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.

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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.

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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.

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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 It’s a mismatch between outdated tools and complex soils.

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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.