A Healthy Soil Microbiome is the Key to Accessing Soil Phosphorus

Phosphorus is one of the three major macronutrients necessary for crop health, along with nitrogen and potassium, and a common input for many growers. But once in the ground, phosphorus can be a challenging nutrient to actually get to the crop. Find out what happens to phosphorus in the soil and how growers can increase their crops’ access to phosphorus while improving farm finances.

With expertise provided by Jessica Schultz, Senior Agronomist at Sound. 

Phosphorus is one of a handful of elements required by all life, not just crops. In both plants and animals, it’s a key component of the energy molecule ATP used to power cellular processes and it’s part of the structural backbone of DNA and RNA. In humans and animals, phosphorus also combines with calcium to form teeth and bones. Plants require phosphorus for photosynthesis, root growth and seed formation, and phosphorus deficiency can reduce yield and delay maturity. 

Jessica Schultz, Senior Agronomist at Sound, is quick to point out just how important phosphorus is to plant growth. ​“It’s easy to slip into the mindset that nitrogen is all that matters for crops, but for plants, growing without enough available phosphorus is like trying to build a house without enough bricks. It’s an essential part of the plant cell, and without enough of it, plants just can’t grow to their full potential, plain and simple.”

For many growers, the first indications that their crop may have a phosphorus deficiency will be visual. Especially early in the season, plant growth will be slow to come up and stunted. Older leaves may begin to turn a dark green to grayish-blue. As deficiency progresses, leaves begin to turn a purple or reddish color and leaves thicken; sometimes stems are impacted too. Phosphorus deficient plants will also have diminished shoot and root growth. 

It’s important that growers supply crops with enough phosphorus early in the growth cycle and respond to deficiency quickly; otherwise, it may not be possible for the plants to recover from the yield-limiting effects of phosphorus deficiency.

Jessica adds that grain fill is another critical point in the plant’s growth cycle. ​“This is when a nutrient deficiency could really impact yield,” she says. ​“The crop still has pretty high demand, so it’s important to develop a healthy root system and symbiotic relationship with soil microbes early on to set your plant up for success in accessing enough nutrients later on in the growing season.” In corn, for example, 50% of total phosphorus is still needed at VT. For growers who are looking to provide their crops with access to plant-available forms of phosphorus all season long, understanding how phosphorus behaves in soil is key to making the most of this important nutrient.

In nature, phosphorus is primarily found in rocks containing phosphate minerals. In areas with high enough concentrations of phosphorus, it may be mined and turned into fertilizer or other products, but otherwise, as the rocks weather and erode, the phosphate dissolves and enters the soil. However, it often doesn’t start out in a form crops can absorb. 

How phosphorus travels from phosphate-rich rocks into the soil and eventually makes its way into a farmer’s crop is part of the phosphorus cycle. To simplify the process, it can help by imagining all soil phosphorus in one of three buckets: unavailable phosphorus, transitional phosphorus or available phosphorus:

• Unavailable phosphorus: This is the biggest phosphorus bucket in the soil, and the phosphorus found here is largely tied up with other minerals and organic compounds in the soil. Slow, natural processes like weathering and decomposition move phosphorus from this bucket to the next.
• Transitional phosphorus: The phosphorus in this bucket is more easily accessible to plants under certain conditions. Short term availability of transitional phosphorus is influenced by pH, soil type and clay content. Phosphorus in this transitional form is made up of highly decomposed organic material and chemically bound phosphorus; depending on the soil pH, phosphorus will bind to different elements — calcium at higher pH or iron and aluminum at lower pH. When bound up like this, phosphorus is not accessible to plants; first, microbes must consume and metabolize the phosphorus in this bucket. Most of the microbes that are important for this process can be found in the plant root zone in response to chemical signals released by the plants’ roots through root exudates.
• Available phosphorus: This bucket is the smallest of all three and contains the phosphorus released from the transitional bucket by microbes to a soluble form that plant roots can absorb. The transformation of phosphorus from transitional phosphorus to available phosphorus relies on those chemical signals sent through the plant roots to activate phosphorus-solubilizing microbes.

Soil tests can help growers understand the availability of phosphorus in their soil and the basic soil characteristics like soil acidity and organic matter content that impact the quantity of accessible phosphorus.

Soil pH can be affected by a variety of factors, including rainfall, soil parent material, organic matter content, and the use of ammonium fertilizers. Acidity impacts the availability of phosphorus by changing which minerals are likely to bind with it in the soil; to reduce the impacts of tie up, a pH between 6 and 7 is ideal. Once a grower knows their soil pH, there are different strategies they can employ to either modify pH or work with its impact. 

Soil organic matter and the additional input of organic materials or manures can also increase the phosphorus pool in the soil and the immediate availability of plant-available phosphorus through decomposition of these materials. However, the long-term availability of phosphorus from these sources will primarily depend on soil microbes bringing their phosphorus content to the plant in plant-available form.

While soil tests are a good tool for growers looking to access more soil phosphorus, it’s important to remember that they may reveal only part of the story.

“Soil testing may indicate whether there is a lack of phosphorus present, or plenty of it,” says Jessica, ​“but since we know phosphorus is very prone to tie-up and reactivity, it’s not always a great indicator of the total availability of phosphorus actually present in the soil or whether the plants are getting enough of the nutrients they need.” Growers may want to pair soil testing with tissue tests to better understand what’s happening between the soil and the crop.

Because the vast majority of the phosphorus in soil is a plant-unavailable form of the nutrient, adding phosphorus fertilizer may seem like a good way to boost phosphorus levels in a grower’s crops. However, because of how easily phosphorus reacts with other minerals in the soil, becoming locked up and inaccessible to the crop, adding fertilizer also adds phosphorus to the unavailable phosphorus bucket. 

“Up to 80% of the phosphorus applied to the soil can become tied up with minerals in the soil and unavailable to crops,” says Jessica. ​“That leaves only 20% available to the plants.” 

But even when growers take phosphorus tie-up into account during fertilizer application, the remaining phosphorus still may not make it to the plant. Rain can wash soil and the phosphorus locked in it out of the field and into local waterways; not only can phosphorus pollution cause harmful algae blooms and negatively impact water quality, it represents the loss of the grower’s investment in the fertilizer to begin with. 

Finally, excess phosphorus applied to the soil can also disrupt the natural phosphorus cycle. When a plant receives all the phosphorus it needs from fertilizer, there’s no need to send signals to the phosphorus solubilizing microbes in the soil. Without the chemical signals from plant roots, the microbes slow their phosphorus conversion process. 

“Soil microbes and plants have evolved a truly symbiotic relationship,” explains Jessica. ​“Microbes receive carbon-complex sugars and starches from the plant root exudates in exchange for phosphorus, an exchange which is triggered by these unique chemical signals. So, when plants stop signaling, the microbes stop trading.” 

Even though there is plenty of phosphorus in the soil, the movement to transitional and available phosphorus slows, and the soil processes and equilibrium are disrupted. In the meantime, a significant amount of applied phosphorus fertilizer continues to be locked in the soil and washed away by the rain. The unavailable phosphorus bucket gets bigger while the available phosphorus bucket stagnates. 

Thankfully, the power to unlock the soil’s phosphorus stores has been perfected over millions of years of coevolution between plants and microbes; for growers, the challenge is how to exploit that relationship and make it work for their crops. 

“Because microbes are responsible for transitioning tied-up phosphorus into plant-available phosphorus,” says Jessica, ​“it’s critical that growers keep the microbe population alive and active. Applying more and more phosphorus is not a great solution, because most of it just gets tied up by minerals in the soil once again, or worse, it leaches out into waterways.”

Transforming that tied-up phosphorus into a form plants can use requires healthy soils, full of living organisms and microbes that will metabolize nutrients like phosphorus for the crop. The process of building healthy soil does take time and effort, especially when compared to the instant boost phosphorus fertilizer can provide, but tapping into the power of the soil microbiome can give growers free in-season access to key nutrients. 

To help growers start boosting their soil’s power, Sound developed SOURCEⓇ, our microbiome activator. SOURCE is a molecule that mimics the plant-to-microbe signal in order to unlock the power of the relationship between plants and microbes, attracting the microbes and life in the soil that make both phosphorus and nitrogen more available to the plant. 

“If growers can keep their microbe populations active,” says Jessica, ​“they can cut back their phosphorus inputs and rely on their soil microbes to do most of the work for them, and SOURCE is a really great way to do that.”

Internal and third-party field data and field trials have borne this out, showing SOURCE increasing both uptake and soil availability of phosphorus by over 18%. As part of our commitment to data-driven science, Sound continues to update the data and research on how SOURCE interacts with key soil characteristics and can benefit a variety of crops.

“With SOURCE, growers can leverage the symbiotic relationships between crops and soil microbes and get access to the phosphorus that’s already in their soil, which reduces phosphorus pollution and increases their profitability and sustainability,” says Jessica. ​“Whatever angle you look at it from, that’s a win.”