Hanna Y. Hanna
Most greenhouse tomato operations in the southern United States are small. These growers have to hold down production costs to compete well in a market dominated by more efficient, larger operations. One way they do this is to reuse the root medium (perlite) for growing the greenhouse tomato plants. However, reusing this medium year after year can lead to serious yield reduction problems. Contaminated perlite can impair the root system and cause weak and less productive plants. Most large greenhouse tomato operations replace the root media once a year, but the smaller grower cannot afford to do this.
Perlite is volcanic lava widely used as a growing medium for many horticultural crops, including greenhouse tomatoes. Before reusing it, it must be cleaned and disinfected to avoid compaction, salt buildup and pest contamination.
The procedure used with this study is a simplified technique involving less labor – time and energy – than previous techniques. It involves using a 24-inch auger mounted on a half-inch electrical drill to bore into the perlite in 5-gallon media bags without moving the bags. This process is necessary to break the root and perlite clumps, restore a loose structure and aid in washing away some of the built-up salts.
To prevent the perlite from dispersing out of the bag, we used a 5-gallon bottomless bucket placed over the media in the bag, and the drill was operated inside the bucket. The perlite was then cleaned and disinfected with a hot-water pressure washer available locally at rental shops. Each bag was drenched with approximately 3.5 gallons of near-boiling hot water (average temperature 194 degrees F) after replacing the pressure nozzle with a spray nozzle.
Information generated from drilling and disinfecting used perlite in 1,880 5-gallon bags indicated that this technique would cost the grower approximately $162 for 5.9 hours for drilling and 6.52 hours for cleaning and disinfecting the media in a 30-foot by 96-foot greenhouse. Replacing the media would cost the grower $753 - $540 to buy new perlite, $56 to dispose of the used perlite and $157 to fill the bags with the new perlite. These data indicate that the described technique can save the grower approximately 78 percent of the annual replacement cost.
Treating perlite with hot water raised media temperature to above limits necessary to kill harmful pests and reduced media salt to within acceptable range for raising container-grown plants.
Tomato plants raised in the recycled perlite produced yields comparable to those raised in new perlite. Used perlite can be cleaned and disinfected repeatedly because it is not organic and is physically and chemically stable.
Most processing expense is labor-related, and smaller producers can save money by doing the job themselves. Renting a hot water pressure washer and buying an auger and a drill can be the only out-of-pocket expenses to complete the job. Many growers have electric drills, and the $20 auger is a popular tool to plant bulbs in flower beds.
Hanna Y. Hanna, Professor, Red River Research Station, Bossier City, La.
(This article appeared in the winter 2005 issue of Louisiana Agriculture.)