A Study of Quantitative Lamp Spectra and Hydroponic Lettuce Growth

Key Participants

Tim Madden, President Biodynamicz, LLC 526 S. Main St. Akron, OH 44311

Roger Buelow, President Light for Life, rbuelow2@gmail.com

Dr. Richard Hansler, Lighting Innovations Institute, John Carroll University; Cleveland, Ohio

Wil Hemker, The University of Akron Research Foundation (UARF); Akron, Ohio contact:

hemker@uakron.edu

Ruth Zito, Horticulturalist NuLight ,LLC, ruthzito@gmail.com

Background:

Light has three principal characteristics: quantity, quality, and duration that affects plant growth.

Light quantity refers to the intensity or concentration of sunlight and varies with the season of

the year. The more light a plant receives, up to a saturation point, the higher the capacity that

plant has to produce plant mass through the process of photosynthesis. Light quality refers to the

color or wavelength reaching the plant surface and can be enhanced through the use of

supplemental lighting. A number of different light sources exist for that purpose. Incandescent

light is high in the red or orange range but generally produce greater heat output. Fluorescent or

cool-white light is high in the blue range of light quality and is used to encourage leafy growth.

These lights are excellent for starting seedlings. Fluorescent "grow" lights have a mixture of red

and blue colors that attempts to imitate sunlight as closely as possible. Solid state light emitting

diode or LED lights have a high efficacy (µmol per Joule), broad spectrum accuracy (in terms of

color), low thermal output and this lighting technology is well-aimed to produce an ideal

environment for growing plants anywhere. Light duration or photoperiod refers to the amount of

time that a plant is exposed to sunlight. Year round production of crops is possible by utilizing

supplemental lighting. To extend day length in crop growing the interest in studying artificial

lighting has grown. This study focused on the comparative characteristics of lamp spectra

qualities of commercial state of the art agriculture lamps of high pressure sodium (HPS),

inductive fluorescence and LED lamps.

Objective:

To compare a reference greenhouse food plant crop’s productivity when grown in like

hydroponic environments when the artificial light source is varied. The commercial light sources

are: Gavita Pro 1000 DE US (HPS lamp), IGrow ED-400W-GB (Inductive Fluorescent),

Heliospectra AB LX 60 Series LED, and Lumigrow Pro 325W (LED lamps). The outcome was

to characterize lamp spectra qualities and plant crop productivity for each commercial agriculture

lamp.

Materials & Methods:

The hydroponic lettuce growing was conducted in a controlled indoor environmental lab, using

Nutrient Film Technique (NFT) hydroponic channels to grow out the lettuce crop. Nutrient Film

Technique is a growing system that constantly re-circulates a continuous stream of water through

grade PVC thermoplastic channels which delivers nutrients to the crop (Figure 1). A baby gem

romaine lettuce variety (Lactuca sativa: v. Green Star), was used with a targeted maturation size

of 170 grams or six ounces. 6 heads of lettuce per channel were grown out for a forty five (45)

day duration for each lamp test condition. The geometrically balanced fertigation system was set

at a nutrient flow of 8-10 ounces per minute which is the common industry standard. Interference

from ambient light sources was minimized by using separate isolated room enclosure grow zones

for each of the four lamps tested. Accurate spectra quantifications were taken for each of the

lamp evaluated. Each light fixture was placed 5 feet above the surface of each NFT channel,

supporting each lettuce plant.

Seeds were germinated in pre-moistened Oasis® expanded foam soil-less media XL 1”X 1”X 1

½” cubes in dilute (600ppm) nutrient solution for four (4) weeks (Figure 2). Seedlings were then

transplanted into the NFT channels (figure 3) and grown out for the full forty-five (45) day

period. Each grow tent temperature was maintained between 68 and 70 degrees Fahrenheit with

40 percent humidity and CO2 at ambient levels. The nutrient solution used for ideal plant health

and growth was supplied by Smithers-Oasis Co. CWP indicators and probes (CWP Instruments

by Marco Industries) were installed in each solution tank to continuously measure PH, PPM, and

solution temperature. The pH was maintained at 6.0 by the addition of dilute phosphoric and

citric acid. PPM was maintained bet 800-825 by the addition of concentrated fertilizer solution

and source water. Solution tanks were manually mixed with R.O. (reverse osmosis) water and

fertilizer. Tanks were cleaned weekly and fresh nutrient solution was utilized.

Designed Outcome:

For each lamp tested the following response variables were measured; plant growth rate was

measured as fresh wet weight per head at harvest time, and dry weight. Plant quality was

observed throughout the growing process on a subjective basis, plant sugar quantity (brix) and

sensory qualities (bitterness, appearance, flavor, texture and overall acceptability) were

compared after harvest for each lamp evaluated. The sensory panel consisted of 5 members and

the sensory qualities were based on the Qualitative Descriptive Analysis method (QDA-see

Table 1) with ratings as follows: 0-3 rating (Poor /Unacceptable) 4-6 (Fair /color, crispness,

sweetness), 7-8 (Good/color, crispness, sweetness, liveliness) 9-10 (Excellent/color, crispness,

sweetness, very fresh, clean taste). The Refractometer Brix Scale for Lettuce is as follows: 4

Poor, 6 Average, 8 Good, 10 Excellent. (See below Table 2.).

Table 1

Attributes Definitions

Color Bright green color, characteristic of lettuce quality

Freshness Liveliness and brightness before losing vigor

Brightness Good condition and a feeling of freshness

Texture Crisp, crunchy, soft-buttery ,soft tender

Taste smooth, bitter, sweet, succulent, mild

Table 2. Lettuce (Lactuca sativa: v. Green Star) plants qualities following 45 days grow period

under four (4) different light sources

Light Source Fresh Weight Dry Weight % Chg Brix Taste/Texture

Heliospectra 6.0 oz. 2.2 oz. 63.3% 6 9

IGrow 6.2 oz. 2.1 oz. 66.1% 3 7

Lumigrow 4.8 oz. 1.9 oz. 60.4% 4 6

Gavita 10.0 oz. 2.6 oz. 74.0% 3 7

Timeline

The lettuce growing was conducted over a 45 day period beginning on December 18, 2014 and

completion on January 30, 2015.

Results

After forty-five (45) days the observed growth rates for the lettuce varied between each light

source. The Heliospectra 600w, Lumigrow 325w and Gavita DE 1000w had measurable higher

biomass accumulation while the IGrow 400w Bloom had a lower biomass but had visible larger

leaf, was less compact and a broader surface. The plant’s morphology had two distinct

characteristics. Lumigrow, Gavita and Heliospectra had greater density and were more compact

with a thicker leaf as compared to the IGrow. The IGrow had greater leaf and stem stretching

with decreased leaf thickness. Increased lateral branching was more prevalent with the

Heliospectra, Lumigrow and Gavita lights. Brix testing results showed that Heliospectra had a

rating of 6 which is in the average range. Lumigrow, IGrow, and Gavita were below average

rating for the Brix test. The sensory qualities for Heliospectra ranked the highest with deep dark

green coloring of the leaf, sweet taste, crisp texture, and good liveliness. IGrow ranked average

with light green leaf coloring, soft tender leaf texture and mild taste, Gavita had good leaf

coloring but bitter taste and average liveliness and last was Lumigrow with dark green leaf color,

crisp texture but less than average bitter taste.

Conclusion:

This comparative leafy green lettuce crop growing study found that the Heliospectra AB LX 60

Series LED lamp grew the tastiest and most attractive lettuce for all four lamps having a similar

average sugar (brix value).

Pictures:

Figure 1. NFT Channel System

Figure 2. Lettuce seed germination flood table system

Figure 3. Growing channel showing

seedlings after transplant

Heliospectra

Heliospectra Week 1

Heliospectra Week 3

Heliospectra Week 6

Lumigrow

Lumigrow Week 1

Lumigrow Week 3

Lumigrow Week 6

IGrow

Igrow Week 1

Igrow Week 3

Igrow Week 6

Gavita

Gavita Week 1

Gavita Week 3

Gavita Week 6