Organic fertilization for greenhouses

2011 Annual 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:

Organic fertilization for greenhouses


Production of vegetable and herb transplants is an integral part of USDA certified organic farm and greenhouse operations. Little information is available on organic fertilization for greenhouse production systems. Preplant incorporated and liquid organic fertilizers are being evaluated as sources of nutrients for seedling and potted plant production in soilless potting mixes. Nitrogen and phosphorus availability are being determined by standard media extraction procedures and incubations of amended potting mixes. A wide variety of preplant-incorporated and liquid fertilizer formulations are being tested alone and in combinations to develop optimum strategies for fertilization and to maintain high productivity while meeting requirements for organic certification. Preliminary results conclude that N and P availability vary widely among organic fertilizers and are not related to the fertilizer guaranteed analysis. Outreach activities were not planned for 2011 as the project is still in the research and analysis stages. This project began during the summer of 2011 and will conclude in December 2012. This study is being conducted in the greenhouses and laboratories located at the University of Connecticut.

Objectives/Performance Targets

  1. To evaluate nitrogen and phosphorus availability from organic pre-plant incorporated fertilizers (PPIF) and organic liquid fertilizers (LF) in soilless potting mixes (SPM). To evaluate combinations of PPIF and LF for commercial production in SPM.


One greenhouse trial was conducted June-July 2011 where combinations of two LF at a single rate and five PPIF at three rates were compared for production of tomato and lettuce transplants. The transplants were grown from seed in 98-cell plug sheets using a commercial soilless potting mix with lime and wetting agent but no manufacturer-added fertilizer. Saturated media extracts (SME) were used to evaluate pH, electrical conductivity (EC) and nutrient concentrations of SPM prior to planting and in conjunction with plant harvest. Solution displacement (“pour-thru”) extractions were used for non-destructive monitoring of pH and EC and nutrient concentrations with growing plants. Concentrations of ammonium, nitrate and phosphate were determined using standard colorimetric methods. A chromometer (CR-400, Konica Minolta Sensing, Inc., Ramsey, NJ) was used to quantify observed differences in leaf coloration seen in digital photos at plant harvest. Values were recorded as lightness/measure of light to dark (L*), chroma value/color intensity (C*), and hue angle/actual color (H*).Shoot fresh weight was measured after detaching the shoot at the cotyledonary node. Plant shoot tissue was dried for 24 hr at 90 – 95 C and ground to pass a 20-mesh screen.

Preliminary results from the June-July greenhouse trial can be seen in the graphs included with the report. It was concluded that N and P availability vary widely among organic fertilizers and are not related to the fertilizer guaranteed analysis starting immediately after the media and fertilizer are mixed together. For example, SME extracts of SPM amended with a fertilizer containing 3% nitrogen contained 6 to 15 times more Nitrate-N than with fertilizers with 5% or 7% nitrogen. Similarly, P concentrations in SME extracts diverged widely among fertilizers. A fertilizer that supplied the lowest quantity of total P provided the highest concentration of SME-P.

Starting in October, the project focus shifted from greenhouse trials to laboratory trials due to lack of environmental control in the greenhouses during a major reconstruction project. New trials using short-term incubations were begun to obtain additional information on nutrient release. Using standard colorimetric methods, concentrations of ammonium, nitrate and phosphate are being determined in samples of growing media initially and after 1, 2, 5, and 7 days of incubation at 20 °C. Seven different preplant fertilizers at a single rate of 0.4 g nitrogen/liter of media were used. These were incorporated into separate batches of two different types of media, a peat and perlite mix and a mix containing peat, perlite, bark, and vermiculite. Also, incubations have been conducted using soil mixes that had a lettuce crop growing in them for 3.5 weeks since the presence of a crop increases biological activity and may impact mineralization. Samples from these trials are still being processed.
The graduate investigator dedicated about 35 hours per week to this project from June through August and about 15 hours per week from September to December to conduct trials, collect and process samples, and analyze the data. In addition, a student laborer provided 96 hours from October to December to assist with incubation trial setup, data collection, and sample processing. Plant tissue and aqueous media extracts from greenhouse trials conducted during the spring and summer of 2011 were processed, including grinding, ashing, and dissolving of plant tissue, and colorimetric analysis of aqueous soilless media extracts. Data entry, statistical analysis, and graphing were also begun and will continue over the winter.

Impacts and Contributions/Outcomes

Since this project is still in the research and analysis stages, no outreach activities or publications were planned for 2011. Publications and presentations are planned for 2012. When complete, this project will benefit greenhouse growers, farmers, and extension personnel who are interested in improving their organic fertilization practices in potted plant and plug production. The work currently in progress is aimed at evaluating the rate of nutrient release from organic fertilizers in SPM and so growers can determine how PPIF and LF can best be combined.


Dr. George Elliott
Associate Professor
University of Connectiuct
1376 Storrs Road, Unit 4067
Storrs, CT 06269-4067
Office Phone: 8604861938