Large Scale Recycling of Used Potting Media with Solarization

Final Report for OS13-075

Project Type: On-Farm Research
Funds awarded in 2013: $3,161.00
Projected End Date: 12/31/2015
Region: Southern
State: Florida
Principal Investigator:
Shawn Steed
UF/IFAS Extension
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Project Information


A large-scale method of recycling used potting soil using solarization for the nursery industry was investigated and proves to be successful.  By using two sheets of four mil plastic, temperatures reached 159 ºF and sufficiently produced enough heat to sterlizie 3.56 yd3 of used potting soil.  Weeds were reduced about 90%.  Recycled soil cost under $5.00 per yd3 in labor to recycle with material costs being paid back in a little over two turns.  There were no growth differences when used at up to 66% of soil mix for growing two species of ornamental plants. 


All ornamental plant nurseries produce plants that do not get sold.  This may be due to a poor quality issue, pest issue, or a lack of sales.  These problem plants will need to be disposed of either on, or in some cases, off site.  In personal conversation with growers, about 10% of potting soil is wasted each year.  Potting media for the production of woody ornamentals in the southern US is currently some combined mix of compost, sand, bark, and wood products.  Typical costs run about $35 per cubic yard of new potting media plus delivery fees.  Approximately 80 three gallon pots can be filled with a cubic yard of potting media.  Some nurseries use locally sourced renewable wastes like coconut coir, peanut hulls, rice hulls, chipped pine trees, etc. Currently, old container potting media is usually dumped in piles in a back fringe of the nursery, leaching remaining fertilizer into the surrounding areas.  The principle investigator has seen some dump piles with hundreds of cubic yards of old media.  Some nurseries have found ways to deal with this issue by incorporating the old media with a percentage of new mix.  The majority of nursery producers do not reuse their old potting media.  This is from fear of contamination of new media with pathogens, nematode infestations and weed seeds that will reduce quality and add costs to production.  Soil physical properties may change with the breakdown of bark and decomposition of peat and may also adversely affect new potting media.  In recent years there have been issues with availability of peat and pine bark.  Sustainability of peat is a debatable question and composted pine bark availability has fluctuated due to demand in energy producing feedstocks. 

Old potting media represents a major opportunity in the overall sustainability of the nursery industry.  This resource that is currently a problematic waste might possibly be reused as a component of container media for freshly potted plants if a low cost, large scale, easily managed solution could be developed with potential drawbacks (ie. weeds, nematodes, pathogens) analyzed and accounted for.

Statement of Proposed Solution

This on-farm research grant proposed to find a large scale, cost-effective solution to the problem of huge piles of wasted container media at ornamental nurseries. This grant will enable nursery growers in the Southeast to evaluate the cost effectiveness of replacing new potting media with recycled media.  Through Extension outreach, this grant will provide growers with a simple structure design and methodology to encourage old media resuse. 

Research has shown the effectiveness of soil solarization of previously used container media (Zinati et al. 2002).   Gamliel et al. (1989) found that high sterilizing temperatures could be reached to a depth of 15 cm and improved growth of greenhouse tomatoes compared to fresh media.  If container media is held at temperatures of 158 ºF or higher for 30 minutes or 140 ºF or higher for one hour, solarization can completely eliminate pests (Stapleton et al. 2008).  It was also shown that soil temperatures in potting soil could be reached in as little as a day when media was incorporated in small bags.

This research has been done on small-scale with satisfactory results. Wide-scale adoption of these practices has not been implemented in the container grown, woody ornamental nursery industry.  For the most part, in the southern United States there is ample, free solar radiation which can provide an energy source for sterilizing soil with solarization.   This proposal sought to develop a method that would reduce reliance on amounts of new container media needed as nursery inputs, recycle on site waste materials in a cost efficient manner, and reduce total costs involved with producing plants, leading to an increase in the sustainability of the entire production system.

Gamliel, A., Katan, J., Chen, Y. and Grinstein, A. 1989.  Solarization for the recycling of container media. Acta Hort. (ISHS) 255:181-188.

Stapleton, J.J., C.A. Wilen, and R.H. Molinar. 2008.  Pest Notes: Soil solarization for gardens & landscapes. UC ANR Publication 74145. UC Statewide IPM Program, University of California, Davis, CA.

Zinati, G.M., H.H. Bryan, and M.M. Codallo. 2002. Solarization as a potential approach for recycling wastes for potting media and as an alternative to methyl bromide for field-grown bedding plants. Proc. Fla. State Hort. Soc. 115:123-127.

Project Objectives:

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.  An economic analysis was conducted to determine the efficacy and payback of using solarization to treat used potting soil.

The second year’s objective were accomplished by conducting a growth study using recycled solarized potting soil to grow out two species of woody ornamental plants and then communicating those results at various conferences and a workshop.   


Click linked name(s) to expand
  • John Pearson


Materials and methods:

Two methods or approaches were made to research potential solutions to this problem using a midscale and large-scale approach at two locations.

We conducted all solarization trials on-farm at Stardust Farms in Lutz, Florida (28°08’16.4″N 82°28’49.2″W). 

Mid-Scale Solarization-Commercial black polyproplene groundcover was used as the base of the solarization pad.  This was laid over soil previously treated with glyphosate to eliminate weeds.  A layer of four mil polyethylene clear plastic was placed over the ground cover to prevent contamination from untreated soil below.  A frame box was constructed with 2 inch x 4 inch boards measuring 12 ft x 7.5 ft x 0.33 ft to yield a treatment area of one yd3.  Untreated used potting soil (approximately 70% pinebark, 25% peat, and 5% sand) was then filled to the top of the wood frame and smoothed with a rake.  Large plant debris and weeds were removed by hand.  The soil was irrigated by hand to visual wetness.  A layer of four mil clear plastic was laid over the top of the frame and tucked under the frame edges.  Another layer of four mil clear plastic was suspended over the entire frame with 1.5 inch PVC pipes approximately 2 inches above the soil surface and sealed with wooden posts laid horizontally upon the ground outside the frame.  A HOBO Micro Station (S-TMB-M006) logged temperatures with three Smart Temp Sensor 12-bit with six meter cables.  Sensors were places at the top just under the surface, two inches deep, and at the bottom of the area, at a four inch depth randomly located within the treatment area.   The solarization started on 24 June, 2013.  On 26 June, 2013 the outermost plastic was raised to approximately one foot to create a more tent like configuration.  On June 28, 2013 the soil moisture level was increased to field capacity and outermost plastic layer configuration was changed to facilitate runoff of rainwater.  On 8 July, 2013 the outermost layer of plastic was removed.   The solarization ended on 15 July, 2013.  Soil was collected from random areas in the treated plot and placed in seedling trays measuring 1 ft x 1.5 ft x 2.5 inches.  Three trays were filled with 1 inch of solarized soil. Three trays were filled with 1 inch of untreated soil directly from the dump pile for a weed growth comparison. Trays were placed in a high tunnel with thirty percent shade. Trays were watered daily with overhead irrigation and grown for two weeks.  Weed seedling numbers were counted after two weeks of growth.

A second trial was run in a similar fashion with the exception of using 0.5 mil plastic as the soil covering and 1.0 mil plastic as the top sheet.  This experiment was started on 17 July and run for 2 days and stopped due to low temperatures within the pile and collapsing plastic.  The top sheet was replaced with 4 mil plastic and ran for 14 days.

A third trial was run in the same manner using 1.0 mil plastic as the soil covering sheet and 4 mil was used as the top sheet.  This experiment was started on 31 July, 2014 and stopped 15 days later.

Large Scale Solarization – A large scale solarization project was begun on 17 July, 2014. Ground cover was laid down on bare ground.  A sheet of 4 mil plastic measuring 24 ft x 24 ft was placed on top of this layer.  3.56 yd3 of the same used potting soil was dumped with a frontend loader onto the plastic and spread by rakes and shovels to a depth of 2 inches.  Large roots, weeds, and plant debris were removed by hand.  Moisture was added to the soil by hand.  The temperature probes were placed at 1.0 inch and two were placed at 2.0 inches in the soil depth and randomly located in the soil pile.  The soil was covered with two layers of 4 mil plastic.  The bottom layer laid directly on the top of the surface of the soil.  The top layer was suspended above the soil with 3 inch galvanized pipes averaging 6 inches in height.  Both top layers of soil were tucked below the bottom layer of plastic and held in place with wooden posts placed horizontally on the ground.  The experiment ran for 14 days.  All costs and time expenditures to solarize the used potting soil were tracked.

The solarized soil was sampled for weeds in the same manner as previously mentioned with the addition of three trays of a fresh commercial potting soil as a weed-free check.  Weed germination numbers were counted after 15 days.  The large scale solarization experiment was repeated on 20 Aug., 2014 and ran for 14 days. A sample of solarized soil and untreated soil was sent for chemical and physical analysis to University of North Carolina Horticulture Substrates Lab.

Economic Analysis

Data was collected including materials and time for the large scale solarization study.  Any savings or costs were compared from purchasing fresh potting media for cost analysis and decision making.  $65.00 per hour was assigned to tractor time and $10.00 per hour was assigned to manual labor to make economic decisions.              

Soil Analysis

Multiple samples of solarized media from both approaches were tested for chemical and physical properties.  The chemical property tests included pH, electro-conductivity, macro and minor elements, salts, and bicarbonates.  These were determined by lab analysis at NC State Soil Substrate Lab.  Physical properties tested were porosity, water holding capacity, and bulk density of solarized media compared to untreated media.  Both treated and solarized soil were assayed and compared for nematodes by Florida Department of Agriculture and Consumer Services.  

Growth Experiment

A growth experiment was conducted incorporating solarized media at different treatment rates with new potting soil.  Treatment rates were new media:solarized media: 100:0 (check), 33:66, and 66:33. Potting media pH averaged 6.5.  Rooted liners (young propagated plants) of two commonly grown plants of the Southeast were planted (Sandankwa viburnum (viburnum suspensum)) about 3 inches high and Lagerstromeia x Natchez (‘Natchez’ crapemyrtle) about 6 inches tall into three gallon pots and placed in common production areas.  All plants received 80g, 17-5-11, 12 month slow release and 40g, 14-5-11 plus minors, 6 month slow release fertilizer (Graco Fertilizer Co.) per three gallon pot.  Weeds were removed by hand.  Experimental design consisted of four plants per replication and three replications per media treatment.  Data gathered from this experiment was average height of plants, width of plants, and visual observation compared to the check for the viburnums. Only height data and visual observations were collected for the crape myrtle. Total plants were 72 per site (2 species x 3 media treatments x 3 replications x 4 plants/rep).   A means comparison was run with ANOVA for significance.

Extension Outreach

The outreach objectives started with a PowerPoint presentation incorporating the research and findings. A how-to fact sheet was published incorporating methods and results.  A regional postcard mailing was sent to all registered nurseries in five counties advertising a demonstration, on-site workshop that was held at the Hillsborough County Extension Office to showcase material set up, and results of the grant findings as well as presenting information on composting to recycle used soil.  A Pre/Post evaluation was created to gain impact results from the workshop participants.  An online presentation was created to be hosted on the Hillsborough County Extension webpage for those interested in solarization to recycle used potting soil.

Research results and discussion:

The first mid-scale test run temperatures reached 134 ºF at the top of the soil pile and 116 ºF at the bottom after two days.  The volume of airspace was increased slightly to trap more heat.  This had no effect on the temperatures.  Soil moisture was increased to field capacity.  This slowed the heat movement to the bottom of the pile.  After removing the top layer of plastic more heat reached the deepest layers of soil, however, maximum temperatures were reduced to about 117 ºF.  This soil was used to test weed seed germination rates compared to untreated soil.  Weed germination after 14 days show an average of 14.7 weeds per tray in solarized soil compared to 240 weeds per tray in untreated soil.  Solarization reduced germinating weeds by 94%.             

The second mid-scale experiment used 0.5 mil covering sheet and a 1.0 mil top sheet, in the thought that this would allow more sunlight in to heat the soil.  This may have been the case but the gauge of plastic was too thin to trap the heat.  The uppermost temperatures reached 135 ºF but the bottom of the pile at a four inch depth only reached 100 ºF.  The top sheet also did not have enough resilience to shed rain water.  After changing the top sheet to 4 mil, temperatures reached 135 ºF.  However the bottom of the pile didn’t exceed 100 ºF.  At night the heat dissipated completely and returned to ambient air temperatures.  This also may have been due to too much water in the soil mix.  It would be advisable to measure water content to determine optimal levels for this type of solarization.

The third mid-scale experiment used a one mil soil covering sheet and a four mil top sheet.  A rain event collapsed the experiment on day two.  On day seven the plastic was adjusted and upper soil temperatures reached 153.79 ºF as a high and 118.83 ºF at the bottom of the pile.  The middle of the pile (two inch depth) reached 122.23 ºF.

In the large scale experiment, soil depth was kept to about two inches. The average height of the tent was about 6 inches above the soil surface.  Highest temperatures were reached at the surface of the soil at 140.78 ºF. The two bottom probe temperatures reached 130.9 ºF and 138.25 ºF.  The average of the soil over the study was about 100 ºF.  Soil from this experiment was sent to the NC State Soil Substrate testing lab for a physical and chemical comparison.  

The second large scale experiment yielded much higher temperatures.  The top most temperature probe measured a high of 159.23 ºF, the middle probe 147.32 ºF, and the bottom probe 144.31 ºF.  This soil was used to run a second weed seed germination trial.  After two weeks in a greenhouse, three trays of newly purchased soil had zero weeds germinate.  The solarized soil averaged 10.6 and the non-solarized soil was 89.0 weeds germinating.  Solarization reduced weeds by 88% compared to untreated used soil.

If solarization is going to be used by growers, careful attention should be made to the topmost layer of plastic and removing water from rain events.  Strong rains will collapse plastic sheets together minimizing temperature gain in the soil. 

Physical (total porosity, container capacity, air space, bulk density) and chemical comparison of solarized and un-solarized soil had little differences (EC, P, K, CA, Mg).  Solarized soil did have more available NO3-N (51.52 ppm) than untreated soil (29.98 ppm) and a lower pH (5.6 versus 6.3).  This may be from breakdown of organic compounds or residual polymer coated fertilizer remaining in used soil.

Nematode counts were compared by Florida Department of Agriculture and Consumer Services, Department of Plant Industry. Nematodes were found in the treated soil, however numbers were greatly reduced (untreated 73 nematodes, treated 10.3 nematodes) and no live nematodes were found in solarized soil.  Assays determined that they were heat killed by solarization. 

To improve the efficacy of the solarization on weed control, it would be beneficial to wet the soil prior to covering with plastic sheets to allow for weed seeds to germinate for a few days.  High temperatures achieved with solarization would eliminate these germinating seeds.

It was confirmed with these series of experiments that soil moisture, depth of soil, plastic thickness, and tenting configuration can impact results of solarizing soil.  However, excellent results can be achieved to eliminate nematodes, nearly eliminate weed seeds, and recycle old potting soils for reuse with minimal cost.    

On 30 September, 2013 the growth study was started.  On 16 July, 2014 data was collected from the growth study. Viburnum suspensum heights and widths were measured and crapemyrtle heights were measured.  There were no significant differences found among the different proportions of new soil combined with solarized soil treatments (100:0, 33:66, 33:66 (new soil:solarized soil)).  At up to 66% recycled solarized soil no differences were found in any of the data measurements proving that the solarized treatment of the potting soil did not affect plant growth.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

S. T. Steed, 2014. Developing a Method for Large Scale Solarization and Recycling of Used Potting Soil. Proc. Florida State Horticultural Society. (in publication)

S. T. Steed, 2014. On-Farm Potting Soil Solarization Increases Sustainability of Environmental Horticulture Industry. Extension Professional Association of Florida Abstract/Presentation.

Steed. S.T. (Presenter) and Barber L. (Host), 2014.  Solarization of Used Potting Soil for Recycling. Radio Program WTIS, St. Petersburg, FL. Recorded October, 2014. Listenership – 48,000, 700 archive listeners per day.

S. T. Steed, 2015. Solarizing Helps Grower Recycle Dumped Potting Soil. The Plant Producer Newsletter. UF/IFAS Extension. Readership – 4000 nursery industry professionals in the state of Florida.

S. T. Steed, 2015. Recycling of Used Potting Soil with Solarization Presentation. UF/IFAS Extension Hillsborough County.

S. T. Steed, 2015. Solarized Potting Soil Effects on Growth of Lagerstroemia x ‘Natchez’ and Viburnum suspensum. Proc. Florida State Horticultural Society. (in publication)

S. T. Steed, 2015. Methods and On-Farm Research Results 2013-2015. UF/IFAS Extension Hillsborough County Extension.

A hands-on demonstration workshop was held at the UF/IFAS Hillsborough County Extension Office. A regional postcard mailing was sent to approximately 1000 registered nurseries in the local five-county area advertising the workshop. This was combined with a presenter on composting for recycling used potting soil. Thirty attendees learned about these two methods to recycle this wasted potting soil back into production. Attendees were also able to visually see a soil solarization demonstration plot. 100% (n=19) of evaluation respondents felt that they could improve their operation by reusing potting soil. 100% improved their level of knowledge about soil composting and soil solarizing after the workshop. Knowledge of composting (1=low, 5= high) improved 66% from 2.63 to a 4.37. Knowledge of soil solarization increased from 2.16 to a 4.32 for a 100% increase. 47% of respondents currently reused old potting soil. 

This research was presented at the Florida State Horticultural Society Annual Meeting (June 2014) to an audience of 40 academic and industry professionals in the Ornamental, Garden and Landscape Section, and approximately 50 UF/IFAS Extension agents and specialists at the Extension Professional Association of Florida (August 2014).

The final research report will be presented at the 2015 Florida State Horticultural Society Annual Meeting in June 2015 to horticultural professionals in the state of Florida, and again at the International Plant Propagation Society-Southern Region Annual Meeting in October 2015, to the southern region U.S. nursery producers, academics, botanical garden managers and an international delegation of the same.

Project Outcomes

Project outcomes:

In our southern location, solarization of used potting soil is an extremely effective and sustainable method to recycle this industry waste.  Very little materials and labor were used to recycle large amounts of potting soil. The only drawback is that sunny weather is a necessity for this method to work. Fortunately, there is plenty of this in the South. The temperatures to sterilize the soil were obtained, in some cases, within the same day.  Nematodes and plant pathogens were effectively treated.  Chemical and physical analysis of treated soil was very similar to un-solarized soil.  This will enable growers to run soil analysis in their dump stockpiles of used soil to determine what ratios they can mix the soil with fresh soil to grow new plants. Basically, in this method a small cost of labor is being substituted for the large purchase price of fresh potting soil at a savings of 87%.  If labor demands are a problem for the nursery then solarization may not be the way to recycle used potting soil.

The on-farm cooperator used this method to sterlize about nine cubic yards of soil to grow a very successful crop of crepe myrtle trees with no adverse effects.  It is expected that if growers can learn from and adopt this method, about 10% of potting soil that would have been wasted in production can be recycled leading to large economic savings on soil and a reduction on the demand of natural resources such as pine bark and peat.

Economic Analysis

Costs for the large scale set up were $233.62 in materials that would be used for the entire solarizing season.  This was deemed to be somewhat high as we assigned $150.00 to the price of galvanized pipes that supported the top plastic sheet.  Labor costs ran $16.63 per solarization run, which included removing finished soil from the solarization pad.  Total costs per yard of soil using our design was about $4.67 to treat used potting soil.  This is a savings of about $30.33 per yd3 of soil or about $107.97 per solarizing run.  Material costs would break even in about 2.16 solarizing turns.  Using solarization is an extremely cost-effective and sustainable method to recycle used potting soil.  The only additional inputs other than the sun are plastic sheeting and labor costs.  Using this method will greatly reduce accumulated amounts of used potting soil on farm sites and conserve natural resources.  Unavailable labor might hinder the adoption of this technique.

Farmer Adoption

After the workshop of 30 attendees, 100% (n=19) of evaluation respondents felt that they could improve their operation by reusing potting soil.  100% of respondents intended to start recycling soil with 26% using composting, 16% using solarization, and 37% using both methods.  Some attendees reported how many yards of soil they thought they would recycle.  Three attendees reported they would recycle 121 yards annually, or a production saving of about $3670.00.  A few other attendees reported that they could reuse 5-10%, 25-50%, 90%, and most of their dumped potting soil annually.  This will be followed up with a six-month evaluation.  Two additional growers are using solarization to recycle used potting soil.  An estimated 4,120 growers, allied industry, regulatory, academic, and Master Gardeners have been reached by this project.

Recommendations for nursery growers using solarization –  This research, presents a good case for nursery producers to visualize used potting soil as a production input instead of a waste to be dealt with.  Used potting soil that was intended for the dumpster or dump pile should have large plant material removed and root balls dumped into a full sun, treatment area for solarization to take place.  At the conclusion of this research a recommendation for farmers would be to wet the soil to be solarized a week prior to starting the process. This will encourage weed seeds to germinate and then be heat killed by the solarization process. By following our recommendations; keeping the soil to 3 inches or less in height, adding water to moistened soil (but not too much that the soil is water logged), using two layers of plastic sheeting, measuring temperatures with thermometers, and having enough sunlight to complete the process, used potting soil can be recycled at less than five dollars per cubic yard.  With some slight modifications and material choices, from our method, it is thought that it will probably cost from two to four dollars per cubic yard, especially if dumped soil is placed into a treatment area instead of going to a dump pile first to be handled again later.


Areas needing additional study

An area of future inquiry might be to assess the green industry producers to ascertain the amount of used potting soil on premises, the quality of ingredients in those piles, and the barriers of resistance to adoption of solarization as a treatment for the soil. Another possibility for additional research would be a more detailed look at whether viruses are killed at solarization temperatures.  A southern region webinar might be a good way to introduce this method to a broader number of growers.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.