Managing 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 normally terminated when most of the nutrient solution is consumed. This document provides a detailed description of how to fill and manage the non-circulating hydroponic unit for commercial lettuce production including the application of fertilizers, the seeding, transplanting, growing and harvesting.

Description

Introduction

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

For a step-by-step explanation of how to construct the growbeds please refer to http://teca.fao.org/read/8992 (“Construction of a non-recirculation hydroponic unit for commercial lettuce production”).

Growbeds are placed in plastic covered rain shelters or greenhouses with screened ends and sidewalls to protect against rainfall and large flying insects. They are filled with 6–8 litres of nutrient solution per plant prior to planting. Thus a growbed designed to grow 50 heads of lettuce should have a liquid capacity in the range of 280–380 litres (Figure 1).

 

Figure 1: Hydroponic lettuce growing in growbeds of non-circulating nutrient solution.

Water

Good water quality is essential for this hydroponic method. It is recommended to test the source water to identify the levels of key water parameters (electrical conductivity (EC), total dissolved solids (TDS), pH, alkalinity and heavy metals).

Be wary of source water with EC values over 2.0 mS and avoid EC values in excess of 3.0 mS of source water. There are methods to correct source water with high salinity through dilution, but it is outside of the scope of this publication.

The alkalinity and pH of the source water should be adjusted based on the manufacturer’s instructions on the hydroponic nutrients. If solids are present in the source water they should be filtered or allowed to settle-out and then decanted. If toxic heavy metals are present, the source water should be avoided.

If possible, avoid water with high salinity because salts concentrate as the nutrient solution is consumed by evaporation and transpiration, and plant growth may be adversely affected. Total salts are measured in milli-siemens (mS) with an electrical conductivity (EC) meter (Figure 2).

Rainwater is an excellent source of water in general, and should be used if the municipal water source does not meet the above criteria.

 

TIP: A good method to test water quality is to compare the growth of lettuce in 4 litre bottles of nutrient solution made with rainwater and municipal water (see: http://teca.fao.org/read/8825).

Figure 2. Different models of electrical conductivity (EC) meters to measure total fertilizer salts.

Fertilizer

Several hydroponic formulas are acceptable, but stock solutions must be prepared based upon the manufacturer’s instructions. Growers may also formulate their own fertilizer formulas.

Please refer to http://teca.fao.org/read/8825 for a selection of different hydroponic fertilizers and a recipe of how to prepare them.

The nutrient stock solutions can also be prepared as described in the following:

Material needed

  • 2 good-quality plastic containers
  • hydroponic nutrient solution (this article uses the example of Chem-Gro (Hydro-Gardens, Colorado Springs, CO hydroponic lettuce formula fertilizer (8-15-36 + micronutrients) but any commercial hydroponic is acceptable
  • magnesium sulphate
  • calcium nitrate
  • two plastic measuring cups
  • two PVC pipes or similar stirring rods
  • electrical conductivity (EC) meter

You can either use a commercial hydroponic fertilizer and prepare it according to the manufacturer’s instructions, or make your own fertilizer solution from inorganic salts. For a detailed description of how to prepare your own fertilizer solution please refer to http://teca.fao.org/read/8825.

Step-by-step description (using Chem-Gro)

Figure 3. Nutrient stock solutions A and B.
  1. Procure two good-quality plastic containers and place them on a firm foundation, such as a cement slab or thick plywood sheet. Make sure that there are no rocks under the containers, because they could crack the plastic and cause a leak.
  2. Label the first container as A and the second as B.
  3. Fill both of the containers with exactly 95 litres of water and make a mark on the container at height of the water.
  4. Remove about 20 litres of water from each container.
  5. Add the fertilizer components using the amounts given below and in Figure 3:
    1. To container A, add 11 kg of Chem-Gro 8-15-36 plus 7 kg of magnesium sulphate (Epsom salts).
    2. To container B, add 11 kg of soluble-grade calcium nitrate.  
  6. Top off both containers with water to the 95 litre mark.

TIP: To prepare smaller amounts of stock solution A, add about 450 g of Chem-Gro 8-15-36 hydroponic fertilizer plus about 270 g of magnesium sulphate to each gallon of final solution. To prepare smaller amounts of stock solution B, add about 450 g of soluble-grade calcium nitrate to each gallon of final solution.

  1. Place a PVC pipe or similar stirring rod in each stock solution container and stir well before using. Each stock solution should have its own stirring rod.
  2. Place one plastic measuring cup (300–500 mL) in each stock solution.
  3. Carry the solutions separately to the growbeds.
  4. Estimate the amount of nutrient solution needed based on the growbed volume, and add it. Use the following guidelines to calculate the volume of nutrient solution required.
    1. The final nutrient solution in the growbed should have an electrical conductivity (EC) of 1.5 mS.
    2. For every 378 litres (100 gallons) of water, approximately 1500 ml is needed of both stock solution A and stock solution B.
    3. EC will rise approximately 0.1 mS for each 2 ml of nutrients (1 ml of stock A plus 1 ml of stock B) that is added to 3.8 litres of water.
  5. Add the stock solutions in equal volumes to the growbeds and stir lightly.
  6. Check the EC and add more nutrient if necessary.

Recommended electrical conductivities range from 1 mS during hot weather to as high as 2.5 mS in cool weather. Too much fertilizer causes salt injury, and too little fertilizer results in poor growth. Grower experience is usually the final basis for determining the exact solution concentration.

Stock solutions do not react when mixed in the dilute growing solution.

TIP: If one does not have an electrical conductivity meter, then about 4 mL of stock solution A and 4 mL of stock solution B should be added to each litre of water in the growbed. For the dimensions presented in this document this 4 mL/litre rate of each stock solution would require about 1.3 litre of stock solution A and about 1.3 litre of stock solution B.

Some remarks about using an EC meter

An EC meter is a very useful instrument for any hydroponic grower. EC meters need periodic calibration. Inaccurate readings may occur with poorly mixed solutions. Higher readings are often found at the bottom of the growbed. EC meters give higher readings when the nutrient solution temperature increases.

Some growers have noticed that EC readings were higher several days after the nutrient solution was prepared because the cold water that was added to the growbeds caused an initially lower EC reading. EC readings of the nutrient solution tend to rise during hot weather, because plants selectively take up more water than nutrients to accommodate increased transpiration. This increases the concentration of total nutrients and raises the EC of the nutrient solution. Conversely, during cool weather, plants selectively take up less water than nutrients, and the EC tends to decrease.

pH

The acidity or alkalinity of the nutrient solution is measured in pH units. If the nutrient solution is too acidic or alkaline, the crops will not grow well and may even die. The recommended pH range for the hydroponic method is 5.5–6.5.

To measure the pH there are several options (Table. 1, Figure. 4).

Table. 1: Alternatives for measuring solution pH

 

pH meter

A pH meter is the most common way of measuring pH. All pH meters need periodic calibration, and pH electrodes tend to have a shorter life than EC meters. An inaccurate reading from a malfunctioning pH meter can wrongly direct a grower to alter the pH of the growing solution, with disastrous results.

pH test kits

Inexpensive pH test kits with a pH range of 4.0–8.5 are also available and may be used alone or in addition to a pH meter.

pH indicator solution

Several drops of indicator solution from the test kit are added to a test tube filled with nutrient solution, and upon colour development, the pH is compared to a colour chart. The indicator liquid has a precision of 0.5 pH units, and this is good enough for most growers and hobbyists.

pH paper strips

Growers may also use pH paper strips to monitor nutrient solution pH. The pH paper strips shown in Figure 14 have a pH range of 0–13 and a precision of 1 pH unit. These strips are sometimes called “litmus paper”.

Figure. 4. Alternatives for measuring solution pH: pH indicator solutions, pH test kit, pH paper strips, and a pH meter.

Nutrient solution pH may be affected by water quality, growing medium, the crop, and other factors. Acids and bases may be used to alter nutrient solution pH, but since they are caustic and can burn skin and eyes, careful handling is required.

TIP: If the pH is too low, a simple method of raising pH is to place finely ground dolomite or limestone (or any other source of CaCO3, e.g. eggshells or washed sea shells) in fine netting, such as a nylon stocking, and immerse this in the growbed and move it until the pH adjusts upward, at which time the dolomite or limestone is removed.

TIP: If the pH is too high, prepare a stock solution of 450 grams ammonium sulphate per 38 litres of water. Add 4 mL of the stock solution per litre of growing solution. The plant will utilize the ammonium nitrogen, and the solution pH will drop. Monitor the nutrient solution after several days and make necessary adjustments.

Growing seedlings

Material needed

  • lettuce seeds
  • plastic bags or small containers for seeds
  • refrigerator
  • Oasis® blocks or a multi-cell tray filled
  • net pots
  • growing medium (fine to medium grades of peat-perlite or peat-perlite-vermiculite media)
  • pencil, envelope, scissors
  • spray mist bottle or a hose mist nozzle

Step-by-step description

  1. Select lettuce varieties

The first task in starting a crop is to select lettuce cultivars (varieties). It is useful to read cultivar descriptions in seed catalogues and order small quantities of seed such as garden packs of several cultivars each of leafy, oakleaf, semi-head and romaine lettuce, and also try both green and red cultivars. Cultivars may need to be changed with each season, and they will vary with the location and specific conditions. For example, Manoa lettuce grows well in cool weather but gets severe tip burn during hot summers. Leafy cultivars tend to be more resistant to tip burn than semi-head and head lettuces.

For lettuce, both raw and pelleted seeds are available (Figure 5). Pelleted seeds might facilitate planting for non-circulating net pot hydroponics better.

Figure 5: Raw (brown) and pelleted (yellow) lettuce seed.

TIP: If seed is ordered from a source far away, it should be sent by air mail or priority mail. If seed is purchased locally, give preference to seed stored under refrigeration or in an air-conditioned room. Do not leave seeds in a hot vehicle, in the greenhouse, or outdoors. Lettuce seeds are “fragile” in the sense that they lose viability when exposed to warm temperatures and high humidity.

  1. Date the seed batches

After procuring the seeds, record the date on each batch. Then place the original seed batches in a sealable plastic container and store this in a refrigerator. These are the “mother batches” which remain in the refrigerator at all times except when filling bags or similar small containers with “working batch” seeds to be transported from the refrigerator to the planting location and then back to the refrigerator (Figure 5).

TIP: Replace mother batch seeds that are more than 3 years old.

  1. Plant the seeds

There are two main options for obtaining seedlings: A) starting your own seeds, or B) using seedlings from a nursery.

A: Starting your own seeds (establishing a plant nursery)

It is essential that only strong healthy seedlings are planted. Moreover, the planting methods applied must avoid transplant shock as much as possible. Thus, the recommendation is to establish a simple plant nursery to ensure an adequate supply of healthy seedlings ready to be planted into the aquaponic units. It is always best to have an excess of plants ready to go into the system, and often waiting for seedlings is a source of production delay. A simple nursery bed can be constructed using horizontal wood lengths lined with polyethylene liner, similar to the construction of the growbeds.

The nursery bed should have good drainage, and be either automatically watered a few times a day, or else hand watered. Use a protected area with some shade to allow the young plants to grow.

Polystyrene propagation trays are placed into the nursery bed and are filled with soil, inert grow media such as rockwool, peat, coco fibre, vermiculite, perlite or a potting mix with a combination of the various types of growing medium. Simpler alternatives for propagation trays are also possible using recyclable materials such as empty egg boxes. Choose propagation trays that allow adequate distance between seedlings in order to favour good growth without competition for light. The following list summarizes the steps.

Eight steps to sow seeds using homemade propagation trays

  1. Fill a seedling tray with growing media such as compost or coco fibre (or pre-treated rockwool).
  2. Sow the seeds in holes about 0.5 cm deep; cover the holes with a light dusting of media without compacting it.
  3. Place the tray in a shaded area and irrigate. Automatic watering systems reduce labour.
  4. After germination and sprouting and once the first leaves appear, begin to harden off the seedlings by placing them in increasingly intense sunlight for a few hours a day.
  5. Fertilize the seedlings once a week with a gentle organic fertilizer high in phosphorous in order to strengthen their roots (optional).
  6. Keep growing the seedlings for at least two weeks after first leaf appearance to ensure adequate root growth.
  7. Transplant the seedlings into the system when adequate growth is achieved and plants are sufficiently strong.
  8. Release the seedlings and their soil plugs using a small blunt instrument and transplant them into net cups using gravel or other media to support them (Figure 6)

Figure 6. Transplanting seedlings into a 5 cm net pot into an expanded polystyrene growbed cover. Alternatively, net pots may also be filled with a peat-perlite growing medium.

TIP: Transplanting is preferred over direct seeding because when transplanting the grower can select the best plants. Also, the time to maturity in the growbeds is reduced compared to direct seeding, thus allowing more crops per year. To reduce the growing time even further wait until at least three to four true leaves have formed.

If you chose to use rockwool cubes or multi-cell tray you can fill the holes with the help of a self-made “envelope seeder” (Figure 7). Trim the top and right side of an ordinary envelope with scissors. Place either raw or pelleted seeds in the envelope. The bottom crease forces seeds to line up in single file. Use a pencil or sharpened stick to guide the seeds into the planting holes of the net pots.

TIP: Plant about 20 percent more seedlings than necessary to ensure that the best seedlings may be selected for transplanting into the growbeds.

Figure 7. Planting pelleted lettuce seeds into rockwool cubes with an “envelope seeder.”

Additional information: Small net pots (top diameter of 5 cm) may restrict growth, particularly if you are growing lettuce to a harvest size of 500 grams because the stem outgrows the diameter of the net cup. This should also be considered if you want to modify the system and grow larger plants.

B: Buying readymade seedlings from plant nurseries

A 2nd option is simply buying ready-to-plant seedlings in large propagation trays from plant nurseries. Remove the substrate from the roots by slowly and carefully dipping the seedling in source water with ¼ nutrient solution treated with a weak systemic fungicide. Then, place the seedling into the net cup (either with other or without pea gravel for support and protection) and then transplant into the growbed.  This can be considered as a good option if:

  • you are looking for a very cost effective production,
  • you need seedlings immediately,
  • there is a disease outbreak in your own nursery.

However, there is a concern of contaminating the grow area with diseases from the nursery.

  1. Moisten the seeds

Lightly mist the seeds after planting with a spray mist bottle or a hose mist nozzle. Some growers cover the seeds lightly with additional fine growing medium or lightly close the planting holes, whereas other growers do not cover the seeds.

Do not place freshly seeded trays in a hot greenhouse immediately after seeding. Instead, try to keep the seeded trays in a cool, moist environment (such as a garage) for at least 36 hours and covered for 24 hours (for covering you can use a 1.5 cm thick sheet of expanded polystyrene or a sheet of plywood). This will help initiation of the germination process. The newly germinated seedlings can tolerate the greenhouse temperatures.

TIP: Another option is to stack the trays on top of each other and cover the top tray. This prevents the medium from drying out and insulates the trays from excessive heat.

TIP: The seedling greenhouse should be a separate structure from the production greenhouse(s). When seedlings are grown in the same structure with the growbeds, there is increased likelihood for transferring diseases and insects to the young seedlings.

Growing

The lower portion (2.55 cm) of the net pots is initially immersed in nutrient solution. The growing medium in the net pot becomes moistened by capillary action, providing water to the seedlings. The nutrient solution level drops below the bottom of the net pots as the plants grow, and the solution is depleted by transpiration and evaporation. This creates an expanding moist air space beneath the growbed cover, protecting the roots from drying out (Figure 8). At this point, direct capillary wetting of the growing medium is no longer possible, but the expanding root system readily reaches the solution in the grow bed as well as oxygen and water from the moist air space.

This system does not require wicks, pumps, aerators, electrical power, or mechanical devices.

Figure 8. The lower portion of the net pots is initially immersed in nutrient solution (left). The nutrient solution level drops below the bottom of the net pots as the plants grow (right).

Important practical advices for growing:

  • The growbeds must be sheltered from rainfall and should be placed inside a greenhouse or rain shelter. Outdoor growing is only recommended for dry locations.
  • The growbed cover should not be lifted while the crop is growing, because roots may be torn and plant growth will suffer.
  • If it appears that the growbed will run out of nutrient solution before harvest, add about 2.5–5 cm of water or half-strength nutrient solution. If this occurs all year long, then consider switching to deeper growbeds.

TIP: Partial shading with a 30–50 percent shade screen during the severe wilting period would be the best remedy, but this is not always possible.

Harvesting

Material needed

  • heavy-duty scissors or a knife
  • plastic produce bags, hard plastic containers, or cardboard boxes
  • optional: 10 percent bleach solution
  • optional: mist spray bottle with potable water

Step-by-step description

Leafy or semi-head lettuce is ready for harvest about 4–6 weeks after transplanting, depending upon transplanting age, season (winter crops take longer than summer crops), and cultivar. Early morning is the best time for harvesting because it prevents the lettuce from wilting.

  1. Wash your hands

Lettuce is eaten raw, and the customer trusts that they are buying a clean and safe product.

  1. Cut the lettuce

Use a heavy-duty, sharp scissors or a knife to cut the lettuce (Figure 9). Sanitize the tool beforehand. Leaves may be trimmed as necessary. 

Figure. 9. Harvesting lettuce
  1. Packaging the lettuce

Place the lettuce in a plastic produce bag, a hard plastic container, or a box if taken to a restaurant. Lettuce can be misted with potable water which can increase shelf-life.

Sanitize hard plastic containers before use. Use new plastic bags, not reused ones. Use new cardboard boxes, particularly if the lettuce has not first been placed in plastic bags.

  1. Clean the net pots

Roots can be removed manually immediately after harvest. Be careful when removing the roots as not to break the plastic net cup.

Net cups should be well cleaned and sanitized between crops to prevent disease by soaking them in a 10 percent bleach solution for 24 hours (Figure 10). Net cups can then be rinsed and dried, and ready to use again.

Figure 10. After removing the remaining plant material and growing medium, the net pots may be soaked in a 10 percent bleach solution (Waite Farm, Mt. View).
  1. Clean the growbed covers

Clean the growbed covers.

Ideally, the remaining nutrient solution is siphoned from the growbed and used to water some other plants growing in soil, because there are nutrients remaining in the solution. This solution should be dispersed over an area rather than dumped in one spot. Normally, the growbed does not need to be rinsed with water. New nutrient solution is added to the growbed, and the growing cycle is repeated.

However, some growers just “top up” the remaining solution with new nutrient solution, and they use EC readings to calculate the amount of additional stock solution to apply to the subsequent crop. As a general rule, growbeds should be drained and refilled at least after every three crops, but sooner is better particularly if the source water contains medium to high salinity.

Insects and Diseases

Please refer to http://teca.fao.org/read/8825  for insect and diseases management in hydroponic systems.

Benefits

  • The grower typically has multiple stages of lettuce growth, so it is possible to continuously supply a market.
  • Growers can harvest a mature crop and transplant a new crop in a growbed on the same day. Thus, growbed occupancy can be in the range of 300–365 days per year.

For general benefits of the hydroponic method please refer to http://teca.fao.org/read/8825.

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

Validation

The suspended-pot, non-circulating hydroponic growing method is not intended for production of long-term crops such as tomatoes and cucumbers, which require large quantities of water and a more complex nutrient schedule.

This technology contributes to the SDGs:

Category

Created date

Mon, 10/07/2017 - 09:22

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)
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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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