UF Researchers: Tree, Cotton Combination Reduces Pollution

August 11, 2004

GAINESVILLE, Fla. — By growing crops and trees side by side, farmers can dramatically reduce groundwater pollution caused by fertilizers, a University of Florida study shows.

Researchers at UF’s Institute of Food and Agricultural Sciences grew cotton in a grove of pecan trees, cutting the amount of nitrate that seeped from the cotton field to the groundwater by slightly more than two thirds.

Widely practiced in the developing world, agroforestry – the practice of growing crops and trees together on the same plot of land – is just beginning to catch on in the United States, where it promises to soften the environmental impact of modern agriculture.

“We know that agroforestry has helped farmers in other countries fight environmental problems such as erosion,” said Shibu Jose, an associate professor of forestry at UF whose findings were published in the May issue of the journal of Forest Ecology and Management. “Now we have data to show that agroforestry can reduce nitrate leaching, which is one of the biggest environmental problems associated with American agriculture.”

Nitrate is a nutrient found in most fertilizers used on crops, gardens and lawns, but in many cases plants often don’t take up all of the chemical, leaving the rest to seep into groundwater or wash away into streams and ponds.

High nitrate levels in groundwater have been linked to “blue baby” syndrome, a potentially fatal illness found in infants in communities dependent on well water. Nitrate-contaminated groundwater also can emerge in springs, where the chemical feeds blooms of algae and bacteria that can choke out other forms of aquatic life.

Proponents of agroforestry have long believed the deep roots of trees could act as a “safety net,” catching nitrate that isn’t taken up by crop plants. But because agroforestry is typically practiced in places where crop fertilizer is rarely used, little research had been done to prove the theory, the scientists said.

The researchers decided to test the effect on two crops traditionally grown in Florida’s Panhandle, planting an acre of cotton in a 43-year-old orchard of pecan trees at UF’s West Florida Research and Education Center Farm near Jay, about 30 miles north of Pensacola. In half of the orchard, the researchers installed underground plastic barriers to keep the roots of the pecan trees from mingling with the roots of the cotton. In the other half, the roots of both plants were allowed to mingle.

Water samples from 120 sites in the orchard were collected from June 2001 through August 2002. While some nitrate seeped below the roots of the plants throughout the orchard, the researchers found far less nitrate in the soil under areas where the roots were allowed to mingle – in some cases, 70 percent less than was found under areas with the plastic barriers.

U.S. farmers have traditionally avoided planting trees and crops together precisely because of the hungry nature of tree roots. Because trees compete with smaller plants for nutrients, water and light, some theorize that crop plants grown under tree cover are less productive than those grown in the open.

But when it comes to competition, the UF study shows not all plants are created equal. With its relatively deep root system, the pecan tree can get water without drying up other plants, the researchers say. And because pecans in an orchard are usually planted far apart, other plants are able to get enough light.

“Success in agroforestry is all about your choice and timing of crops,” said Sam Allen, a postdoctoral researcher at UF and an author of the study. “Right now we’re working to find what crops work best in agroforestry in this climate and determining the best methods for growing them.”

Though agroforestry is attractive primarily for its potential environmental benefits, proponents of the technique expect it to catch on for another reason – money. A new grove of pecans must mature for years before it produces a marketable crop, and the same is true of most other fruit-bearing trees. Landowners who plant timber can wait decades before getting a return on their investment. Agroforestry can help those landowners generate a profit in the short term, advocates say.

“Agroforestry methods are practiced only by a small number of farmers in the U.S., but when farmers understand the benefits of this approach, I believe we will see much more interest,” said P.K. Nair, a distinguished professor in UF’s School of Forest Resources and Conservation and director of the university’s Center for Subtropical Agroforestry.

No one is yet growing cotton and pecans together in commercial production, the researchers say. But some farmers, seeking to reduce their environmental impact, have begun growing other crops under tree cover.

One such farmer is Jerry Conrad, a nursery owner who began raising camellias on 9 acres of semi-forested land in the early 1980s. Conrad said the trees on his property keep his camellias cool in the summer, protect them from wind damage and attract birds that eat plant-damaging pests.

“A 1000-acre cornfield is abhorrent to nature, because in nature no one plant grows in isolation,” he said. “When you grow a single crop on such a huge scale, you’re inviting disease and pests and plenty of other problems. But if you work with the diversity of nature, these problems will often take care of themselves.”