How to Gain Access to the Soil’s Phosphorus Vault

A grower’s success relies on a complex, interdependent web of organisms, minerals, and nutrients; a change in one of these factors can affect countless others. 

Not only is phosphorus one of the three macronutrients necessary for plant growth, along with nitrogen and potassium, it’s a vital nutrient for all living organisms. In humans and animals it plays a role in the formation of teeth and bones, and in plants, it’s a crucial nutrient for photosynthesis. In all organisms, phosphorus is a necessary component of DNA and RNA, cell membranes, and the energy-rich ATP molecule that fuels both animal and plant activity. Phosphorus deficiency can lead to impaired growth of both plants and roots, reduced yield, and delayed maturity, making it essential that growers secure enough phosphorus for their crop. 

Not only is providing plants with the right amount of nutrients important, growers must also make sure the nutrients are supplied in forms that can be taken in and used. It doesn’t matter how much phosphorus is applied if the crops can’t access it. Unfortunately, phosphorus is particularly prone to binding with other elements in the soil, locking away it’s plant-powering potential.

Phosphorus is most commonly available to plants in the form of orthophosphate molecules, which have a negative charge. These molecules can form strong bonds with soil particles and with elements like calcium, aluminum, and iron, making them inaccessible.

Understanding how much phosphorus is available to your plants can be challenging. Sound’s Senior Product Marketing Manager, Erik Wolfe, says that while soil tests are a great tool, they don’t always tell the full story for phosphorus. ​“Your soil test might indicate a lot of phosphorus, but your plants might not be able to access very much of it,” he says. ​“And as soil pH changes, the element that will bind up phosphorus changes — iron and aluminum at a lower pH, then calcium at higher pH.”

Erik says he thinks of phosphorus in the soil like a bank: ​“Up to 80% of applied phosphorus can be quickly bound and tied up by elements like iron, calcium, and aluminum, making it unavailable for plant uptake. It’s like having a bank account where you can only use 20% of your money.” Once that 20% has been used by the crops, it’s as if the account is empty.

A common solution is just to apply more phosphorus, knowing that some will be tied up physically in the soil. But when phosphorus leaves the field, it can have serious impacts on the environment. Because phosphorus binds to mineral elements in the soil and to soil particles themselves, heavy rain can wash phosphorus into surface water. In rivers and lakes, excess phosphorus causes harmful algae and cyanobacterial blooms, reducing the amount of dissolved oxygen available, and seriously impacting the diversity and food supply of aquatic wildlife.

If instead a grower wants to try and reduce phosphorus tie-up, they may try banding or chelating. Banding is a method of application that places the nutrient below the surface of the soil but near the seeds or seedling plantings, which makes the fertilizer more available to the plants than to weeds. For phosphorus, banding reduces contact with the soil and the potential for fixation within the soil, but requires soil pH values between 5.5 and 6.5. Chelating agents, on the other hand, are additives that create strong bonds to the aluminum, iron, and calcium and reduce the potential for those metals to bind to phosphorus.

But why not take advantage of the phosphorus already in the soil? Using resources already in the soil decreases input cost and improves a farm’s sustainability. First, though, something needs to break down those bonds. If we look closely, just below the surface, nature has hidden the key to unlocking the treasure trove of phosphorus trapped in the soil.

The relationship between plants and the soil microbes that live in their root zones has evolved over millions of years. In return for the sugars, starches, and amino acids that are released by the plants’ roots, the tiny root-bound microorganisms remove nutrients like phosphorus and nitrogen from the soil and concentrate them in forms the plants can use.

“Phosphate and nitrogen are both essential nutrients for plants to carry out their life cycles, and microbes have coevolved with plants for a long time,” explains Aden Kinne, Associate Scientist at Sound. It’s a classic symbiotic relationship that benefits both plants and microbes, and it’s also the solution to the problem of phosphorus lock-up that SOURCE™ capitalizes on. SOURCE is a microbiome activator that stimulates the nitrogen-fixing and phosphorus-solubilizing microbes in the soil to make both nitrogen and phosphorus more available to the plants.

“The mechanism in nature is for phosphorus solubilizing microbes to convert phosphorus into orthophosphate forms that can then be taken up by the plant through the roots,” says Erik. ​“That’s how it happens in nature — it’s the microbes that facilitate this whole process — and SOURCE taps into that.”

To test SOURCE’s microbe stimulating power, the research team isolated the phosphorus solubilizing microbes in soil and placed them in a media with no microbe-available phosphate. ​“Only the microbes which could solubilize phosphate and make it bioavailable could live,” Aden says. Comparing how the microbes reacted to different chemistries, it was clear that SOURCE intensified the microbes’ phosphate-solubilizing abilities.

Field trials have borne out these results, with SOURCE increasing both soil availability and uptake of phosphorus by over 18%. For growers, SOURCE offers a way to access the untapped reserves of phosphorus that is typically unavailable to their plants.

“A grower can either over-apply, try to reduce tie up with banding or chelating, or help the microbiome unlock the phosphorus that’s already there,” says Erik. ​“The solution is not necessarily more phosphorus, the solution is an active microbiome. Because SOURCE increases the activity of phosphorus solubilizing microbes, that is going to lead to more phosphate solubilization.”

Not only are growers getting additional value out of their soil’s phosphorus reserves, saving input costs, and increasing profitability, they’re also reducing phosphorus-laden runoff.

“We’re really excited about SOURCE because it can help farmers reduce application of nitrogen and phosphorus inputs and increase sustainability,” says Aden.