Meeting the fertilization needs of greenhouse lettuce

Ornamental plant growers considering producing an edible greenhouse crop may want to try lettuce. Neil Mattson, associate horticulture professor at Cornell University, said lettuce is a plant with moderate fertility needs.

“Grown hydroponically, lettuce has somewhat lower fertility needs than a greenhouse tomato crop,” 

Mattson said. “Grown as a container crop, lettuce is relatively similar to petunia. However, lettuce has somewhat greater calcium needs. Growers can produce a relatively good crop of lettuce in containers, if they use a complete fertilizer at a moderate strength of 150 parts per million nitrogen.”

Mattson said head lettuce can be produced in containers similar to a bedding plant crop. The seed would be planted into a plug tray for three to four weeks. Transplanting the plugs into larger containers, the crop could be finished in four to six weeks depending on light and temperature levels.

He said baby leaf lettuce can be grown in flats. The seed is directly sown into the growing medium and grown for three to four weeks until plants reach suitable size.

Leaf tipburn is a physiological disorder that can occur when growing greenhouse lettuce. It can greatly impact the salability of a crop.

“The main reason that tipburn occurs is the lettuce is growing too fast under high light,” Mattson said. “For lettuce, the target daily light integral is 17 moles per square meter per day. The light level should be lower if there is poor air flow. If the light level goes higher than 17 moles, the rapid growth of young leaves is affected. There may be an inadequate calcium supply, especially as the lettuce heads begin to mature and close. If there is not enough air flow and not enough transpiration by the young leaves, then not enough calcium can reach the leaves through the xylem sap. This can cause tipburn to occur. It’s a case of pushing the plants too fast.”

Mattson said in many cases, tipburn is not a result of a lack of calcium supplied to the plants, but an inability of the plants to transport enough calcium to the young leaves.

“For container-grown lettuce, there is typically enough calcium if the growing medium has a lime charge and if the fertilizer water solution contains more than 50 ppm calcium,” he said. “Many common bedding plant fertilizers, including 20-20-20, 20-20-20 and 21-5-20, do not contain calcium. These fertilizers are typically used with tap water sources that contain moderate alkalinity. In many cases, these tap water sources also contain sufficient calcium.”

Mattson said it is important for growers to test their water sources to make sure adequate calcium is being supplied, either from the water source or added into the fertility program. If calcium needs to be added, calcium nitrate is most commonly used. However, calcium nitrate is not compatible with most complete fertilizers.

“Usually if a grower has to add calcium, it can be done using a separate stock tank or a separate injector,” Mattson said. “One strategy is to use a separate injector for the calcium nitrate in a series with a 20-10-20 fertilizer that is being added with a second injector. Adding 50 ppm calcium from calcium nitrate should be sufficient.

“An alternative method of calcium application, if a grower has only one injector is to rotate between two separate stock tanks, one for calcium nitrate and one for the bedding plant fertilizer. A grower would then rotate between the two fertilizers. For example, for two days he would use the 20-10-20 fertilizer and on the third day he would use the calcium nitrate applied at 150 ppm.”

Mattson has been able to grow a relatively good crop of container-grown lettuce using granular organic fertilizers incorporated into the growing medium.

“We incorporated poultry-based organic fertilizer (Sustane 8-4-4) into the growing medium at a rate of 8 pounds per cubic yard for both the seed germination and transplant growing mixes,” he said. “That provided good fertility, but for optimum yields I would also suggest making some liquid organic fertilizer applications, maybe two to three times a week as the plants get older.”

Mattson said the organic granular fertilizer he used is temperature-dependent and is broken down by soil microbes. Sustane 8-4-4 has a 45-day release period, but under very warm greenhouse temperatures Mattson has noticed quicker release rates. He said there are other slow release organic fertilizers with different release periods. For example, Verdanta EcoVita lists a 75-100 day release period.

One strategy that Mattson recommends growers do periodically is to monitor the electrical conductivity (EC) and pH levels.

“Monitoring EC will help growers determine if the plants are receiving sufficient fertility,” he said. “If a grower is incorporating a slow release fertilizer, this is a good indicator of when additional fertilizer needs to be added. An under-fertilized plant will show yellow lower leaves from nitrogen deficiency.”

Mattson said monitoring pH is important as it impacts nutrient availability. He said lettuce isn’t commonly susceptible to iron deficiency, but it will start to show up when the pH starts to increase above 6.5-7.

“Monitoring EC and pH is especially important in hydroponics,” he said. “A good grower who is producing his crop in a growing medium in containers will monitor the pH every week or two. The pH may change over the course of a week by maybe one unit.

“Growing hydroponically, a grower should be monitoring the pH every day and make adjustments. Depending on the type of fertilizer and the quality of the water, the pH in a hydroponic set up could change two units in a day.”

Mattson said light and temperature are going to be the drivers for how long it takes to finish a lettuce crop. Whether a grower is producing the crop in containers with growing medium or hydroponically shouldn’t have any effect on the length of production.

He said plant spacing can also impact the size of the lettuce head. If plants are grown in small containers and spaced pot-to-pot, the lettuce heads may not reach full size.

For greenhouse lettuce, Cornell University researchers developed a hydroponic production model that enables growers to produce a lettuce crop from seeding to harvest in 35 days if temperature and light intensity are at optimum levels.

“When the light level isn’t optimized, a lettuce crop can take more than 100 days from seeding to harvest,” Mattson said. “High pressure sodium lamps would be the best lamps to use if a grower is looking to provide supplemental light in a greenhouse to increase the daily light integral. For the Cornell model we adjust the amount of light in the greenhouse based on the amount of outdoor light. Seventeen moles per square meter per day is the daily light integral we are aiming for with the model. The optimum temperature for plant development is about 75ºF during the day and 65ºF at night.”

For more information

Neil Mattson, School of Integrative Plant Science

T: (607) 255-0621 

Hort Americas, LLC
Chris Higgins
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