Organic fertilization for greenhouses

Final Report for GNE11-020

Project Type: Graduate Student
Funds awarded in 2011: $12,556.00
Projected End Date: 12/31/2012
Grant Recipient: UCONN
Region: Northeast
State: Connecticut
Graduate Student:
Faculty Advisor:
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Project Information


Certified organic production in greenhouses is hampered by lack of scientifically based information regarding nutrient management. A plethora of organic fertilizers are available, but little is known about nutrient availability from these materials. Furthermore, even less is known about the release of organic nutrients in soilless potting mixes (SPM), which are widely used for greenhouse production. This project was developed to evaluate representative organic fertilizers as nutrient sources for production of seedling ‘plugs’ for transplant, either to the field or for growing on in containers. Tomato seedlings grew better with a combination of preplant incorporated fertilizers (PPIF) and liquid fertilizer applied at each irrigation (LF) than with either alone. Over the range of concentrations tested, response to increasing PPIF fit a quadratic model, while response to LF was linera. Combinations of PPIF and LF were evaluated for tomato, lettuce and mizuna. Different combinations produced maxium growth for different species. However, some combinations were consistently among the best for all species. Saturated media extracts (SME) of SPM prior to planting showed significant differences among PPIF in ammonium (NH4), nitrate (NO3) and phosphate (PO4) availability. SME of SPM sampled after cropping showed significant differences among PPIF and LF in residual NH4, NO3 and PO4. SPM amended with PPIF and incubated without plants showed significant differences among PPIF in NH4 and PO4 release, and in conversion of NH4 to NO3. Nitrification was evident in a SPM containing sphagnum peat moss, bark and perlite but not in a SPM containing only peat and perlite. Selection of organic fertilizers for greenhouse production should be based on general crop nutrient requirements and fertilizer nutrient release characteristics as affected by SPM biological properties. Further research will be required to evaluate nutrient release and nitrification from LF and nutrient-rich bulk components of SPM.


Consumer demand for local, sustainably produced food has led to increased interest in certified organic production in controlled environments. The USDA National Organic Program (NOP) has market recognition and is the most widely adopted form of third-party certification for production. Based on personal observation in Pennsylvania, New Jersey and Connecticut, greenhouses are an integral part of diversified farm operations in the Northeast region. Vegetable farms, especially those using organic practices, often grow their own seedling ‘plugs’ for transplant into field production or for retail sale at roadside stands and farmers’ markets. Wholesale purchase of vegetable and herb transplants, especially certified organic transplants, is usually impossible due to high cost and limited supply. Leanne Pundt, Extension Educator for the University of Connecticut (personal communication), confirms that traditional farming operations in Connecticut are eyeing greenhouse production of potted plants, especially organic edibles, as an innovative market opportunity, while established greenhouse operations with small acreage are experimenting with alternative crops such as cool-season greens to replace unprofitable fall poinsettia production.
Unfortunately, there is a lack of scientific information available regarding the use of organic fertilizers in soilless potting media (SPM). SPM are generally preferred for greenhouse production and are acceptable under the NOP as long as the mix is formulated with approved materials. A survey of organic greenhouse growers in Maine showed that 55% considered management of fertility, potting mix, and pH to be a production challenge for organic ornamentals. Twenty-seven percent said it was their greatest production challenge. Only insect and disease management caused more concern. The typical organic grower surveyed grew a combination of potted vegetables, herbs, and ornamentals either for direct retail sale on-site or for transplant into field production (Burnett and Stack 2009). The prohibition of any form of synthetic fertilizer under the National Organic Program requires a radical change from conventional fertilization practices and currently requires much experimentation by growers to provide adequate nutrition for high productivity (Mattson and Burnett 2010). Conventional greenhouse producers have available a wide array of both soluble and controlled-release fertilizers that can be readily applied to provide nutrients as needed. There is also a plethora of information on delivery systems and nutritional monitoring practices (Dole and Wilkins 2004). Conventional greenhouse growers in Maine see fertilization to be their primary barrier to converting to organic ornamental bedding plant production (Burnett and Stack 2009). Strictly organic fertilizers contain N solely in chemically reduced forms, whether relatively insoluble forms such as proteins, or soluble forms such as amino acids or ammonium. In field soil used for crop production, conditions are generally favorable for mineralization, i.e., the hydrolysis of complex reduced forms to ammonium, and subsequent oxidation of ammonium to nitrate. The capacity for mineralization and nitrification in SPM is relatively low compared to soil and is highly variable among different types of SPM (Elliott 1986). However, the NOP currently allows use of natural sodium nitrate (“Chilean nitrate”) in limited quantities as a N fertilizer. Phosphorus in organic fertilizers may be derived from organic matter or from natural minerals, as long as it is not treated to solubilize phosphate. However, the NOP allows use of phosphoric acid in limited amounts to stabilize fish emulsion fertilizers.
The goal of this project is to evaluate organic fertilizers as sources of nutrients for greenhouse production of seedling transplants in containers with soilless potting mixes.

Burnett, S.E. and L.B. Stack. 2009. Survey of the research needs of the potential organic ornamental bedding plant industry in Maine. HortTechnology 19:743?747.
Mattson NS, Burnett S. 2010. Organic Substrates and Fertilizers. Greenhouse Grower. July 2010. viewed 2011-02-25.
Elliott GC. 1986. Urea hydrolysis in potting media. J. Amer. Soc. Hort. Sci. 111:862 866.

Project Objectives:

Objective 1: Evaluate pre-plant incorporated (PPIF) and liquid (LF) organic fertilizers as sources of nutrients for greenhouse production of seedling transplants in containers with soilless potting mixes (SPM). Objective completed June 2011 to May 2012. Growth of seedling basil, lettuce, mizuna and tomato was evaluated in response to PPIF and LF at different rates, alone and in combination.
Objective 2: Evaluate nutrient availability from organic fertilizers. Objective completed June 2011 to December 2012. Saturated media extracts (SME) from potting mixes used for growth trials were analyzed for ammonium (NH4), nitrate (NO3) and phosphate (PO4) to evaluate both initially available and residual nutrients after cropping. Samples of SPM amended with different PPIF were incubated and analyzed for NH4, NO3 and PO4 to evaluate release of nutrients.
Objective 3: Dissemination of knowledge obtained. A poster summarizing this project was presented at the 1st Northeast Organic Research Symposium held in conjunction with the Northeast Organic Farming Association of New York’s Winter Conference in Saratoga Springs in January 2012. These two events attracted a total of about 1400 farmers, gardeners, and others interested in organic farming. A summary of the poster was also published in the conference proceedings. Information from this project was used to update the University of Connecticut’s Cooperative Extension IPM website. Additional presentations and publications are anticipated.


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  • Dr. George Elliott


Materials and methods:

General procedures
Plant growth trials were conducted at the University of Connecticut (UConn) Floriculture greenhouse range. The greenhouse section used was devoted to organic production for the duration of this project. The covering is single-layer glass. The glass was coated with shading compound from May to September. Heat was supplied by steam, with a thermostat control set at 60 F. Passive ventilation was supplied by manually controlled ridge and side vents, opening at about 75 F. Active ventilation was provided by sidewall fans, controlled by a thermostat set at about 80 F.
All production inputs were compliant with the National Organic Program. Seeds were certified by the Maine Organic Farming and Gardening Association Certification Services. Commercial fertilizers and potting mixes were certified by the Organic Materials Review Institute.
Potting mixes were obtained from commercial distributors. Exact composition of commercial mixes is proprietary information. Berger OM2 Organic Germinating mix contains fine peat moss, fine perlite, fine vermiculite, dolomitic limestone, calcitic limestone and organic wetting agent. Sunshine #2 Natural & Organic contains sphagnum peat moss, perlite, dolomitic limestone and an organic wetting agent. Fafard Natural & Organic FOF 30 contains sphagnum peat moss, bark, perlite, dolomitic limestone and an organic wetting agent.
For Trial 1, seeds were sown in packs of 6 cells about 1 cm3 cut from a 288 plug tray (Dillen Products, Akron OH). For Trials 2 and 3, packs were cut from 98-cell plug trays with cell dimensions 3.5 cm top diameter x 4.1 cm deep, volume 28 cm3 (Landmark Plastic Corp., Akron OH). Typically 3 to 4 seeds were sown per cell. Seeds were germinated in a mist bed with bottom heat at about 70 F. Packs were removed from the mist bed after emergence, and cells were thinned to single seedlings. Packs were placed on capillary mats to maintain uniform moisture in the medium and to avoid nutrient leaching.
Exact composition of commercial fertilizers is proprietary information, but product labels provide guaranteed analysis (N-P2O5-K2O) and sources of nutrients (Table 1). Granular products for preplant incorporation (PPIF) were comminuted in a blender prior to use. Liquid fertilizers (LF) were stored at about 40 F. Liquid fertilizer was applied at each irrigation beginning at the first true leaf stage. Seedling packs were placed in trays about 1 cm deep and subirrigated until the surface of the mix was visibly moist.
The saturated media extraction (SME) procedure (Warncke 1986) was used to extract nutrients from potting mixes prior to planting or after plants were harvested. Concentrations of ammonium, nitrate and phosphate were measured using standard colorimetric procedures (Chaney & Marbach 1962, Cataldo et al. 1975, Murphy & Riley 1962)

Plant growth trial 1. The test crop was tomato (Solanum lycopersicum) ‘Bellstar’. Berger OM2 mix was amended with MCGR to provide 0.2, 0.4, 0.6 0.8 or 1.0 g/dm3 total N. Liquid fertilizers GP and PIN were applied at 75 or 150 mg/L total N. Controls with no preplant or no liquid fertilizer were included.

Plant growth trial 2. Test crops were tomato ‘Bellstar’ and lettuce (Latuca sativa) ‘Red Saladbowl’. Berger OM2 mix was amended with NATU, MICR, MCGR, SSTA or SUS5 to provide 0.2, 0.4 or 0.6 g dm3 total N. A control with no PPIF was included. Liquid fertilizers GP and PIN were applied at 100 mg/L N for the first application and 150 mg/L for subsequent irrigations.

Plant growth trial 3. Test crops were mizuna (Brassica rapa) and lettuce ‘Red Saladbowl’. Berger OM2 mix was amended with NATU, MICR, MCGR, PAR4, SSTA or SUS8 to provide 0.6 g dm3 total N. Liquid fertilizers BOM, ESP, GP, NK and PIN were applied at 100 mg/L total N.

Incubation trial 1. Sushine #2 N&O (SUN) and Fafard FOF30 (FAF) potting mixes were amended with NATU, MICR, MCGR, PAR4, SSTA, SUS5 or SUS8 to provide 0.4 g/dm3 total N. Unfertilized controls were included. Aliquots approximately 15 cm3 were transferred to 50 mL centrifuge tubes with caps. Samples were incubated at 20 C. Immediately after mixing, and 1, 2, 3, 4 and 7 days thereafter, samples were extracted by shaking 10 min using 20 mL of 1 M potassium chloride. Extracts were filtered using medium retention filter paper. Ammonium, nitrate and phosphate were measured. In addition, standard 804 cell packs (ITML, Myers Industries, Akron, OH) with a volume of about 15 cm# per cell were filled with amended mixes and sown with lettuce ‘Red Saladbowl’. Liquid fertilizer was applied as PIN at 150 mg/L total N. After 6 weeks of growth, plant shoots were harvested and the potting mix shaken free from the roots and mixed. Samples of cropped media were incubated and extracted as for the uncropped media.

Incubation trial 2. Potting mixes and fertilizers were the same as the first incubation trial. Pots 12 cm tall with a volume of 445 cm3 were filled with mixes. Mixes were wetted up by application of 50 mL aliquots of deionized water until leaching was observed. At intervals of 1, 4, 8, 11, 15, 18, 22 and 25 days thereafter, mixes were leached with 250 mL deionized water applied in successive 50 mL aliquots. All leachate was collected, weighed to estimate volume, and filtered with medium retention filter paper. Pots were covered with plastic film wrap and incubated at 20 C between leachings. Ammonium, nitrate and phosphate concentrations were measured and the quantity of each was calculated as the product of concentration and volume.

Data were analyzed with SAS 9.2 and 9.3 using PROC MIXED and PROC GLM.

Cataldo, D.A., M. Haroon, L.E. Schrader, V.L. Youngs. 1975. Rapid colorimetric determination of nitrate in plant tissue by nitration of salicylic acid. Commun Soil Sci Plant Anal 6:71 80
Chaney, A.L., E.P. Marbach. 1962. Modified reagents for determination of urea and ammonium. Clin Chem 8:130 132.
Murphy, J. and J.P. Riley. 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27:31 36.
Warncke, D. 1986. Analyzing greenhouse growth media by the saturation extraction method. HortScience 21:223.

Research results and discussion:

Plant growth response
Plant growth trial 1. Maximum growth was obtained with combinations of PPIF and LF (Figure 1). Response to PPIF fit reasonably to second order polynomial, with maximum growth at 0.4 to 0.6 g/dm3 total N. Response to LF was linear, with PIN producing significantly greater shoot fresh weight than GP.

Plant growth trial 2. Over all treatments, tomato growth was significantly affected by PPIF type and rate, but not by LF (Figure 2). Shoot fresh weight increased with increasing total N for all PPIF, but over all rates SSTA produced slightly lower shoot fresh weight than other types. LF interacted with PPIF type and rate. With SUS5, NATU and MICR at 0.6 g/L total N, PIN was more effective than GP. Lettuce shoot fresh weight was significantly affected by PPIF type and rate as well as LF (Figure 3). With MICR, NATU, SSTA and SUS5, shoot fresh weight increased with increasing total N, but with MCGR it decreased linearly. Over all PPIF types and rates, PIN produced greater shoot fresh weight than GP, but maximum growth was obtained with SSTA or SUS5 at 0.6 g / dm3 total N with no difference between PIN and GP.

Plant growth trial 3. The response to PPIF was similar for both crops, as MICR, NATU and SSTA produced the largest plants, although not significantly greater than SUS8 for mizuna (Table 2). PIN produced the largest plants for lettuce, while PIN and NK produced the largest plants for mizuna. Interactions of PPIF and LF were significant, and differed between species. With lettuce, maximum shoot weight was obtained with MICR, NATU, SSTA or SUS8 plus PIN. With mizuna, maximum shoot weight was obtained with MICR or SSTA plus NK.

Nutrient availability
Plant growth trial 2. Initial SME showed distinct differences among PPIF (Figure 4). Ammonium increased with rate for all products, but MICR and SSTA were higher than MCGR, NATU and SUS5. Nitrate was not detected with MCGR, NATU or SUS5, at very low levels with SSTA but at 15-32 mg / L with MICR. Phosphate increased with increasing PPIF rates, with SSTA, SUS5 and MICR producing much higher concentrations than MCGR or NATU. The phosphate data were recalculated on the basis of total P provided, since the N:P ratio of the PPIF varied from about 2:1 for NATU to 16:1 for SSTA (Figure 5). The relationship between total P incorporated in the mix and reactive P in SME varied greatly among PPIF, as SSTA, the fertilizer that provided lowest quantity of total P had the highest concentration of P ins SME, while NATU, which provided the highest quantity had the second lowest concentration.. For both crops, residual ammonium levels in SME of potting mix sampled after plants were harvested were very low and unaffected by PPIF or LF treatments (data not shown). Nitrate levels were unaffected by treatment in the potting mix cropped with tomatoes (data not shown). With lettuce, residual nitrate-N was very low, but increased from 6.5 to 13.9 mg/L as PPIF increased from 0.2 to 0.6 mg/L N. Phosphorus levels were affected by both PPIF and LF, with similar results for the two crops (Table 3). However, differences were small. Accumulation of phosphate in the control with no PPIF shows that LF must have been contributing significant amounts of P to the potting mix.

Plant growth trial 3. Initial SME showed significant differences among PPIF consistent with the previous trial (Table 4). Ammonium concentrations were higher with MICR and SSTA than the other fertilizers. Nitrate was detected with MICR but was below the effective limit of detection with other fertilizers. Phosphate concentrations were higher with MICR and SSTA than the other fertilizers. Residual nutrient concentrations were affected by both PPIF and LF and were generally consistent between the two crops. (Table 5). Ammonium was much higher with ESP than other LF, and was very low with PIN and NK. Nitrate was higher with MICR, PAR4 and SSTA than other fertilizers in potting mix cropped with lettuce, but greater with MCGR and PAR4 with mizuna. BOM and NK had the highest nitrate concentration with both crops. Although the effect of PPIF on phosphate was highly significant, the differences were small. The effect of LF was more pronounced. Phosphate was highest with ESP and GP, and very low with BOM and NK. These results indicate that LF contributed most of the residual P, as P was depleted with the two LF fertilizers that did not provide it.

Incubation trial 1. Ammonium and phosphate accumulation differed among fertilizers but was similar between the two uncropped potting mixes (Figures 6 and 7). Ammonium concentrations were initially highest with MICR and SSTA. With the other fertilizers, ammonium levels increased greatly after 2 or 3 days incubation. After 7 days, differences persisted but the range of values was much reduced. Nitrate was not detected in either potting mix (data not shown.) Cropping affected nutrient accumulation differently in the two potting mixes. Ammonium levels were very low and unaffected by fertilizer in FAF, but ranged from moderate to very high with significant differences among fertilizers in SUN (Figure 8). In particular, NATU, MCGR, PAR4 and MICR accumulated ammonium-N concentrations in excess of 100 mg/L after 7 days of incubation. Increasing ammonium-N with incubation indicates continued mineralization of organic N from the nutrient sources. Nitrate was present in cropped samples (Figure 9). Concentrations were much higher in FAF than SUN. Differences among fertilizers in FAF may reflect differences in nitrification. Differences among fertilizers and with time in SUN are not significant, as all values are near or below the effective limit of detection for the assay. Unfertilized controls had very low concentrations, indicating that LF during cropping did not contribute significant amounts. Phosphate concentrations in cropped potting mixes were slightly lower than uncropped (Figure 10). Differences among fertilizers were not consistent between FAF and SUN. With SUN, the unfertilized control had very low phosphate-P levels, but with FAF concentrations ranged from 10 to 27 ppm. Evidently LF during cropping had different effects in the 2 mixes.

Incubation trial 2. Cumulative leaching of ammonium differed among fertilizers and between potting mixes (Figure 11). About twice as much ammonium leached from SUN as from FAF. Ammonium leaching from SUN was linear over time with all fertilizers. From FAF, ammonium leaching was initially rapid then leveled off with MICR, NATU and SSTA. Nitrate was not detected in leachate from SUN (data not shown), but significant quantities on nitrate leached from FAF (Figure 12). Cumulative leaching differed among fertilizers. The sum of nitrate plus ammonium leached from FAF was similar to ammonium leached from SUN. Phosphate leaching differed between mixes (Figure 13). Unaccountably, phosphate leaching was consistently observed with the unfertilized control for FAF. Phosphate leaching was otherwise similar in the 2 mixes. Leaching patterns and total quantities differed among fertilizers. With SSTA, leaching was initially rapid but then leveled off, whereas with MICR leaching was linear over time. MICR and SSTA leached the largest quantity of phosphate-P in both mixes, whereas MCGR and PAR4 were scarcely different from the unfertilized controls.


Selection of fertilizers to provide nutrients for production of crops in soilless potting mixes is a vital management decision for growers. Growers who use conventional soluble fertilizers can rely on label information describing guaranteed analysis and sources of nutrients in choosing among different products. For organic growers, the decision is made more complex because nutrient availability is not related to guaranteed analysis, and different products have radically different sources of nutrients. Research trials cannot evaluate the plethora of organic fertilizer products available commercially, much less the unique, locally available sources of nutrients used by organic growers.
The results of these trials indicate that some products are consistently effective as sources of nutrients for seedlings grown in soilless potting mixes. Furthermore, supplying nutrients from a combination of PPIF and LF is clearly more effective than either alone. Specific combinations of PPIF and LF are most effective for different crops.
Nutrient availability differs greatly among organic fertilizers. Preplant SME indicated that ammonium and phosphate were more readily available in some products, e.g., SSTA or MICR, than others, e.g., MCGR or PAR4. Guaranteed analysis is of little use in comparing organic fertilizers, especially for available phosphorus. Phosphate availability as indicated by preplant SME was not related to total phosphorus provided by fertilizer. Differences in phosphate availability among PPIF as well as plant growth response to specific PPIF – LF combinations suggest that phosphorus may be a limiting nutrient for plant growth with some organic fertilizers. Post-harvest SME showed little difference among PPIF in residual nutrients, but large differences attributable to LF. Selection of complementary PPIF and LF should be based on evaluation of the ability of PPIF to supply readily available nutrients initially and the ability of LF to supply supplemental nutrients to support continued growth. Incubations revealed that differences in nutrient availability among fertilizers is also affected by potting mix. In particular, nitrification was evident in a potting mix containing sphagnum peat moss, bark and perlite, but was not detectable in a soilless potting mix containing only sphagnum peat moss and perlite. Furthermore, some fertilizers supported much higher rates of nitrification than others.
The impact of differences in nutrient supply is crop-specific, as different crops have different nutritional requirements. For example, in the second growth trial SSTA produced lower tomato shoot fresh weight than other PPIF, but with lettuce it was not significantly different from SUS5 in producing the largest plants. Lettuce response to increasing rate of PPIF was not affected by LF, but tomatoes produced greater shoot fresh weight with PIN than GP. In this case, the principle difference between the two LF is that PIN provides approximately 1/3 of the total N in the form of sodium nitrate, while nitrate is less than 0.02 percent of the nitrogen in GP. In the third growth trial, lettuce and mizuna both produced maximum shoot fresh weight with MICR, NATU and SSTA as PPIF. However, lettuce produced the greatest shoot fresh weight with PIN, which has a 3-1-1 analysis. Mizuna produced the greatest shoot fresh weight with PIN or NK, which has an analysis of 10.7-0-2. These results indicate that the P supplied by PIN was important for lettuce, but that P supplied by PPIF was adequate for mizuna.

Research conclusions:

Potential outcomes for this project include: improved efficiency of nutrient management in certified organic production. This research will provide technical support for advisors and fertilizer manufacturers. No data are available regarding these outcomes.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary

Education/outreach description:

A poster summarizing this project was presented at the 1st Northeast Organic Research Symposium held in conjunction with the Northeast Organic Farming Association of New York’s Winter Conference in Saratoga Springs in January 2012. These two events attracted a total of about 1400 farmers, gardeners, and others interested in organic farming. A summary of the poster was also published in the conference proceedings. Information from this project was used to update the University of Connecticut’s Cooperative Extension IPM website ( Additional presentations and publications are anticipated.

Project Outcomes

Project outcomes:

The most effective sources of fertilizer should be least costly per unit nutrient. Thus, the information provided should assist growers in selecting the most cost-effective fertilizers. Furthermore, effective nutrient management results in greatly increased productivity with reduced environmental impact. No data are available on the economic impact of nutrient management for certified organic production.

Farmer Adoption

No information is available regarding adoption of scientifically based nutrient management practices by organic farmers.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

Further information is needed regarding the following issues for organic nutrition : 1) nutrient release from LF, especially since the National Organic Program has announced that the use of Chilean nitrate will no longer be permitted for certified organic production. New LF fertilizers are available to replace products that contained Chilean nitrate. 2) The effect of fertilizer and media components biological process such as nitrification and phosphate solubilization. 3) nutrient availability from nutrient-rich bulk potting mix components such as anaerobically digested dairy. These topics are under active investigation in follow-up research from this project.

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.