Large Scale Recycling of Used Potting Media with Solarization

Large Scale Recycling of Used Potting Media with Solarization

Summary

Successful methods were achieved to recycle used potting soil using a mid and large-scale solarization approach. The process is inexpensive and uses minimal labor. Temperatures reached 159 F under two layers of 4 mil plastic in full sun. These temperatures were hot enough to kill nematodes, most plant pathogens and a majority of weed seeds. Soil physical properties were not substantially altered during the solarization process. The recycled soil is now being tested at different rates with new potting soil to determine growth feasibility for woody plant production.  

Objectives/Performance Targets

The objective of this research was to return used potting media, which is currently a waste product, back into the production cycle. This would reabsorb some of the production loss of growing container plants and reduce reliance on purchasing new potting media thereby reducing costs and waste. This process would need to be cost efficient and effective in order to be adopted by many growers. Solarization was the method chosen to accomplish this. Our first year of the research grant, we met our performance targets by conducting two on-farm trials, a mid-scale (one yd³) and a large-scale (3.5 yd³) solarization study. Potting media quality was determined after solarization by conducting chemical and physical analysis of pre and post-treated soil. Weed germination counts and nematodes analysis were also conducted pre and post-solarization. At the end of this first year, we have now moved into our growth study. This study will look at the performance of the solarized treated soil being used back into the production cycle versus using new potting soil.

• Picture of Mid and Large-Scale Solarization Plots

Accomplishments/Milestones

Based on our study, the solarization process has been a successful and inexpensive method of treating large batches of used potting soil to be recycled back into production. After conducting our first mid-scale solarization trial, we learned the best configuration of arranging the layers of plastic to build the greatest heat gain in the plot. We were then able to move to a large scale (3.5 yd³) configuration and reached temperatures of 140 F within a day. Soil assays have found that the solarization is effectively killing nematodes. We also see a reduction of germinating weed seeds averaging 91% compared to our untreated soil. The physical attributes of the solarized soil has not been affected through the process. Estimations of the cost of labor in our large-scale solarization experiment to recycle this waste is about $5.00/yd³. Costs of the materials, which can be reused multiple times, are about $233.00. 

• Weed seed germination in used potting soil untreated (left) versus solarized (right) at 14 days.

Impacts and Contributions/Outcomes

During the warm season between May and September there are large amounts of solar radiation available to do work. When coupled with a very simple arrangement of clear plastic layers and a few days time, used potting soil can be treated and reabsorbed back into production. In short, a recycled savings of about $30.33 per yd³ of soil or about $108.00 in our configuration, per large scale solarization run can be achieved. The material costs of setting up an area to solarize would be paid back in the third use. At this stage, the on-farm research is showing that with minimal labor and materials, soil can be treated to kill nematodes and most weed seeds from what was previously a waste pile. This will enable growers to reuse their waste potting soil to recoup profits. This method also achieves environmental benefits by reducing the amount of new potting soil containing pine bark and peat potentially needed by the industry. Grower estimates are that 10% of potting soil is dumped in the production of plants. This research could potentially have a considerable impact on production if the plant growth study shows minimal negative effects on plant health.

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