Unlocking Phosphorus in Soil: Access Soil’s Phosphorus Vault

A grower’s success relies on a complex, interdependent web of organisms, minerals, and nutrients; factors like the availability of phosphorus in soil can significantly influence this delicate balance, with changes to one element affecting 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 in soil is particularly prone to binding with other elements, locking away its 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 in soil, knowing that some will be tied up physically. But when phosphorus leaves the field, it can have serious impacts on the environment. Because phosphorus in soil 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.

Soil pH plays a critical role in determining how much phosphorus is available to plants. Phosphorus is most soluble and plant-accessible when soil pH is in the optimal range of 6 to 7. At lower pH levels, phosphorus binds tightly to aluminum and iron, forming compounds that are largely insoluble. 

On the flip side, at higher pH levels, phosphorus becomes bound to calcium, again reducing its availability. These chemical reactions can lock up large amounts of phosphorus in the soil, even when soil tests show it is present in abundance. Maintaining the right pH range is crucial to avoid these binding processes and to maximize phosphorus efficiency.

Managing soil pH requires a proactive approach, and growers have several practical options. In acidic soils, applying lime can help raise the pH, neutralizing acidity and reducing the availability of aluminum and iron that bind phosphorus. For alkaline soils, adding organic matter such as compost or manure can buffer pH and improve phosphorus availability by enhancing microbial activity and releasing organic acids. Regular soil testing is also essential to monitor pH levels and guide amendments. 

By keeping soil pH in the sweet spot, growers can ensure that phosphorus remains accessible, improving nutrient uptake and reducing the need for additional fertilizer inputs.

Phosphorus is a critical nutrient for plant growth, but its unique behavior in the soil and environment sets it apart from other elements like nitrogen. Unlike nitrogen, phosphorus does not have a gaseous phase in its cycle, meaning it primarily moves through soil, water, and living organisms. This lack of volatility means phosphorus is tightly bound to the soil and reliant on specific processes to become available to plants. The phosphorus cycle begins with mineralization, where organic matter in the soil decomposes, releasing phosphorus into a form plants can use. From there, soil microbes play a central role, breaking down organic compounds and helping to release phosphate ions. However, phosphorus can also be immobilized, becoming tied up within soil particles or bound to elements like calcium, iron, and aluminum, making it inaccessible to crops.

Soil microbes are nature’s hidden workforce, driving much of the phosphorus cycle. They not only help mineralize organic phosphorus but also release organic acids that can solubilize bound forms of phosphorus, making it more bioavailable. However, the cycle is a delicate balance. Immobilization can lock phosphorus away when conditions, such as soil pH, change. Unlike nitrogen, which can enter the atmosphere and return to the soil through precipitation, phosphorus remains in the terrestrial and aquatic systems, cycling slowly and with little flexibility. 

This fixed nature of phosphorus highlights the importance of managing its availability in soils, as runoff into water bodies can lead to harmful environmental consequences. By understanding the key stages of the phosphorus cycle, growers can better harness the power of soil microbes and natural processes to unlock this essential nutrient and reduce dependency on chemical inputs.

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 released by the plants’ roots, the tiny root-bound microorganisms work to unlock nutrients like phosphorus in soil and nitrogen, concentrating 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. 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.

Phosphorus in soil exists in two main forms — organic and inorganic — and understanding their interaction is key to optimizing plant growth. Organic phosphorus comes from the decomposition of plant and animal material, including crop residues, manure, and microbial biomass. While this form is abundant in many soils, it is not directly available to plants. Instead, it serves as a reservoir that must be broken down into simpler compounds before plants can use it. Inorganic phosphorus, on the other hand, includes readily available forms like orthophosphate, which plants can absorb through their roots. These inorganic forms are crucial for immediate plant nutrition but can quickly become tied up in the soil, forming insoluble compounds with calcium, aluminum, or iron, depending on the soil’s pH.

The interaction between organic and inorganic phosphorus is driven by soil microbes, which act as the bridge converting one form to another. Microbial activity is essential for mineralizing organic phosphorus, breaking it down into orthophosphate that plants can absorb. At the same time, microbes can immobilize inorganic phosphorus, incorporating it into their biomass and temporarily making it unavailable to plants. This dynamic process ensures a continuous supply of phosphorus, though its efficiency depends on factors like soil temperature, moisture, and microbial diversity. By fostering healthy microbial populations and managing organic inputs, growers can enhance the natural cycling of phosphorus in their soils, ensuring both short-term availability and long-term sustainability.