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Researchers from University of Florida found that nitrogen and phosphorus from commercially-available inorganic fertilizers enter surface water more easily than those same nutrients from biosolids.

What are biosolids?

Every year, Floridians produce approximately 340,000 dry tons of domestic wastewater biosolids or the solid byproduct that accumulates in the wastewater treatment plant. A significant portion of the state’s biosolids are used to fertilize beef cattle pastures. The remainder is landfilled.

How we know this.

The research team set up boxes containing approximately 20 pounds of soil and transplanted bahiagrass, a typical livestock grazing grass. Two types of fertilizers were applied to the grass – either biosolids or commercially available inorganic fertilizers. The research team then mimicked a one-hour rainfall based on a 25-year average in central Florida. Each rainfall ended after 30 minutes of runoff was generated for each box.

Results found that 38% of applied phosphorus and 46% of nitrogen from the inorganic fertilizer were lost and therefore released into the runoff. That compared with losses of approximately 3% of phosphorus and 6% of nitrogen for biosolids-amended soils.

Why this matters.

Too much nitrogen and phosphorus in our waterways can lead to harmful algal blooms, like the blue-green algae blooms we observe in Lake Okeechobee. When fertilizers are applied to agriculture fields prior to a heavy rain, some of the nutrients from the fertilizer get washed into our waterways. This is known as nutrient runoff. Because Florida is a rainy state with a thriving agriculture industry, nutrient runoff is inevitable.

These findings show that biosolids release fewer nutrients after a big rainfall event compared to commercially-available inorganic fertilizers.

The researchers acknowledge that if not applied properly, biosolids do pose a risk to soil, air and water quality. Last year, in response to growing opposition to biosolids application, the DEP formed the Biosolids Technical Advisory Committee to evaluate the current management practices and regulations that govern the application of biosolids.

What’s next?

More research in the field with biosolids and commercial fertilizer is needed to better understand how nitrogen and phosphorus behave in sandy soils.

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