Large Scale Recycling of Used Potting Media with Solarization

Project Overview

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

Annual Reports


  • Additional Plants: native plants, ornamentals


  • Education and Training: extension, on-farm/ranch research
  • Energy: solar energy
  • Pest Management: soil solarization
  • Soil Management: organic matter, soil analysis


    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.   

    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.