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

Final Report for GS04-032

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
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Project Information


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
[email protected] 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.


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  • Elizabeth Larrea


Materials and methods:

Treatment Overview: Four experiments were conducted in the NCSU greenhouses using the tomato cultivar ‘Celebrity’. This cultivar was selected because it is popular with organic growers in the Piedmont area of NC. Organic certification requires the use of organically grown seed. No organically grown seed of ‘Celebrity’ was available but untreated seed was obtained from Harris Seeds (Rochester NY) for Experiments 1-3 and from Johnny’s Selected Seeds (Winslow, Maine) for Experiment 4. Different seed lots were utilized for all experiments. Percentage germination of seed lots utilized for the four experiments ranged from 85% in Experiments 1 and 2 to 89% in Experiments 3 and 4. All research was conducted in the Marye Anne Fox Science Teaching Laboratory greenhouses on N.C. State’s main campus. Greenhouses were glasshouses with evaporative cooling pads and automated sunshade cloth.

Treatments (Table 1) in the first experiment were included in Experiments 2-4. Two additional treatments were added in Experiment 2, and another 6 treatments were included in Experiments 3 and 4. The purpose of the additional treatments is described more thoroughly below, but the general objectives of Experiments 2-4 were to test additional commercial organic mixes coming on the market and to optimize the percent of the various components of the locally used mix.

Experiment 1 was conducted from March 17 - 27, 2004 (early spring) and utilized an organic (SG), a conventional (F4P) commercial mix and a “grower’s mix” (GM20) utilized by organic growers in the Piedmont area of NC for transplant production. Components of the original 3 mixes, as well as the subsequent 7 mixes are further described in Table 1. All experiments utilized a complete randomized block design. A log transformation was performed on germination percentage data. An analysis of variance was conducted using the Waller Duncan test in SAS (SAS Institute Inc., Cary, NC) for windows version v8.

Experiment 2 was conducted May 24 -June 4, 2004 (late spring), and included the three treatments from Experiment 1 as well as a variation of the original grower’s mix (GM10). In Experiment 1 seeds sown in GM20 exhibited reduced germination compared to the commercial mixes. The purpose of the additional treatment in Experiment 2 was to test the hypothesis that reducing the percentage of vermicompost from 20% to 10% would increase germination.

Experiment 3 was conducted from July 15 - July 24 (summer). This experiment included the four treatments utilized in Experiments 1-2 as well as treatments in which single organic fertilizers were mixed with the peat / perlite base in order to determine the individual effect of each component on seed germination. A treatment was also added in which feather meal and vermicompost were mixed with the peat / perlite base to determine the effect of these two fertilizers in combination. Two additional commercial organic mixes, which had become available for grower use, were also added for comparison. In this experiment, elemental S was added to all mixes containing vermicompost in order to lower pH.

Experiment 4 was conducted March 10 – 22, 2005 (early spring, essentially the same conditions as Experiment 1). This experiment repeated all treatments utilized in Experiment 3 during the spring season when the greatest number of organic transplants would be produced by growers.
Mixing of Substrates in Non-Commercial Treatments

Custom blends were mixed for fifteen minutes in a one cubic yard cement mixer. Vermicompost was passed through a ¼ inch sieve prior to being added to the mixer to reduce clumping.
Seeding, Plant Care and Data Collection

Experiment 1: A commercial organic mix, Sun Gro Sunshine Growers Organic (Sun Gro Horticulture Canada Ltd., Vancouver, British Columbia), a conventional commercial mix, Fafard 4P (Conrad Fafard, Inc., Agawam, MA), and a custom organic mix, GM20, were placed in six-cell inserts cut apart from a 72-cell plug tray. Five replicates of six cells were included for each treatment. Two seeds were placed into ¼ inch holes in each cell and additional media of the same treatment was added to cover the seeds. All media were watered to runoff upon completion of seeding. Subsequently plants were subirrigated daily by placing a saucer under seeded flats, applying 200 mL of water for 1 hour, and then removing the saucer and excess water. Germination data was collected at the same time daily from March 23 -27, 2004.
Pour thru analysis utilizing the methods described by Cavins et al. (2000) was conducted immediately prior to seeding and at the end of the germination period to determine the electrical conductivity (EC) and pH of the leachate as an indication of rootzone conditions. Analyses were conducted using a pH/EC/TDS meter by Hanna Instruments (Woonsocket, RI). Leachate samples were submitted to Agronomic Division, North Carolina Department of Agriculture (NCDA) for elemental analysis. Analyses were completed utilizing the procedures detailed in Plank (1992).

Experiment 2: On May 24, 2004, six replicates of 72 cell plug trays were filled with four potting mixes (GM10, GM20, Sun Gro Organic, and Fafard 4P) for a total of twenty-four trays and 1728 seeds. The media was initially wetted as in Experiment 1, but then placed in a mist bed set to irrigate every eight minutes for eight seconds for one day prior to seeding (May 26). At seeding, additional media was added to cover the seeds; trays were watered in and returned to the mist beds. Only 1 seed was planted per cell, as opposed to 2 seeds per cell in Experiment 1. Germination data was collected daily from May 31st -June 4th.
In addition to the pour-through sampling conducted as in Experiment 1, samples of each potting mix and unmixed vermicompost were submitted to NCDA for analysis as waste material (Table 7). Waste analysis was completed utilizing the procedures detailed in Plank (1992).

Experiment 3: Procedures were as in Experiment 2 except that four replicates of 10 different substrates were tested (Table 1). Seeding took place July 15, 2004 and germination data was collected July 20-24.
In addition to pour through samples, samples of each potting mix as well as unmixed vermicompost and feather meal were submitted to NCDA for analysis as waste materials (Table 7).

Experiment 4: Procedures were the same as in Experiments 2 and 3, with four replicates of the same substrates as in Experiment 3. However, pre-wet trays were not placed in the mist beds before seeding as was done in Experiments 2-3. Trays were filled on March 10, 2005 and seeded March 11. Vermiculite rather than the treatment substrate was placed over the seed.
Throughout the experiment, trays were hand watered with a 2 gallon per minute foggit nozzle (DRAMM Corporation, Manitowoc, Wisconsin) two to three times daily during the germination period. Substrates received different amounts of water, depending on the appearance of the surface. Germination data was collected from March 18 - March 22.
Pour through and waste samples were collected and submitted to NCDA as in Experiments 2 and 3.

Tissue Analysis: Tissue analysis was completed for Experiments 2, 3, and 4. Beginning ten days after initial seedling emergence, two seedlings per tray were cut off at the soil level weekly and submitted to NCDA for tissue analysis.

Growth Analysis: Seedlings were harvested four weeks after initial seedling emergence and placed in a drying oven for five days at 160 degrees Fahrenheit. Dry weights were measured for each replicate.

Literature Cited

Cavins, T.J., B.E. Whipker, W.C. Fonteno, B. Harden, I. McCall, and J.L. Gibson. 2000 Monitoring and Managing pH and EC Using the PourThru Extraction Method. NCSU Horticulture Information Leaflet 590.

Plank, C.O. ed. 1992. Plant Analysis Reference Procedures for the Southern Region of the United States. Southern Cooperative Series Bulletin 368. http://www.cropsoil.uga.edu/~oplank/sera368.pdf

Research results and discussion:

Seed germination in Commercial Mixes: Seed germination in the Sun Gro Organic was comparable to the conventional mix (Fafard 4P) in all experiments (Table 2). Two additional organic potting mixes were utilized in Experiments 3 and 4. In Experiment 3, the Fafard Organic 10 was comparable to both the Sun Gro and the Fafard 4P while seed germination in the Fafard Organic 20 was significantly reduced. In Experiment 4, seed germination was reduced in Fafard Organic 10 and Fafard Organic 20 compared to Sun Gro and Fafard 4P.

Seed germination in Custom Mixes: Seed Germination in custom mixes varied. Seed germination in the original organic grower’s mix, GM20, was significantly reduced compared to the commercial potting medias in all experiments except Experiment 4. Reducing vermicompost from 20% to 10% increased germination in Experiment 2 to levels not significantly different from the commercial mixes, however; in Experiment 3, seed germination in GM10 was reduced compared to all other mixes included in that study (Fafard 4P, Sun Gro Organic, and GM20). In Experiment 4, both GM10 and GM20 were comparable to Fafard 4P and Sun Gro Organic. Thus, reducing the percentage of vermicompost increased germination in one of three experiments.
Seed germination in the FM mix and the KM mix was not significantly different from Fafard 4P and Sun Gro Organic. Seed germination in the FMVC mix was reduced compared to Fafard 4P and Sun Gro Organic in Experiment 3 but not in Experiment 4 while germination was not reduced in the VC mix in either experiment.
In Experiment 4, seed germination was much more consistent among treatments than in the other three experiments with reduced germination in only Fafard Organic 10 and Fafard Organic 20.

EC: There was a wide range in EC readings among experiments and among treatments (Table 3). EC was measured both at the time of seeding and during week two of each experiment once all germination data had been collected. There was a large drop in EC readings during that time. The EC for the Sun Gro were consistently the lowest among all treatments and ranged from an average initial EC of 0.20 in Experiment 1 to 0.69 in Experiments 4. The initial for GM20 was much lower in Experiment 1 than any other experiment with an EC of 0.69 in Experiment 1 and 4.52, 3.86, and 3.99 in Experiments 2 through 4, respectively. The EC during week 2 for all treatments and experiments are listed in Table 4.
Results of waste analysis show that the GM20 mix had the highest level of soluble salts of all potting mixes with 5.28 mS/cm in Experiment 2, 5.65 mS/cm in Experiment 3, and 6.21 mS/cm in Experiment 4. Sun Gro Organic had 0.15 mS/cm soluble salts in Experiment 3 and 0.05 mS/cm in Experiment 4 while Fafard 4P had 0.14 mS/cm and 0.19 mS/cm in Experiments 3 and 4 respectively. Elemental analysis data for Experiments 2, 3 and 4 for all treatments are shown in Table 7.

Dry Weights: Transplants grown in GM20 and GM10 had significantly higher dry weights compared with the commercial mixes, Fafard 4P and Sun Gro Organic in Experiments 1, 3, and 4 (Table 2). Reducing vermicompost from 20% to 10% produced no significant difference in dry weights in Experiments 2-3 and significantly higher growth in Experiment 4.
The average dry weight of transplants grown in Fafard 4P was comparable to those grown in Sun Gro Organic in Experiments 1 and 2, significantly greater than that of Sun Gro in Experiment 3 and significantly less in Experiment 4. In Experiment 3 Fafard Organic 10 produced transplants with a dry weight greater than that of Sun Gro Organic and not significantly different than Fafard 4P. In Experiment 4, the reverse occurred with transplants grown in Fafard Organic 10 having dry weights significantly greater than those grown in Fafard 4P but not significantly different than those grown in Sun Gro Organic. Fafard Organic 10 produced transplants with significantly greater dry weights than Fafard Organic 20 in both Experiments 3 and 4. Transplants grown in Fafard Organic 20 had dry weights not significantly different from those grown in Sun Gro Organic in Experiment 3 and those grown in Fafard 4P in Experiment 4.
There was variability in growth of transplants grown in the additional mixes formulated to omit ingredients of the original grower mix. Transplants grown in the KM mix had dry weights significantly lower than most other treatments in both Experiments 3 and 4. Transplants grown in the VC mix were significantly lower in growth in Experiment 3, but not 4. The FM mix produced transplants with average dry weights significantly lower than other treatments in Experiment 4, but not Experiment 3.

Plant nutrient analysis: Transplants were deficient in nitrogen by the fourth week of production in all commercial and custom mixes during at least one experiment (Figures 3.1-3.3). The only phosphorus deficiency found in a custom mix containing vermicompost during any experiment was in the FMVC treatment in Experiment 3. Fafard 4P only became deficient in phosphorus in Experiment 3 while Sun Gro Organic was deficient in potassium by week 3 and in phosphorus by the fourth week in all experiments. Potassium deficiencies were also found in the final weeks of production for most custom mixes during Experiment 3 and for mixes not containing vermicompost in Experiment 4. For the Sun Gro mix and the FMVC mix, deficiencies were seen as early as week 2. No micronutrient deficiencies were found in any potting mix during the transplant production period.

Elemental analysis: Elemental analysis results indicate considerable variability in all major nutrients between the three batches of vermicompost utilized (Table 3). The batch utilized for Experiments 1 and 2 contained the lowest nutrient levels (6,618, 547, and 478 parts per million (ppm) of nitrogen, phosphorus, and potassium respectively). Batches utilized for Experiments 3 and 4 were fairly similar in N and P (Experiment 3: 16,358, and 17,034 ppm respectively of nitrogen and phosphorus compared to 17,788 and 16,171 ppm in Experiment 4). However, potassium was much higher in the material utilized in Experiment 4 (2,173 ppm) than that utilized for Experiments 3 (791 ppm) and 2 (478 ppm). It is not clear why nutrients were lower overall in the first vermicompost batch tested, but it is probably safe to conclude that vermicompost from hog waste is a better source of nitrogen and phosphorous than of potassium. One batch of both feather meal and kelp meal were used for all experiments. Feather meal potassium content decreased over time with a potassium content of 1,254 ppm for feather meal utilized during Experiment 3 and 772 ppm for feather meal utilized during Experiment 4.
Of the treatments utilized in all four experiments, seed germination was significantly greater in the commercial organic (Sun Gro Organic) and conventional (Fafard 4P) treatments than in the grower’s mix (GM20). However with careful watering and use of vermiculite to cover seeds as in Experiment 4, it is possible that an acceptable level of seed germination can be obtained in the GM20 mixture. None of the variations of the original custom mix utilized consistently provided for improved germination over the GM20. Neither Fafard Organic 10 nor Fafard Organic 20 provided for seed germination greater than the Sun Gro Organic mix. The experiment with the fewest differences in germination among treatments was Experiment 4. This was the only experiment in which treatments were hand watered throughout the germination period. In Experiment 1, water was provided via sub-irrigation while Experiments 2 and 3 utilized mist beds for irrigation. For Experiments 1 through 3, equal amounts of water were provided to all treatments regardless of substrate conditions. However, by hand watering in Experiment 4, water was provided to the substrate as needed, potentially providing improved conditions for seed germination and resulting in the more consistent seed germination among treatments.
Another possible explanation for the more consistent germination among treatments in Experiment 4 is the use of vermiculite to cover seeds rather than the use of the relevant potting media. The use of vermiculite or a similar material is often recommended to cover seeds for germination as it is a media that will maintain high humidity and moisture while allowing maximum aeration at the seed level (Cantliffe, 1998). Thus, it is possible that improved and consistent germination was due to improved conditions as a result of the use of vermiculite.
There may be some relationship between germination and the level of soluble salts in the media utilized, however, due to the overall variability of results in these experiments, any pattern remains unclear. EC measurements both at the time of sowing (Table 3) and two weeks later (Table 4) were inconsistent between experiments. The clearest relationship between EC and percent germination can be found in Experiment 3 in which the four treatments with the greatest percent germination - Sun Gro, FM Mix, KM mix, and Fafard 4P - had the lowest initial EC.
Differences in transplant growth both among treatments and experiments can be largely attributed to the nutrient status of plants. For example, transplants grown in the FM mix in Experiment 3 had an average sample dry weight of 3.50 grams (g), while growth was greatly reduced in this mix in Experiment 4 producing samples with an average dry weight of only 0.2775 g. As can be seen in Figure 3, phosphorus and potassium deficiencies occurred very early in the feather meal mix in Experiment 4 with those deficiencies presumably leading to the reduction in growth. Similarly, seedlings grown in the VC mix had an average dry weight of only 0.125 g in Experiment 3, but increased to an average dry weight of 3.7225 g in Experiment 4. This appears to be due to the fact that tissue nitrogen content was much lower in Experiment 3 than in Experiment 4, with samples being deficient by week 2 in Experiment 3. Transplants grown in the KM mix were very small in both experiments with an average dry weight of 0.10 g and 0.395 g in Experiments 3 and 4, respectively. The kelp meal mix was generally lacking in nutrients, with the greatest deficiency being in nitrogen in which samples were deficient in nitrogen by week 2 in both experiments in which the treatment was included.
Differences in plant nutrient status in transplants grown in the same mix but in different experiments may be largely due to the variability of nutrient content in the organic fertilizers utilized. For example, different batches of vermicompost were utilized for each experiment with the exception of one batch being utilized for Experiments 1 and 2. The nutrient content of the vermicompost varied greatly between batches with the first batch containing much lower levels of nitrogen, phosphorus, and potassium than subsequent batches. Paul (2005) found similar variability in her work with vermicompost and recommended analysis of vermicompost prior to use to determine nutritional content.
The results of this experiment indicate that organic potting mixes blended by growers utilizing incorporated organic fertilizers may best be formulated to provide a starter charge rather than attempting to incorporate sufficient nutrition for the entire production period. While results of this study and those of Handreck (1986) indicate that additions of hog waste vermicompost in a potting mix may provide sufficient phosphorus, there appears to be a need for soluble fertilizer additions in order to provide plants with sufficient nitrogen and potassium, especially since transplants are sometimes kept more than 4 weeks before transplanting. This approach would also be more similar to that utilized by conventional growers and would have the added benefit of allowing growers greater control over transplant growth in the final stages of production, particularly in cases where it may be necessary to hold seedlings prior to transplanting longer than originally anticipated due to inclement weather or other issues. In fact, slow release nitrogen fertilizers are typically not used for vegetable transplant production as plant height may become excessive with constant nitrogen availability (Vavrina, 2002).

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

Larrea, Elizabeth. 2005. Optimizing substrates for organic tomato transplant production. M.S. Thesis North Carolina State University.

Larrea, E.S., Peet, M.M. and C.D. Harlow. 2005. Selecting Substrates for
Organic Transplant Production. Presentation 32nd National Agricultural
Plastics Congress. March 5-8, 2005, Charleston, SC. P58-64.

Larrea, E.S., Peet, M.M. and C.D. Harlow. 2005. Selecting Substrates for
Organic Transplant Production. Proceedings 32nd National Agricultural
Plastics Congress. March 5-8, 2005, Charleston, SC. P58-64.

Larrea, E.S., Peet, M.M. and C.D. Harlow. 2006. Optimizing Substrates for Organic Tomato Transplant Production Part One: Seed Germination. In Review.

Larrea, E.S., Peet, M.M. and C.D. Harlow. 2006. Optimizing Substrates for Organic Tomato Transplant Production Part Two: Seedling Growth. In Review.

Larrea, E.S., Peet, M.M. and C.D. Harlow. 2006. Optimizing Substrates for Producing Tomato Transplants Utilizing Organic Practices: Seedling Germination, Growth, and Nutrition and Media Characteristics in Commercial and Custom Blended Mixes. Poster Presentation. 27th International Horticultural Congress. August 13-19, Seoul Korea.

Project Outcomes

Project outcomes:

Results of this research indicate that grower blended organic potting mixes perform as well or better than many commercial organic mixes that are available for transplant production. Careful management of these mixes is necessary, however, as organic components incorporating into the mixes can be quite variable and seed germination may be reduced. Testing of materials is recommended prior to use. Careful management of water is also necessary to provide adequate germination conditions and in order to avoid unnecessary leaching nutrients incorporating into the potting mix. The use of vermiculite to cover seeds is recommended in order to improve seed germination. While sufficient phosphorus was provided in several of the mixes tested, nitrogen and potassium additions would be needed to produce healthy transplants for more than the first 2 – 3 weeks.
Several graduate students who are beginning research in this area have contacted us for guidance on their projects.

Farmer Adoption

A number of organic growers in North Carolina are using potting mixes produced on-farm to produce transplants, however the number has not been quantified to this point. This research was included in grower presentations on organic greenhouse production given by Peet in Jackson Mississippi, Arizona, and Nashville Tennessee. As stated in the Outcomes section of this report, the organic materials utilized in this study can be quite variable, and testing organic materials as waste samples to quantify the nutrients provided and salt levels is desirable. Additionally, careful watering is needed to avoid leaching of pre-plant nutrients from the potting mix and to provide adequate conditions for seed germination. The use of vermiculite to cover seeds appears to aid in improving seed germination in grower blended potting mixes.


Areas needing additional study

Studies investigating the improvement in seed germination through the utilization of vermiculite to cover seeds when utilizing grower blended potting mixes would be beneficial. Additional research on the properties of organic materials that affect germination, specifically potential compounds that may inhibit seed germination, would also be beneficial.

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.