Optimizing substrates, composts, and fertilizer additions for organic transplant production

Project Overview

Project Type: Graduate Student
Funds awarded in 2004: $10,000.00
Projected End Date: 12/31/2005
Grant Recipient: North Carolina State University
Region: Southern
State: North Carolina
Graduate Student:
Major Professor:
Mary Peet
North Carolina State University

Annual Reports


  • Agronomic: soybeans, wheat
  • Vegetables: broccoli, cabbages, cucurbits, peppers, sweet corn, tomatoes


  • Crop Production: conservation tillage
  • Education and Training: demonstration, display, extension, farmer to farmer
  • Pest Management: allelopathy, biological control, chemical control, cultural control, integrated pest management, mulches - killed, mulches - living, mulching - vegetative, mulching - plastic, physical control, trap crops, weed ecology
  • Production Systems: agroecosystems
  • Soil Management: earthworms, green manures, nutrient mineralization, organic matter, soil analysis, soil quality/health


    Conventional and organic potting mixes were tested to compare effects on seed germination and seedling growth of tomato. Materials tested showed variation in chemical composition with time and batch indicating a need to conduct nutrient analyses on substrates prior to use. Germination was sometimes reduced in media containing higher quantities of organic amendments emphasizing the need to optimize germination conditions through careful watering and the use of vermiculite to cover seeds. While a portion of transplant nutrition can be provided through pre-plant incorporation of organic fertilizers into potting media, all media tested would require additions of soluble fertilizer to provide complete nutrition.

    Tables, figures or graphs mentioned in this report
    are on file in the Southern SARE office.
    Contact Sue Blum at 770-229-3350 or
    sueblum@southernsare.org for a hard copy.


    U.S. farmland managed under certified organic farming systems expanded substantially during the last decade and indications are that consumer preferences for organic products will continue to grow as will this segment of U.S. agriculture (Greene, 2001). However, research to support this segment of agricultural production, and specifically organic vegetable transplant production methods is extremely limited. Most information available to organic farmers regarding transplant production is either based on research done with conventional systems or anecdotal information rather than focused scientific investigation.
    Scientific studies conducted with organic potting mixes have typically involved evaluating particular media amendments but have rarely worked with a mix of ingredients with the aim of providing both adequate seed germination and sufficient nutrition throughout the transplant period. Organically grown transplants conforming to specifications by the National Organic Program are rarely available commercially, so they must be produced on-farm as are the potting mixtures themselves. Commercial organic mixes are expensive, and not always locally available. Although ‘recipes’ and anecdotal information are available, most growers develop their transplant potting mixes through trial and error, as few guidelines and even fewer published experimental reports are available.
    One limiting factor in organic potting media is that mixes that provide adequate nutrition for plant growth do not necessarily provide optimal conditions for seed germination, as salt levels may be above optimal. Seed germination is the first step in crop production and adequate seed germination is necessary for transplant production to be space and cost effective. In order for seeds to germinate, they must be exposed to adequate conditions including water, temperature and oxygen (Bradbeer, 1988). Both the physical and chemical properties of the soil affect seed germination. Specifically, high salt content in a sowing media can inhibit germination (Mayer and Poljakoff-Mayber, 1989).
    Conventional transplant mixes are soilless and typically consist of peat, perlite, vermiculite, and pine bark in various proportions along with wetting agents and ‘starter’ fertilizer charges. Soilless media components have little or no available nutrient content. Once seedlings are established, soluble fertilizers are added weekly to maintain the transplants until they are taken to the field. Organic transplant mixes also may contain peat, perlite, vermiculite, pine bark and coir, but substitute composts and organically allowable fertilizers for the conventional wetting agent and starter charge. The proportion of compost added, as well as the selection of additional media and fertilizers, varies greatly, both in commercial organic mixes and in those mixed on-farm. Typical recommendations call for compost to comprise less than half of the total volume of the mix, and often only 20-30%. Although compost is a common media ingredient among organic growers (Kuepper, 2002), there is wide variation in the types and quality of compost used. A number of studies have been conducted to determine the appropriateness of including various types and proportions of compost for transplant production. Raviv et al. (1998) found that fertilized tomato seedlings produced in a mix of 30 percent compost (from cattle manure), 30 percent peat, and 40 percent vermiculite had greater dry weights than plants produced in peat and vermiculite (60:40) alone. While composts from a number of plant and animal sources have been utilized as sources of additional nutrients in organic transplant mixes, vermicompost is widely used. Vermicompost, a type of compost rich in organic matter and plant available nutrients generated as earthworms break down organic waste (Dominguez, 2004) is frequently added to organic potting mixes and has been reported to affect seed germination. Atiyeh et al. (2000a), found that substitution of commercial potting media with 20 – 40% vermicompost significantly increased the germination rates of tomato seed while Buckerfield et al. (1999) found that seed germination in radish decreased or was delayed as the percentage of vermicompost added to a substrate increased. The specific characteristics of vermicompost (N-P-K, salts, pH) depend both upon the source of wastes that are worked by the worms and the maturity of resulting vermicompost (Handreck, 1986, Hidalgo and Harkess, 2002).
    Gagnon and Berrouard (1994) investigated the effects of organic fertilizers on the growth of tomato transplants by adding organic fertilizers to plants grown in a 3:1 peat – compost medium. Of the fertilizers studied blood meal, feather meal, meat meal, crab-shell meal, fish meal, and dried whey sludge were found to produce the best plant growth. Feather meal is a by-product of the poultry processing industry and is used as a slow-release organic nitrogen fertilizer. Due to local availability and low cost, feather meal currently is being widely utilized by local organic growers in North Carolina. Hadas and Kaustsky (1994) found feather meal to contain12% nitrogen. Hartz and Johnstone found feather meal to contain 14.2% nitrogen. Gagnon and Berrouard found feather meal to have nutrient content of 13.6-0.3-0.2 N-P-K. Hadas and Kautsky (1994) studied the rate of nitrogen mineralization of feather meal in soil, and found that 45, 55, and 65% of fertilizer N were released after 1, 2, and 8 weeks. Organic growers sometimes add kelp meal to transplant mixes and use it as a foliar fertilizer. Manufacturers of kelp meal claim that it contains small amounts of nitrogen and potassium, plant growth regulators and trace elements, is a chelating agent, and enhances microbial activity.

    Literature Cited

    Atiyeh, R.M., C.A. Edwards, S. Subler, and J.D. Metzger 2000b. Earthworm-processed
    organic wastes as components of horticultural potting media for growing marigold
    and vegetable seedlings. Compost Science and Utilization, 8:215-223.

    Atiyeh, R.M., N. Arancon, C.A. Edwards, J.D. Metzger. 2000a. Influence of earthworm-
    processed pig manure on the growth and yield of greenhouse tomatoes.
    Bioresource Technology 75:175-180.

    Atiyeh, R.M., S. Subler, C.A. Edwards, G. Bachman, J.D. Metzger, and W. Shuster. 2000c.

    Effects of vermicomposts and composts on plant growth in horticultural container
    media and soil. Pedobiologia 44:579-590.

    Bradbeer, J.W. 1988. Seed Dormancy and Germination. Chapman and Hall, New York,
    NY. 27-28.

    Buckerfield, J.C., T.C. Flavel, K.E. Lee, and K.A. Webster. 1999. Vermicompost in solid and liquid forms as a plant-growth promoter. Pedobiologia 43:753-759.

    Cantliffe, D. J. 1998. Seed germination for transplants. HortTechnology 8:499-503.

    Domínguez, J. 2004. “State-of-the-Art and New Perspectives on Vermicomposting
    Research.” Earthworm Ecology. Ed. C.A. Edwards. 401-424.

    Gagnon, B. and S. Berrouard. 1994. Effects of Several Organic Fertilizers on Growth of Greenhouse Tomato Transplants. Canadian Journal of Plant Science 74: 167-168.

    Greene, C. 2001. U.S. Organic Farming Emerges in the 1990’s: Adoption of Certified
    Systems. U.S. Department of Agriculture, Economic Research Service, Research
    Economics Division, Agriculture Information Bulletin No. 770.

    Hadas, A. and L. Kautsky. 1994. Feather meal, a semi-slow release fertilizer for organic
    farming. Fertilizer Research 38:165-170.

    Handreck, K.A. 1986. Vermicompost as Components of Potting Media. Biocycle 27:58-62.

    Hartz, T.K., and P.R. Johnstone. 2006. Nitrogen Availability from High-nitrogen-containing Organic Fertilizers. HortTechnology 16:39-42.

    Hidalgo, P. and R.L. Harkess. 2002. Earthworm castings as a substrate amendment for
    Chrysanthemum production. HortScience 37:1035-1039.

    Kuepper, G. and K. Adam. 2002. Organic Potting Mixes for Certified Production.
    Appropriate Technology Transfer for Rural Areas. Horticulture Technical Note. http://attra.ncat.org/attra-pub/potmix.html.

    Mayer, A.M. and A. Poljakoff-Mayber. 1989. The Germination of Seeds. 4th ed.
    Pergamon Press, New York, NY. 220-224.

    Paul, L.C., and J.D. Metzger. 2005. Impact of Vermicompost on Vegetable Transplant Quality. HortScience 40: 2020-2023.

    Raviv, M., R. Reuveni, and B. Zion. 1998. Improved Medium for Organic
    Transplants. Biological Agriculture and Horticulture 16:53-64

    Vavrina, C. 2002. An Introduction to the Production of Containerized Vegetable
    Transplants. Fact Sheet HS849. Horticultural Science Department, Florida
    Cooperative Extension Service, Institute of Food and Agricultural Sciences,
    University of Florida. http://edis.ifas.ufl.edu/HS126

    Project objectives:

    The goal of this study was to provide guidance for growers in terms of efficient and cost-effective combinations of ingredients for an organic growing media and the period of time over which these mixes should be able to support seedling growth without the costly addition of soluble organic fertilizer. Various organic amendments were examined in terms of their effectiveness for seedling production and in terms of the period in which mineral nutrition was provided to the plant by that particular amendment.

    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.