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.Why farmers add phosphorus in the first placePhosphorus is one of the three primary nutrients plants require for growth, along with nitrogen and potassium. Without enough phosphorus, crops struggle and production suffers.AdvertisementAdvertisementAdvertisementAdvertisementFor 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.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. increased from approximately 5.8 million metric tons per year to over 8.5 million metric tons annually.AdvertisementAdvertisementAdvertisementAdvertisementIn 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.More in ScienceIn 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.When soil phosphorus becomes a water problemIf phosphorus stayed locked in the soil, farmers would have wasted money on fertilizer they didn’t need. And excess phosphorus in soil can …
