Perlite and Hydroponics: Possible Substitute for MeBr?

Worldwide, methyl bromide is used primarily as a soil fumigant to eliminate soilborne pathogens and to control weeds. Since methyl bromide will no longer be available to U.S. growers after 2005, scientists are persistently seeking alternatives to this chemical that has been so widely used for decades.

George J. Hochmuth has been researching the idea of growing crops hydroponically in a soil-less mixture, eliminating the need for methyl bromide. Collaborators include Tim Crocker—who along with Hochmuth, is an extension specialist with the University of Florida's Horticultural Sciences Department—and Bob Hochmuth, with the university's Suwannee Valley Research and Education Center.

"On research plots at the University of Florida, we've been growing muskmelons in walk-in and low tunnels using perlite in a soil-less culture system," George Hochmuth reports. "Our data show that this is a possible alternative to fumigating the soil with methyl bromide. We've also successfully grown strawberries in an outdoor hydroponic system using perlite bags."

Each year, about $4 billion worth of horticultural crops are produced worldwide with soil-less cultures. According to Hochmuth, although these technologies are widely used around the world, they aren't used as extensively in the United States. This is primarily because U.S. land available for agriculture is becoming more and more expensive. In 1988, U.S. growers produced about $32 million worth of crops with soil-less mixtures, while in 1992, growers in Holland alone accounted for about $1.6 billion. Today, the estimated value of Florida's greenhouse vegetable industry is $20 million. However, with the impending ban on methyl bromide, U.S. growers need viable options to stay in the business of producing the nation's winter supply of fresh fruits and vegetables.

"The impending loss of methyl bromide will almost certainly lead to reduced yields under soil-based fruit and vegetable production in environments such as we have in Florida where soil pathogens, if left uncontrolled, can severely affect crop growth," Hochmuth says. "The good news is that in our research experiments, we got higher yields in tunnels planted with perlite than in those planted with soil."

Perlite is a unique volcanic mineral that has been used for years to amend professional potting soils made from peat moss. It retains and holds substantial amounts of water, which can be released as needed. The research was collaborative with the Schundler Company, Metuchen, New Jersey—a member of the Perlite Institute, New York City, which funded the research along with Airlite Products, Vero Beach, Florida.

Strawberries

Produced on 6,000 acres and valued at over $100 million each year, strawberries are important to Florida growers. Essentially all Florida strawberries are grown using plastic mulch culture in fields, with little chance for crop rotation. Each year, the soil must be fumigated with methyl bromide to control diseases, weeds, and nematodes before new mulch is applied.

"We placed perlite and a peat mix in layflat bags about 3 feet long and 10 inches in diameter. We placed the bags end to end on a level area of soil covered with black plastic," Hochmuth says. After three fertilizer treatments, Hochmuth and colleagues planted six plug plants per bag and managed disease, insects, and mites by integrated pest management. On a per-acre basis, perlite bags allowed twice the numbers of plants that could be accommodated by typical field culture with plastic mulch.

"We harvested an average of just under a pound per plant, not significantly different from the yield achieved with methyl bromide fumigation in soil," he reports. "Considering that on a per-plant basis we achieved the same yields, we actually produced twice the yield of the field system since we had twice the number of plants, per acre, in the perlite bags." However, Hochmuth says that further refinement of the amount and timing of the controlled-release fertilizer applications is needed before large-scale adoption of this technique.

Muskmelons

Using perlite, Hochmuth and colleagues planted Galia muskmelons in walk-in tunnels and low tunnels during the winter/spring and fall/winter growing seasons in 1997 and in the winter/spring season in 1998. Colleagues include Eric A. Waldo, Daniel J. Cantliffe, and Steven A. Sargent, all with the University of Florida at Gainesville.

Walk-in tunnels are quonset-style structures covered with a single layer of polyethylene film, heated and cooled passively without powered equipment; and low tunnels are simply row covers.

"In half of the rows under both types of tunnels, we used perlite for our growth medium. For the other half of the plants, we used raised beds of soil, polyethylene mulch, and drip irrigation," Hochmuth explains. They also placed thermal tubes in half of the walk-in tunnels. These tubes, which are about 12 inches in diameter and hold water, act as solar collectors during the day and release the energy as heat during the night.

According to Hochmuth, the tunnels and thermal tubes protect crops from cold temperatures, which can extend the growing season. "This gives the grower the advantage of premium, off-season prices."

Soil-less Culture

Using soil-less mixtures such as perlite, Hochmuth says, eliminates the need for methyl bromide: since there is no soil, there are no soilborne pathogens or weeds. "In addition, as issues such as land availability and water use become more important, soil-less culture may prove to be an acceptable alternative to traditional soil-based crop production," he says.

Hydroponics offers many benefits. For example, nutrient runoff from growing crops can be captured and reused by growers as fertilizer for pastures or other crops. This can prevent the problem of excess fertilizer leaching into groundwater from the soil.

And as land prices increase, growers need to optimize use of their land. Soil-less mixtures maximize crop yield per acre.

With soil-less mixtures, plants get water and nutrients through a nutrient solution and are physically supported by a soil substitute such as rockwool or perlite.

Most soil-less mixtures have good aeration and drainage capacities. "Perlite is also sterile, has a neutral pH, and is readily available, nontoxic, safe to use, and relatively inexpensive," says Bruce Schundler, president of the Schundler Company. "Also, it can be used in the greenhouse or in outdoor plantings. It's ideal for water conservation and expands from four to 20 times its original volume when heated quickly. Plants grown in perlite take up water as needed; they don't suffer from too much or too little water."

This is because the surface of a particle of perlite contains tiny cavities that hold moisture and nutrients which are available to plant roots. The particle's shape lends itself to numerous air passages, which provide optimum aeration and drainage. Since it is sterile, perlite is free of diseases, seeds, and insects.

"Perlite's ability to cling to plant roots and root hairs helps reduce transplant shock and production time. It is being used for propagation and seed cultivation, plug production and transplants, interiorscape and planter growing, composting, hydroponic cultures, turf and lawns, and placement around shrubs, trees, and landscaping," Schundler reports.

In addition to aiding drainage, perlite eliminates soil crusting in heavy clay soils. Also, plant roots more easily penetrate the perlite growing media.

Tomato plants grown hydroponically in perlite have produced average yields 7 percent higher than tomatoes grown in other soil-less mediums. In Holland and, to a lesser degree in the United States, commercial cut flowers, strawberries, and orchids are being grown in 100 percent hydroponic perlite, Schundler says.

"A grower who wants to use perlite to grow crops should follow a few basic guidelines," Schundler iterates. "The first criterion is to carefully plan the nutrient solution system and the second is to frequently monitor irrigation, temperature, and nutrient levels throughout the season, then adjust the systems as necessary."

Growers should develop backup systems and procedures to ensure an adequate water supply to administer the nutrient solution system. "Careful consideration should be given in selecting the optimum nutrient program appropriate for the particular crop being grown" Schundler says.

Walk-in Tunnels

What exactly is a walk-in tunnel? A portable, walk-in, greenhouse-like structure without a permanent electrically powered heating or ventilation system, covered with one layer of plastic, and sited on field soil, says Hochmuth. "These tunnels can't protect crops from temperatures to the same degree that heat-equipped greenhouses can. But, they cost a lot less to build and operate, and they're effective.

"Walk-in tunnels have been used extensively throughout the Middle East, Asia, and southern Europe to grow vegetables. And some tomato growers in the northeastern United States have used this practice since about 1992.

"Laying clear polyethylene tubes filled with water along crop rows has effectively moderated cool temperatures and led to increased early yields of peppers, compared to black polyethylene tubing," Hochmuth reports. "We add chlorine bleach to keep algae from growing in the water."

To deter insects, Hochmuth's team used an insect screen that covered the entire side of the tunnel from the ground to about 70 inches high and was buried in the ground to provide additional anchoring for the tunnel structure.

Understanding the dynamics of humidity and the temperature of air and growing media in the tunnels is of utmost importance to the success of the project, Hochmuth says. The effect of outside temperatures on inside temperature and humidity levels is also important.

"We had to reduce high humidity levels in the tunnels because fungal and bacterial diseases as well as some insect pests thrive in this environment. We simply lowered the sides of the tunnels when night temperatures were predicted to be below 50°F and raised the sides on cloudy days when outside temperatures were above 65°F. On sunny days, we raised the sides when outside temperatures were above 50°F. For our row-cover tunnel crops, we'd apply floating row covers over crops when outside temperatures were expected to drop to around 36°F."

This regime was used for muskmelons; strawberries will thrive under much cooler conditions, he says.

"Based on our research results with winter production of Galia muskmelons and with strawberries, we feel that using protective structures and soil-less mixtures is a viable option for growers faced with the loss of methyl bromide," Hochmuth says. "At the University of Florida, we're using this system as well as soil-based cultivation systems to grow a variety of crops without methyl bromide."

Last Updated: April 9, 1999