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To prevent fertilizer from escaping cranberry bogs, just add salt

Aluminum sulfate traps phosphorus in sediment so it doesn’t drain into lakes and ponds

by Emma Hiolski
December 5, 2017

Cranberries corralled for harvest float on the surface of a flooded field.
Credit: Casey Kennedy
Farmers harvest cranberries by flooding fields, but the water can carry excess phosphorus downstream into lakes and ponds.

Cranberries often fade from memory in the aftermath of Thanksgiving feasts. Scientists with the U.S. Department of Agriculture and the University of Massachusetts Cranberry Station, however, focus on the scarlet berries year-round: In particular, these researchers want to prevent phosphorus applied to cranberry bogs as fertilizer from draining into nearby waterways.

Algae feast on the essential nutrient when it runs off from farm fields into lakes and ponds, causing nuisance or harmful growth. Massachusetts accounts for a quarter of U.S. cranberry production, and growers there face heavy regulations designed to reduce the downstream impacts of phosphorus.

As part of their effort to help farmers meet regulatory requirements, the researchers at USDA and U Mass are reporting that aluminum sulfate treatment can trap phosphorus in the sediment of irrigation ponds and cranberry bogs, preventing it from draining away (J. Environ. Qual. 2017, DOI: 10.2134/jeq2017.04.0134).

On cranberry farms, most phosphorus is applied to bogs as fertilizer or is found naturally in irrigation pond water. At harvest time, farmers flood their bogs with water from those ponds to facilitate easy collection of the buoyant berries. Any excess phosphorus drains with the water out of the bogs after harvest.

A cranberry farmer operates a small, flat barge on a pond while another man dumps a sack of aluminum sulfate into the water through a specialized port.
Credit: Casey Kennedy
Kennedy dumps aluminum sulfate into a pond to test whether it would bind excess phosphorus and remove it from the water.

The researchers, led by Casey D. Kennedy of USDA’s Agricultural Research Service, tested the ability of various salts to remove phosphorus from pond water. In water, phosphorus is primarily bound to sediment particles; certain chemical salts can neutralize negative charges, so when treated with those salts, sediment particles clump together, settle out of the water, and become unavailable to plants and algae.

Lab experiments showed that aluminum sulfate, compared with iron and calcium salts, bound phosphorus in water most effectively at low concentrations (5-15 mg/L). Cranberry plants have also been shown to be tolerant of high-aluminum soils.

The team then tested the feasibility of using aluminum sulfate on cranberry farms. The scientists rented a modified barge and dispersed 15 mg/L aluminum sulfate into two ponds: about 450 kg into an irrigation pond and about 400 kg into a former cranberry bog.

Aluminum sulfate treatment worked best in the shallow waters of the former cranberry bog, removing 94% of the phosphorus from the water compared with 78% from the irrigation pond. Aluminum concentrations in the shallow pond also did not exceed U.S. Environmental Protection Agency acute toxicity limits.

Hydrologist Brian L. Howes of the University of Massachusetts, Dartmouth, is familiar with the issues faced by cranberry producers and applauds the work. “It provides a positive new approach by reducing phosphorus release from cranberry bogs to levels near those of natural freshwater bogs and wetlands,” he says.

Kennedy says the team hopes to develop the method for application to post-harvest cranberry bogs, though he points out that removing phosphorus from the water will not be a long-term solution. For that, he says farmers will need a variety of strategies, including reduced fertilizer use. “There’s not going to be a silver bullet.”



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