Hydroponic Lettuce Production in Phenolic Foam

By David Kuack and Vijay Rapaka

Inorganic growing substrate materials that have been used for hydroponic crop production include rockwool, perlite, vermiculite, expanded clay and pea gravel. Phenolic foam is a relatively new inorganic substrate that offers many desirable production properties.

Phenolic foam cubes

The Oasis Horticube Growing Medium is a sterile phenolic foam. Like rockwool, Horticubes have no cation exchange capacity, no buffering capacity and no initial fertilizer charge.

Horticubes come in a variety of sizes, including: 1-inch Thin-Cut (276 cubes/sheet), 1-inch (162 cubes/sheet), 1¼-inch (104 cubes/sheet) and 1½-inch (50 cubes/sheet). All of the sheets measure 10- by 20-inches and fit into standard 1020 trays. Each sheet is pre-scored on the bottom and top to allow for easy separation of the cubes at transplant.

The 1-inch Thin-Cut Horticube was developed primarily for hydroponic lettuce production. This high density configuration accommodates 276 seeds in a standard 1020 tray. Each cube is pre-punched with a dibble hole that is uniform in depth and has center to center spacing. This allows for the use of automated seed sowing equipment. Horticubes work equally well with both nutrient film technique (NFT) and a raft (float) growing system.

Using the North Carolina State University porometer, the 1-inch Thin-Cut Horticube has a water-holding capacity of 80 percent and air porosity of 20 percent. A comparable rockwool product, which has grooves at the bottom of the sheet, has a water-holding capacity of 60 percent and air porosity of 40 percent.

Sowing the seed

Horticubes can be seeded dry and do not need to be watered prior to sowing the seed. Seed can be sown using a vacuum seeder or manually by placing the seed in the dibble holes. The specially designed hole is tapered to ensure the seed sets properly in each cube.

After the seed is sown irrigation can be done manually with a hose and water breaker (i.e., wide fan nozzle) or automatically by passing the Horticube sheets through a watering tunnel. The sheets should be thoroughly saturated.

Water-holding capacity

A single Horticube sheet holds about 4 liters (1 gallon) of water. However, it takes more than 4 liters of water to ensure total saturation of the foam because of water channeling through the dibble holes and grooves on the Horticube sheet. To ensure thorough saturation about 10 liters (2.6 gallons) should be applied so that the water pours through the bottom of the sheet. As rule of thumb, water each sheet for 2 minutes at regular tap water pressure.

If the seedlings are going to be irrigated/fertigated by overhead irrigation, place the Horticube sheets in solid bottom trays with drain holes. Never use a solid bottom tray without drain holes. If sub-irrigation is going to be used, place the Horticube sheets in trays that have solid sides and web bottoms.

Like rockwool, the Horticube sheets can be rewetted. Both of these media should not be allowed to go completely dry between waterings.

Once the Horticubes are thoroughly saturated, the cubes should stay moist during the course of germination.

Lettuce seed sown in Horticubes does not have to be topdressed with vermiculite. The seed also does not require a dark treatment for germination. The best germination usually occurs when the Horticube temperature is below 70°F. The seed usually germinates in two to three days.

Watering and fertilizing seedlings

Generally lettuce seedlings in Horticubes do not require misting or watering during germination. However, on bright hot summer days consider a brief misting (5 seconds once a day) on Day 2 and Day 3. Apply clear tap water with no fertilizer.

Once the lettuce seed has germinated the mist frequency needs to be adjusted. A typical misting program consists of starting from Day 4 to Day 7, three times a day for 10 seconds. From Day 7 to finish, mist four times a day for 10 seconds. If the seedlings are going to be either hand-watered or on a sub-irrigation system, irrigate only once a day.

Start fertilizing the seedlings on Day 4. All of the different nutrient formulations developed for lettuce production will work with Horticubes. Growers should customize their specific formulations depending on water supply, lettuce cultivars, production system, climate and season. The nutrient solution pH should range from 5.5 to 6. The recommended electrical conductivity during propagation is 1.0 mS/cm. The recommended electrical conductivity during production is 1.2 to 2.2 mS/cm.

Transplanting seedlings

Lettuce seedlings should be ready to transplant 10 to 14 days after sowing depending on seasonal climate conditions. During summer months it takes about 10 days from sowing to transplant and during winter months it takes about14 days. The criteria for transplant are development of two true leaves and root penetration through the bottom of the Horticubes.

At transplant the pre-scored sheets can be easily separated into individual cubes. The easiest way is to break the individual cubes from the top down along the scoring.

Production and harvest

Lettuce seedlings in Horticubes transplanted into a NFT or raft system perform equally well. During production the recommended electrical conductivity of the nutrient solution should be 1.2 to 2.2 mS/cm. With a NFT system the water flow rate should be 1 to 1.2 liters per minute. Analysis of the nutrient solution should be done on a regular basis in order to make formulation adjustments.

Hydroponically-grown lettuce produced in Horticubes can be harvested with the root system intact. Leaving the root system intact can help to extend the shelf life of the lettuce.

For more: Smithers-Oasis North America; (800) 321-8286; www.oasisgrower.com or Hort Americas, LLC at +1 469 532 2383.

David Kuack is a freelance technical writer in Fort Worth, Texas, dkuack@gmail.com. Dr. Vijay Rapaka is Manager—Grower Research, Smithers-Oasis Co., Kent, Ohio, vrapaka@smithersoasis.com.

Visit our corporate website at http://www.hortamericas.com