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 , 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 , to then determine how much more, if any, to add to particular fields.
Why farmers add phosphorus in the first place
Phosphorus is one of the three primary nutrients plants require for growth, along with nitrogen and potassium. Without enough phosphorus, .
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, , 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 .
Years of application have led to something no one initially planned for: accumulation.
How much phosphorus has built up?
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. 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 . 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 . In parts of Florida, for example, some agricultural soils contain phosphorus concentrations more than .
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.
When soil phosphorus becomes a water problem
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 , reducing diversity that is important for good soil health.
Unfortunately, phosphorus doesn’t always remain in place. Rainfall, irrigation and drainage – either dissolved in water or attached to eroded soil particles – into nearby canals, streams, rivers and lakes. Once there, it becomes .
The result can be explosive algal growth, , 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 .
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 . Cutting back on nitrogen without lowering phosphorus can .
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 , particularly where warm temperatures and nutrient pollution overlap.
Rising global temperatures are exacerbating the problem. than colder waters, increasing the likelihood that phosphorus pollution will trigger eutrophication and dead zones.
Flawed testing hid the problem
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 used today were developed decades ago and were designed to work reasonably well across many soil types. . Some are sandy; others are rich in organic matter formed from centuries of decayed plants.
And those , delivering inaccurate findings of how much phosphorus plants can actually access. For instance, in soils that have , 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.
A smarter way forward
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 . 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 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, . 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.
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