The Benefits Of Recycled Perlite

December 13, 2005 - 11:32

Perlite is volcanic lava widely used as growing media for many horticultural crops, including greenhouse tomatoes. The original rock found in nature is crushed to small pieces and heated to approximately 2,000° F to force inside moisture to evaporate and expand the granules like popcorn. This lightweight media is inert, pathogen free and easy to handle; it will not decompose.

Most greenhouse tomato operations in the Southern United States are small. Growers in the region have to reduce production costs and increase plant yield or price to compete in a market dominated by more efficient, large corporations. Yield reduction of greenhouse tomatoes planted in the same media year after year without processing is a serious problem. Contaminated perlite can impair the root system and lead to a weak and less productive plant. Most of the large greenhouse tomato operations replace the root media once a year to avoid yield reduction or crop failure. Reusing perlite without processing is potentially risky because of media compaction, salt buildup and pest contamination, but replacing used perlite with new media for each new tomato crop is costly. Recovering the expense by selling the fruit at a higher price may not work well in a competitive market.

Steam sterilizing used perlite before planting a new crop has been recommended to safeguard against pathogen contamination; however, this treatment requires expensive steam generators and may not be adequate to restore perlite’s loose structure and reduce media salt.

A Method For Reuse

We developed a practical procedure to process used perlite for repeated use in greenhouse operations with limited space. The procedure can be applied as needed after terminating the previous tomato crop and before planting a new one. The economics of this procedure and its effect on media salt and tomato yield were evaluated in six consecutive tests conducted at the Louisiana State University Agricultural Center, Bossier City, La.

Studies were conducted in a 30x96-foot double-polyethylene greenhouse typically used to raise 640 tomato plants in 320 polyethylene bags. In every study, tomato plants were grown in 5-gal. poly bags filled with either new or recycled perlite. To recycle perlite, all vegetation of the previous tomato crop was removed from the greenhouse, and the media was sifted, cleaned and disinfected using the procedure developed. To minimize labor input, a simple, movable hopper was designed that can be built by growers. The hopper was equipped with a wire mesh screen for sifting used perlite to restore its loose structure and isolate most of the tomato roots. It was built on wheels and can be rolled between tomato rows in the greenhouse.
Used perlite containing tomato roots was poured out of the poly bags onto the hopper mesh screen and shaken well. Á The loose perlite was collected in same size poly bags at the bottom, and the remaining roots were separated on the screen-top and discarded. The perlite was then treated with hot water at temperatures reaching 200° F to leach out excess salt and disinfect the media. A hot water pressure washer was used after replacing the pressure nozzle with a water breaker. Each bag was drenched with approximately 31?2 gal. of hot water.

An inverted gutter system equipped with side channels to collect runoff is now popular in greenhouse operations to reduce pollution. The same system can be used to collect and reuse the already disinfected runoff resulting from treating used perlite with hot water.

Time for sifting and hot water treatment of used perlite in 320 poly bags was recorded and labor costs were calculated. New perlite sufficient to fill 320 poly bags was purchased, and labor wages to fill the bags with the new perlite were also calculated. The time to remove 320 poly bags filled with used perlite from the greenhouse and disposal was estimated.

Benefits Of Reuse

Results of these tests indicated that sifting and removing old tomato roots and re-filling 320 poly bags with the sifted media required 24.9 person-hours. Cleaning and disinfecting the sifted media with hot water required 6.2 person-hours and 1,120 gal. of municipal water priced at $1.41 per 1,000 gal. The hot water pressure washer rent and fuel consumption cost was $75. The sifter cost prorated over five years was $40 per year. Assuming wages were $7 per hour, it would cost $334.35 to process used perlite for recycling the example greenhouse (see Figure 1, right). The cost to replace the used media in 320 poly bags, on the other hand, was $540 to buy the new perlite, $56 to dispose of the used perlite and $156.80 to fill the same old poly bags with new media, for a total of $752.80.

Processing used perlite had a significant effect on reducing media salt. In fact, perlite EC, nitrate nitrogen and potassium were reduced by 43, 50 and 47 percent, respectively. Media pH increased after the hot-water treatment because the tap water had a higher pH than the perlite. Tomatoes planted in the recycled perlite produced more early fruit and had higher total marketable yields and heavier fruit than tomatoes planted in the new perlite (see Figure 1, right).

Results of this study indicate that replacing perlite at the onset of every tomato crop is expensive, and processing perlite for recycling can reduce the replacement cost by 56 percent. Most of the processing expense is labor related, and smaller producers are more than willing to do the job themselves without considering it an added expense. The rent of a hot water pressure washer, fuel consumption and water cost could be the only “real” out-of-pocket expenses to do the job.

Long-Term Effects

Previous research has indicated that tomato plants grown at an EC of 6.0 produced 64 percent less yield than those grown at an EC of 1. It was documented that the optimum EC for raising container-grown tomato plants was 2.0-3.5, and the optimum nitrate nitrogen and potassium contents were 100-199 and 150-249 mg per liter, respectively. Results of this investigation have indicated that media EC, nitrate nitrogen and potassium were higher before processing but dropped to within the optimum levels after processing.

In our study, perlite temperature measured at a depth of 8 inches in the poly bags with a long stem Traceable Thermometer reached 160° F and 185° F after the hot-water treatment in the spring and fall season, respectively. These temperatures exceeded the 122° F temperature defined by some researchers as lethal for several fungi, and the 113° F temperature defined as lethal for nematodes.

In conclusion, cleaning and disinfecting used perlite for recycling can save the smaller grower a significant amount of money and be repeated as needed for many years. Additionally, it does not reduce tomato yield. The collective effect of salt reduction, media disinfection and the presence of an optimum level of nutrients may have improved plant yield in the recycled perlite. We have recycled perlite twice a year in our commercial greenhouses using this sift and disinfect technique since 1997 without any undesirable effect on tomato production.
Efficient management of limited-space greenhouse tomato operations by reducing production cost and improving plant yield should help smaller producers survive a very competitive market.

About The Author

H.Y. Hanna is professor of horticulture at the Louisiana State University Agricultural Center, Bossier City, La. He can be reached by E-mail at hhanna@agcenter.lsu.edu.

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