Hanna Y. Hanna and Douglas T. Smith

Perlite is a processed volcanic mineral widely used as a propagating and growing medium for many horticultural crops, including tomatoes. The initial rock found in nature is crushed to small pieces and heated to 2000 degrees F. The trace of moisture trapped inside the small pieces evaporates and puffs the granules just like popcorn. Perlite is a lightweight mineral, easy to handle, pathogen-free and will never decompose. Growers have to replace it every year or two to minimize the risk of crop failure because of salt and pathogen buildup. The expense to dispose of the old material and replace it with new perlite shipped from distant markets can be significant.

LSU AgCenter researchers have developed a procedure to clean, sterilize and recycle perlite. Following the tomato harvest and removal of all vegetation from the greenhouse, perlite is sifted to remove tomato roots and restore its original loose structure. For better handling of the medium and to minimize labor input, researchers built a simple movable hopper (Figure 1) equipped with a wire mesh screen for sifting the perlite to isolate and remove tomato roots. The hopper is built on wheels and can be rolled between the tomato rows in the greenhouse.

Perlite mixed with old tomato roots is poured out of the grow bag onto the hopper screen and shaken well. The loose, root-free perlite is collected in a receiving bag at the bottom. It is then treated with hot water (Figure 2) at temperatures reaching 200 degrees F using a hot water pressure washer to leach out excess salt and kill pathogens and insects. Each bag is drenched with one to two gallons of hot water in a couple of minutes. Measuring temperature in a random sample of the hot water treated bags revealed that perlite temperature ranged from 170 degrees F to190 degrees F and remained hot for several hours after treatment. This procedure can be repeated once a year after harvest.

New versus recycled

Researchers tested recycled and new perlite media for growing greenhouse tomatoes over the last three years and found no significant differences in tomato yield. The cost to sift and disinfect the perlite in a 30-by-96 foot greenhouse was estimated at about $250. That figure is based on approximately 30 hours to do the job at $5 to $6 per hour plus the hot water pressure washer rental at around $60. Replacing the old perlite once a year to avoid crop failure would require the same amount of labor to dispose of the old medium and re-bag the new perlite at a purchase price of about $515 for the same size greenhouse.

Recycled perlite benefits

Fungus gnats, a major problem in the greenhouses at the Red River Research Station, disappeared completely after the regular use of hot water. We believe that hot water treatment reduced thrips, white flies, spider mites and a disease that strikes greenhouse tomatoes called botrytis. Pest control with hot water can lead to less pesticide use, lower production costs and a product more desirable for some consumers looking for pesticide-free products.

Another benefit is not having to use household bleach as a disinfectant for the growing medium. Chlorine bleach can cause eye and skin irritation. Also, it can damage tomato plants severely if not leached out completely from the growing medium.

Traditionally, tomato seedlings are raised in growing trays for five weeks before transplanting into the permanent grow bags. Because the hot water-treated medium is pathogen free, and salts are leached out, it is possible to skip this step and plant the new germinated seedlings (10 days old) directly into the permanent grow bags. This practice can lead to healthier and more productive tomato plants as long as the perlite temperature remains between 60 degrees F and 80 degrees F to minimize seedling loss.
 
Hanna Y. Hanna, Professor, and Douglas T. Smith, Maintenance Repair Master, Red River Research Station, Bossier City, La.

(This article was published in the summer 2002 issue of Louisiana Agriculture.)