end up polluting local waterways, contributing to low-oxygen dead zones where marine life cannot survive. 

The challenge for growers with sufficient levels of phosphorus in their soil but deficient crops is how to access the phosphorus they already have and make the most of any additional applications. 

Luckily, the keys to unlocking these phosphorus reserves are already in a grower’s soil. The complex soil ecosystem has resulted in microorganisms that have developed mutually beneficial relationships with plants, and growers can use these partnerships to their advantage.

Some soil organisms like phosphorus-solubilizing and nitrogen-fixing microbes directly convert unavailable forms of these nutrients into forms that the plant can use in exchange for some of the carbohydrates that plants produce via photosynthesis. Phosphorus-solubilizing microbes can free the phosphate molecules that would otherwise be locked in the soil, allowing them to be taken up by plants. In order to attract and activate these beneficial organisms, plants release chemical signals through their roots, letting the microbes know they’re open for business. 

Another incredibly important soil microorganism when it comes to phosphorus are arbuscular mycorrhizal fungi (AMF), whose primary function in the soil is to help crops access more plant-available phosphorus, says Anne Kakouridis, Research Scientist at Sound. 

“AMF act like an extension of the root system,” she explains. The fungi are about the size of a root hair, but they extend the reach of the plant much farther than the roots alone can.

“While AMF don’t dissolve inaccessible phosphorus themselves, they transport sugar from the plant to the phosphorus-solubilizing microbes and then pick up the plant-available phosphorus and carry it back to the plant,” Anne says. ​“AMF are very efficient at their job; even though they don’t dissolve the phosphorus themselves, it’s been shown across many, many studies that plants with AMF have much higher phosphorus levels than those without.” 

For crops like corn, where phosphorus is not typically considered the primary limiting nutrient, Nate says he’d like to see more growers taking soil phosphorus levels into consideration. ​“It’s easy for phosphorus to become an afterthought,” he says. ​“But I suspect that it affects more growers than we realize.”

When it comes to applying any nutrients, not just phosphorus, Nate says it’s important to consider what the plants actually need to take up, not just applying based on removal rates. ​“The plant requires more phosphorus than just what is going to be removed,” he says. ​“Because phosphorus is prone to getting locked in the soil, application conversations often circle what will be taken out of the soil. Instead, as growers, we should think about how to foster better phosphorus conversions in our soil to improve plant availability.” 

The first step is a soil test to understand factors like pH and soil type that impact phosphorus availability and conversion. Growers may also want to include tissue testing to get a fuller picture of their plants’ phosphorus uptake. 

Then, Nate says growers can begin to build out a plan to promote the microbial activity that will free soil phosphorus, based on their specific situation and needs. While trying to manage soil pH and a well-balanced nutrition plan are important, soil health plays a significant role in supporting phosphorus conversions. Nate says practices like cover cropping and no-till can help support healthy soils by increasing organic matter and reducing compaction. 

“Manure is great for adding organic matter, which is beneficial for both the soil and microbial health,” he adds.

No-till practices are particularly beneficial for AMF, says Anne, since soil disturbance can break up the very fine strands of the fungi. 

While it may seem counter intuitive, applying too much additional phosphorus can actually disrupt the natural phosphorus cycle in the soil. When provided with sufficient phosphorus from fertilizer, plants stop sending signals to soil microbes and the phosphorus solubilization process ceases. Some applied phosphorus will continue to be locked in the soil, but the crop will be unable to access it. 

As the intermediaries between phosphorus solubilizing microbes and plants, too much applied phosphorus also puts AMF out of a job. 

“If plants can get sufficient phosphorus on their own from the grower, they won’t give any sugar to the fungi and will actually kick them out,” says Anne. Since the plant is getting enough nutrition, this may not seem like an issue, but the absence of AMF has consequences beyond just nutrient availability, she says. 

“Even in cases where plants have enough nutrients and have ceased their relationship with the AMF, they do worse overall,” Anne explains. While the fungi’s primary role is phosphorus transportation, they also help plants get access to more water and protect plant roots from pathogens.

“AMF even pull down carbon into the soil and help the plants maintain a higher rate of photosynthesis,” Anne says. ​“But despite providing all these benefits, plants seem to only maintain their relationship with AMF based on nutrient status.”

Understanding how and when corn plants use phosphorus, and how to spot deficiencies, helps growers provide appropriate nutrition and support to the crop throughout the season.

In corn, growers are most likely to see deficiency symptoms, including stunted growth, reduced root size and yellowing or even purpling of older leaves, in a young plant’s first 40 to 50 days of life, before diminishing. But just because the symptoms disappear doesn’t mean the deficiency has. 

“As the plant grows, phosphorus is a very necessary nutrient, especially in the middle and later portion of its life, after tassel and in later reproductive stages,” says Nate. ​“In fact, over 50% of the phosphorus the plant needs in a season is needed after tassel. But pay particular attention to signs of deficiency in the early stages of the corn crop, because that’s when it’s most likely to show its face.” 

As the plant develops, it will take inventory of the available nutrients in order to plan for future growth, and around V4 or V5 is when the corn determines ear size. Nate says that after that, the plant is mostly just filling the order. Growers with a deficiency shouldn’t despair though: there’s still value in applying additional phosphorus later in the season. 

Last year, on his own farm, Nate says he ended up with this exact problem. He noticed signs of what looked like a phosphorus deficiency in one corner of a field, and confirmed with a plant tissue test.

“I applied a N/P/K blend and a new technology of phosphorus fertilizer that prevents phosphate molecules from binding to other particles in the soil, protecting them and keeping them more available to the plants,” he says. ​“Within about a week, the visual difference between those plants and the rest of the field disappeared. The plants responded very, very quickly.”

Although Nate says he took a small yield hit in that section of his field, ​“There is absolutely still value in salvaging a crop and addressing that phosphorus deficiency as fast as you can.”

No two fields are the same, to say nothing of different growers’ operation, so the most important thing growers can do is understand their soil, says Nate.

Once a grower knows what they’re working with in the ground through soil tests, developing a plan to promote microbial activity in the soil will help growers maintain the natural phosphorus cycling and get access to free, in season phosphorus for their plants.

“Anything that will improve soil health will also benefit those soil microbes,” says Nate. That includes regenerative practices like cover cropping, not till, and applying manure.

Growers can and should also make use of new technologies to help address phosphorus tie up, including advanced phosphorus fertilizers and SOURCE, Sound’s microbiome activator.

SOURCE is a chemistry that mimics the plant to microbe signals that attract a variety of beneficial soil organisms, including those that turn unavailable phosphorus into forms that plants can use. With a wide application window from V4 to R3, SOURCE can help growers address nutrient deficiency throughout the season while replacing 25 pounds of phosphorus and 25 pounds of nitrogen. Best of all, it works with what growers are already doing.

“There are still question marks around how much phosphorus — and other nutrients — growers are getting out of manure, when they are getting it, when it’s being converted, and when it’s becoming plant-available,” says Nate. ​“SOURCE works with what you already have, so if you’re using manure, it can help boost nutrient conversion and create a healthier soil profile.” 

“There are many things growers can do to try and optimize phosphorus conversion in the soil, but even when you do everything right, if the microbial activity isn’t there it just won’t happen,” says Nate. ​“That’s where a product like SOURCE can come in to activate those beneficial microbes.”