Construction of a non-recirculation hydroponic unit for commercial lettuce production

Summary

Hydroponics is the most common method of soil-less culture (growing agricultural plants without the use of soil), which includes growing plants either on a substrate or in an aqueous medium with bare roots. Non-circulating hydroponic methods, importantly, do not require electricity or a pump. With the method presented in this document, the entire crop can be grown with only an initial application of water and nutrients. No additional water or fertilizer are needed. The crop is usually terminated when most of the nutrient solution is consumed. This document provides a detailed description of the construction of the growbeds and grow sheets used in the non-circulating hydroponic unit for commercial lettuce production.

Description

1. Introduction

For an introduction of the concepts of hydroponic method please refer to http://teca.fao.org/read/8825 (“Simple non-circulating hydroponic method for vegetables”).

The concept of growing vegetables for home-garden use presented in this document can be expanded to commercial-sized growbeds which will be describe in the following. Hydroponic lettuce is grown in growbeds filled with nutrient solution (water plus a complete hydroponic fertilizer) instead of in soil (Fig. 1). 

A schematic drawing of a suspended-pot, non-circulating hydroponic system.

Fig.1.  A schematic drawing of a suspended-pot, non-circulating hydroponic system

Growbeds are situated in plastic-covered rain shelters or greenhouses with screened ends and sidewalls to protect against rainfall and large flying insects. The growbeds are filled with 5.7–7.8 litres of nutrient solution per plant prior to planting. Thus, a growbed designed to grow 50 heads of lettuce should have a liquid capacity of about 350 litres. The growbed dimension would be about 1.20 × 2.40 m, assuming a 12 cm depth. This is a convenient measurement, as many of the materials come in these sizes.

 

Fig. 2: Hydroponic lettuce growing in growbeds of non-circulating nutrient solution.

This document describes how to construct a non-recirculation hydroponic unit for commercial lettuce production.

For the management of these units, including the nutrient solution formulation, please refer to http://teca.fao.org/read/xxxx (“Managing a non-recirculation hydroponic unit for commercial lettuce production”)

Growbeds

Common growbed dimensions are 1.20×2.40 m and 1.20×4.80 m, but other dimensions may be used. The reason is because the polystyrene beadboard is commonly sold in this dimension.

TIP: Given that most people are unable to reach lettuce plants located more than 1 m away, use of growbeds wider than 1.80 m is not recommended.

TIP: Given that growbeds should be level to within 2 cm, the growbeds should not to be too long if the ground is uneven at all (for example, on a concrete slab). Smaller growbeds are easier to level if the ground is uneven (for example, on stones).  

Materials needed

  • wood lengths (5×15 cm, 2.40 m length)
  • wood lengths (5×15 cm, 1.10 m length)
  • Nails (8 cm) or screws (6 cm)
  • 1.20×2.40 m plywood sheet (1.3 cm or thicker) (Or:  recycled metal roofing, 1.20×2.40 m)
  • Black polyethylene sheeting
  • Wood pallets (used for growbed supporting structure)
  • Weed barrier fabric (> growbed size)

Alternative: As an alternative to wood, you can also use metal profiles (rectangular and cylindrical shaped) for the growbed support frame. The decision of which material to use can be based on availability and cost, noting that metal has a longer lifespan than wood.

Step-by-step description

1. Rectangular frame

A rectangular frame of 1.20 m wide × 2.40 m long × 14 cm high is constructed with 5×15 cm lumber by fastening either with 8 cm nails (or: 6 cm deck screws).

Fig. 3: Scheme of the rectangular frame

2. Placement

Growbeds can be place on the floor (Option A) or supported at waist-height (Option B). Option A is cheaper and simpler, but harvesting is less convenient and comfortable.

A: On the floor

Lumber frames (without any additional plywood or metal bottom) may rest directly on the floor (i.e. compacted soil, concrete floor, rooftop, patio, etc.) but make sure that no sharp objects are on the surface of the floor that could puncture the liner.

B: Optional: Growbed supporting structure

Growbeds can be constructed at a convenient working height (75–90 cm). A full 1.20×2.40 m growbed weighs more than 360 kg. Growbeds should be supported at least every 1.20 m on stacked concrete blocks or have a well braced lumber frame. You may construct a growbed support structure from cross-braced, upright pallets. Recycled metal roofing is then attached to this framework, making a tabletop (Figure 4). 

Fig. 4: Growbed support structure from cross-braced, upright pallets with recycled metal roofing as a table top (Waite Farm, Mt. View).

3. Growbed bottom

Fasten a 1.5 cm or thicker plywood sheet to the lumber frame. This becomes the bottom of the growbed. In place of plywood bottoms you can use recycled metal roofing, but take care to prevent sharp edges from cutting the plastic growbed liner. Alternatively, lumber frames without the bottom plywood sheet may rest directly on the metal roofing of the supporting structure without any attachment.

Lumber frames (without any additional bottom) may also rest directly on level ground. Optionally, weed barrier fabric should be placed under the frame to cushion against rocks and prevent weeds from penetrating the plastic liner (Figure 5).

Fig. 5: A lumber frame rests directly on weed barrier fabric placed on a level surface.

4. Black polyethylene sheeting

Growbeds are lined with a layer of black polyethylene sheeting with a thickness of 0.8–1.4 mm (Figure 6). It is recommended to use two layers as a failsafe to prevent leakage if the top layer rips.

If no black polyethylene sheeting is available, other colours can be used. However, clear plastic is problematic as it allows for algae growth which affects dissolved oxygen (DO) availability. Also, you should avoid clear construction grade polyethylene because sunlight deteriorates exposed plastic not protected by the growbed cover.

Fig.6: Growbed is lined with two layers of 6 mm black polyethylene sheeting

TIP: It is easier to lay and fit the two layers consecutively rather than both at once. Polyethylene rolls are typically sold in 3 m and 6 m widths, but it is easier to work with the 3 m width.

When cutting the plastic, allow for several inches of overhang on the ends and sides. Lay the polyethylene loosely in the growbed. Air pockets may develop if the plastic is fastened before water is added, and this often causes leaks. The plastic must be fitted snugly against the sides and bottom of the growbed because unsupported plastic sheeting is prone to leak. Preliminary fitting is accomplished by using the side of one’s hand (it may be helpful to imagine a slow-motion martial arts “chop”).

5. Add some water

Add about 5 cm of water. The water may cause the polyethylene to pull away from the sides of the growbed, so it is important to add the water before the polyethylene is stapled in place. The polyethylene is then given a final fitting to the growbed sides and bottom to ensure that it rests firmly against the lumber.

Fold and trim the plastic at the ends of the growbed. Use a staple gun to fasten the polyethylene to the outside frame rather than to the top of the growbed; stainless steel staples are preferred.

TIP: If a growbed leaks while a crop is growing, add a few handfuls of fine vermiculite to the growbed. Rub the vermiculite between your hands to mill it into very fine particles. The vermiculite can plug small holes and retard the leak. To repair a leaking growbed, a new sheet of plastic can be added over the two existing sheets after the crop is harvested.

Grow sheets

Materials needed

  • Sheets of 5 cm thick polystyrene (white beadboard, 32 kg/m3 density) or extruded polystyrene (Styrofoam™)

TIP: Extruded polystyrene should be used preferably to expanded polystyrene, assuming it is available and similarly priced. Over time, expanded polystyrene holds a lot of water which leads to algae growth and makes sheets more susceptible to breaking.

Alternative cover materials: Alternative options for growbed cover materials include plywood and plastic. Thin sheets of plywood (0.6 cm) are easier to cut holes, however, they are susceptible to break more easily under the weight of heavy plants such as tomatoes. Several growers have successfully constructed growbed covers by covering lumber frames with plastic weed barrier fabric; holes for the net pots are burned in the fabric with a hot pipe. A coat of white latex paint is recommended for plastic and plywood covers. This is especially true for dark-colored covers, which heat up in direct sunlight.

Step-by-step description

1. Sheet size

The growbed cover should be easily removable and fit loosely on top of the growbed. Sheets of 5 cm thick extruded (or expanded) polystyrene (white beadboard, 32 kg/m3 density) or extruded polystyrene (Styrofoam™) are preferred because they are lightweight and it is easy to cut holes in them for the net pots. Individual sheets should be cut to 0.60 × 1.20 m to facilitate handling (Figure 7).  Larger sheets, such as 1.20 × 2.40 m, are often broken while handling because they are somewhat fragile.

First, score the polystyrene about 0.5 cm deep with a utility knife, then snap the sheet at the score line.

 

Fig.7: Cutting polystyrene sheets

2. Mark the sheets

Mark the cover sheets according to Figure 8.

Equidistant plant spacing is preferred, but this is not always possible if 0.60 × 1.20 m growbed cover sheets are used. A suggested plant spacing (density of about 22 plants per square metre) on a 0.60 × 1.20 m growbed cover sheet is to first mark three rows along the 0.6 m side (located at 10, 30, and 50 cm) and then mark four plants per row on the 1.20 m side (12 plants per sheet). The suggested layout of the first and third rows will be at the 10, 40, 70, and 100 cm marks The layout of the middle row will be at the 20, 50, 80, and 110 cm marks (Figure 8).

Fig. 8: Layout of marked cover sheets for a density of 22 plants per square metre.

Additional information: Plant density

The grower must determine the optimal planting density (the number of plants per square foot or the number of plants per growbed.)

For larger crops, such as tomatoes and cucumbers, a spacing of about 16–20 plants per square metre is recommended. For smaller crops like herbs and lettuce, 20­–25 plants per square metre is recommended.

Densities greater than two plants per square foot for larger head cultivars grown to mature head stage will often result in slower growth. Higher density plantings are usually done with smaller cultivars of lettuce such as ‘Lollo Rossa’ or when plants are harvested at a younger stage.

Growers are advised to compare several planting densities for their growing situation on a small scale before committing to a specific density for commercial-scale production. Parameters to consider include quality, weight, size, shape, diseases and growth rate.

3. Cut the holes

Use an electric drill with a 2 inch hole saw to cut holes at the layout marks (Figure 9). TIP: Preferably, cut the holes about ¾ of the way through from one side, and then complete the operation from the other side of the sheet. This gives a better cut and prevents the plugs from sticking in the hole saw.

 

Fig. 9: An electric drill with a 2 inch hole saw is used to cut holes in the top cover sheets at the layout marks. A template with a suggested plant spacing on a 2 × 4 ft tank cover sheet with a density of 22 plants per square metre is shown.

4. Arrange the sheets

Plastic pots need to be placed on the floor of the growbed to provide additional support to the middle of the sheets. This prevents bowing or sagging of the polystyrene (Figure 8).

 

Fig. 10: A plastic pot placed on the floor of the growbed provides additional support to the middle of the sheets, preventing bowing or sagging of the extruded polystyrene.

TIP: A uniform spacing arrangement in the growbeds can be achieved in a 1.2 m wide growbed by alternating cover sheets so that the first sheet has the 10 cm mark of the first and third rows from one side of the growbed and the adjacent sheet is flipped end-to-end with the 20 cm mark of the first and third rows on that same side of the growbed (Figure 10).

Fig. 10: A uniform spacing arrangement is achieved by alternating the 0.6x1.2 m growbed cover sheets in an end-to-end fashion.

For more information you can also contact: greenercities@fao.org.

Validation

Various leafy vegetables may be grown on a commercial scale with this unique and efficient technique. However, the method is intended only for crops that require less than 8 litres of water per plant for the entire growing season, from transplanting to harvest. Potential crops include leafy, romaine, and semi-head lettuces, cilantro, green onions, kai choy, pak choi, and watercress.

This technology contributes to the SDGs:

Category

Created date

Mon, 10/07/2017 - 09:24

Source(s)

College of Tropical Agriculture and Human Resources (CTAHR), University of Hawaii at Manoa

The College of Tropical Agriculture and Human Resources (CTAHR) is the founding college of the University of Hawaii. With six academic departments and approximately 170 permanent faculty, CTAHR provides exceptional educational, research, and extension programs in tropical agriculture and food systems, family and consumer science, and natural resource management for Hawaii and the international community.

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Food and Agriculture Organization of the United Nations (FAO)

Food and Agriculture Organization of the United Nations (FAO)
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Achieving food security for all is at the heart of FAO's efforts - to make sure people have regular access to enough high-quality food to lead active, healthy lives.
FAO's mandate is to raise levels of nutrition, improve agricultural productivity, better the lives of rural populations and contribute to the growth of the world economy.

Organización de las Naciones Unidas para la Agricultura y la Alimentación (FAO)
El mandato de la FAO
Alcanzar la seguridad alimentaria para todos, y asegurar que las personas tengan acceso regular a alimentos de buena calidad que les permitan llevar una vida activa y saludable, es la esencia de las actividades de la FAO.El mandato de la FAO consiste en mejorar la nutrición, aumentar la productividad agrícola, elevar el nivel de vida de la población rural y contribuir al crecimiento de la economía mundial.

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Atteindre la sécurité alimentaire pour tous est au coeur des efforts de la FAO - veiller à ce que les êtres humains aient un accès régulier à une nourriture de bonne qualité qui leur permette de mener une vie saine et active.
Le mandat de la FAO consiste à améliorer les niveaux de nutrition, la productivité agricole et la qualité de vie des populations rurales et contribuer à l’essor de l’économie mondiale.

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