When people think about agricultural pollution, they often picture what is easy to see: fertilizer spreaders crossing fields or muddy runoff after a heavy storm. However, a much more significant threat is quietly and invisibly building in the ground.

Across some of the most productive farmland in the United States, a nutrient called phosphorus has been accumulating in the soil for decades, at levels far beyond what crops actually require. While this element is essential for life-supporting root development and cellular chemistry to grow food, too much of it in the wrong places has become a growing environmental liability.

I’m part of a research effort to figure out how much phosphorus is already in the soil, to then determine how much more, if any, to add to particular fields.

Phosphorus is one of the three primary nutrients plants require for growth, along with nitrogen and potassium. Without enough phosphorus, crops struggle and production suffers.

For decades, applying phosphorus fertilizer has been a kind of insurance policy in American agriculture. If farmers weren’t sure how much was already in the soil, adding a little extra seemed safer than risking a shortfall. Fertilizer was relatively inexpensive, and the long-term consequences were poorly understood.

Unlike nitrogen, which easily escapes from soil into the air or groundwater, phosphorus sticks to soil particles. Once it’s added, it tends to remain in place. That trait made phosphorus seem environmentally benign.

However, phosphorus can still be carried off fields when rain or irrigation water erodes phosphorus-rich soil, or some of the built-up phosphorus dissolves into runoff.

Years of application have led to something no one initially planned for: accumulation.

Since the mid-20th century, farmers across the United States have applied hundreds of millions of tons of phosphorus fertilizer. From 1960 to 2007, phosphate fertilizer consumption in the U.S. increased from approximately 5.8 million metric tons per year to over 8.5 million metric tons annually.

In more recent decades, fertilizer use has continued to rise. In corn production alone, phosphorus applications increased by nearly 30% between 2000 and 2018. Crops absorb some of that phosphorus as they grow, but not all of it. Over time, the excess has piled up in soils.

In many regions across the United States, soil phosphorus levels are now far higher than what crops actually require. In parts of Florida, for example, some agricultural soils contain phosphorus concentrations more than 10 times above levels considered sufficient for healthy plant growth.

Scientists call this buildup “legacy phosphorus.” It’s a reminder that today’s environmental challenges are often the result of yesterday’s well-intentioned decisions.

If phosphorus stayed locked in the soil, farmers would have wasted money on fertilizer they didn’t need. And excess phosphorus in soil can hinder the uptake of essential plant micronutrients and alter the soil microbial community, reducing diversity that is important for good soil health.

Unfortunately, phosphorus doesn’t always remain in place. Rainfall, irrigation and drainage can transport phosphorus – either dissolved in water or attached to eroded soil particles – into nearby canals, streams, rivers and lakes. Once there, it becomes food for algae.

The result can be explosive algal growth, known as eutrophication, which turns clear water a cloudy green. When these algae blooms die, their decomposition consumes oxygen, sometimes creating low-oxygen “dead zones” where fish and other aquatic life struggle to survive. This process is primarily driven by phosphorus leaching, as seen in the Florida Everglades.

Another prime example is the largest dead zone in the United States, covering about 6,500 square miles (16,835 square kilometers), which forms each summer in the Gulf of Mexico. Cutting back on nitrogen without lowering phosphorus can worsen eutrophication.

Some algal blooms also produce toxins that threaten drinking water supplies. Communities downstream may be told not to drink or touch the water, and face high treatment costs and lost recreational opportunities. National assessments document toxins associated with algal blooms in many states, particularly where warm temperatures and nutrient pollution overlap.

Rising global temperatures are exacerbating the problem. Warmer waters hold less oxygen than colder waters, increasing the likelihood that phosphorus pollution will trigger eutrophication and dead zones.

Given the risks, a natural question arises: Why don’t farmers simply stop adding phosphorus where it isn’t needed?

Part of the answer lies in how the amount of phosphorus in the soil is measured. Most soil tests used today were developed decades ago and were designed to work reasonably well across many soil types. But soils are incredibly diverse. Some are sandy; others are rich in organic matter formed from centuries of decayed plants.

And those traditional soil tests use acids to extract phosphorus from the soil, delivering inaccurate findings of how much phosphorus plants can actually access. For instance, in soils that have more than 20% organic matter, like those found in parts of Florida and other agricultural regions, the tests’ acids may be partially neutralized by other compounds in the soil. That would mean they don’t collect as much phosphorus as really exists.

In addition, the tests determine a total quantity of phosphorus in the soil, but not all of that is in a form plants can take up through their roots. So soil where tests find high phosphorus levels may have very little available to plants. And low levels can be found in soil that has sufficient phosphorus for plant growth.

When farmers follow the recommendations that result from these inaccurate tests, they may apply fertilizer that provides little benefit to crops while increasing the risk of pollution. This isn’t a failure of farmers. It’s a mismatch between outdated tools and complex soils.

The solution isn’t to eliminate phosphorus fertilization. Crops still need it, and many soils genuinely require additional nutrients. The challenge is knowing when enough is truly enough.

Researchers, including me, are developing improved testing methods that better reflect how plants actually interact with soil. Some approaches mimic plants’ root behavior directly, estimating how much phosphorus crops can realistically take up from any given field or type of soil – rather than only measuring how much exists chemically.

Other tests look at the amount of phosphorous a field’s soil can hold before releasing excess nutrients into waterways. These approaches can help identify fields where farmers can use less phosphorus or pause it altogether, allowing crops to draw down the legacy phosphorus already present.

The phosphorus problem is a slow-moving one, built over decades and hidden below ground. However, its effects are increasingly visible in the form of algal blooms, fish kills and contamination of drinking water supplies. Farmers can measure and manage soil nutrients differently and reduce pollution, save money and protect water resources without sacrificing agricultural productivity.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Dinesh Phuyal, University of Florida

Read more:
Toxic algae blooms are lasting longer in Lake Erie − why that’s a worry for people and pets

25 years of Everglades restoration has improved drinking water for millions in Florida, but a new risk is rising

Fertilizer prices are soaring – and that’s an opportunity to promote more sustainable ways of growing crops

Dinesh Phuyal does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.