When selecting a substrate for growing hydroponic leafy greens, lettuces and microgreens, the type of production system should be one of the first factors considered.
The type of hydroponic production system a grower is going to use can impact his choice of substrate.
“Any system in which the water is being recirculated like nutrient film technique (NFT), a grower needs to make sure the substrate doesn’t crumble and fall apart and clog the irrigation lines and filtration system,” said international agricultural consultant Hugh Poole. “If a grower is using coir, peat, perlite or some other loose substrate, there could be some issues with the recirculating systems. This should not be an issue with deep water raft systems.”
Rens Muusers, technical account manager for the U.S. at Grodan, said whether a hydroponic crop is grown in a greenhouse or a vertical farm the substrate does much the same thing.
“There’s not much difference between a greenhouse and a vertical farm,” Muusers said. “The substrate has the same type of functionality regardless of the type of facility it is used in. One of the main considerations is what kind of production system is going to be used and how does the substrate fit into that system.
“One of the first things to think about is how and for how long the plants are going to be propagated. If the seedlings will be kept longer in a propagation area, then a larger amount of substrate will be needed to allow adequate space for the plant roots to develop. Holding the plants longer in propagation will also likely require additional production area to allow for proper spacing of the plants.”
Maximizing production space
Muusers said the length of time growers hold seedlings in the propagation area will impact the size of the plugs that are used.
“This is a major reason why growers are choosing to use 1-inch or 1½-inch plugs,” he said. “Growers using 1½-inch plugs would be keeping their plants in the propagation area longer to optimize space utilization. The longer the plants are kept closer together in propagation the longer the crop can benefit from the microclimate. Additionally, if the plants are held longer in propagation results in less space needed in the final growing area.
“Transplanting small plants into the final growing spacing is not an efficient use of space. Holding the seedlings longer before transplanting them into NFT channels or a deep water pond requires less space in the final growing area to grow the same amount of plants.”
Muusers said sometimes transplanting larger plugs can speed up the production of a crop.
“When transplanting plugs from a propagation area that has an optimum microclimate into a greenhouse, is like putting the plants out into a desert,” Muusers said. “In indoor vertical farms, theoretically the climate is managed in a more optimal way. Growers can manage the microclimate around the plants, but they have to consider how cost effective that is. The space in a vertical farm is going to be more costly than in a greenhouse so it is even more critical to optimize space utilization in a vertical farm. For any particular type of growing space utilization is critical. All of these steps impact the substrate that is used. Depending on the spacing, the substrate needs to be able to manage those kinds of processes as well.”
Producing fresh cuts
Siebe Streekstra, account manager West at Grow-Tech Inc., said choosing a substrate for leafy greens production depends on how the crop is harvested.
“If the leafy greens are going to be cut, a grower can choose whether to grow in a plug or in a thick mat,” Streekstra said. “I work with a grower in southern California who harvests leafy greens six times from one planting. The plants are grown on a thicker BioStrate mat and it works really well. The grower is operating in an indoor controlled environment facility and is able to control the environment much better than in a greenhouse. Some growers prefer to use plugs because they provide them more of a buffer, but in general the mat is usually cheaper.
“I expect more people are using plugs for leafy greens. There are more cubic inches available in a plug than in a mat to hold roots, nutrients and water. The more stable the roots, the better it is for growth longevity. If growers are looking to harvest fresh cut leafy greens, they can usually go through and cut every two weeks. This enables growers to take six to eight harvests in a period of 16 weeks. If the roots have more space it helps the plants to grow back quicker. With plugs there are fewer concerns about having enough water and nutrients.”
Different needs for different crops
Muusers said the length of production time for a crop will influence the substrate choice.
“There are significant differences in production time when growing leafy greens and lettuce versus growing microgreens,” he said. “Microgreen crops require seven to 14 days max. That’s a really short crop. A lettuce crop can require as long as 45 days.
“There is a relationship between the length of the crop time and the amount of substrate that has to be used as well. For microgreens, a thin Grodan Cress Plate is an ideal substrate volume for the crop. But this substrate wouldn’t work for a head of lettuce, which requires more volume during the growing cycle. Growers typically use plugs for lettuce.”
Although microgreens are a short term crop, Streekstra said they’re not as simple as they seem.
“If microgreens are being grown in a greenhouse, one has to be aware of the outside temperature and humidity when choosing a substrate. I’m working with a greenhouse grower in Las Vegas who is producing microgreens. It is hard to grow microgreens because the circumstances are so harsh. The humidity is very low and the temperatures are so high that the evaporation is immense. The grower uses a thick BioStrate mat substrate in order to keep up with plant transpiration. Growers who are producing under harsh climate conditions should consider using a thicker medium.
“This Las Vegas grower is actually moving into an indoor controlled environment production facility where he can control the temperature and humidity. That should make production much easier and consistent. Growing microgreens in a greenhouse, kind of depends on where the operation is located. In general, people in greenhouses don’t control their circumstances as well as in an indoor production operation.”
Growing microgreens organically
Poole said growers producing microgreens should be able to grow the crop organically.
“This is a crop that finishes in seven to 14 days,” he said. “Microgreens should be able to utilize the nutrients that are in the seed. This is a crop that can be grown without any fertilizer inputs. Regardless of the substrate, whether it is peat, coir, hemp or paper towels, I don’t see why for such a short-term crop that the substrate can’t be organic.
“I’m not sure that an organic fertilizer would even help this short-term crop significantly. Most organic fertilizers need time for microbial activity to break down the nutrients and make them available to the plants. I expect that the production systems wouldn’t have sufficient time to make an impact on nutrient availability. This is a biologically driven system that is dependent on temperature. With traditional fertilizers the nutrients are available when the fertilizer is added to the system. Just because an organic fertilizer is added doesn’t mean that the nutrients will be available at the critical times that the plants need them.
Poole said a filtration system should be installed if organic fertilizers are used in recirculating hydroponic systems.
“The filtration system will help to ensure that the irrigation lines don’t become clogged with fertilizer particles and organic matter,” he said.
For more: Hugh Poole, FloraSynergy; (864) 359-7090; hapoole@Interact2Day.com. Rens Muusers, Grodan, email@example.com; http://www.grodan.com. Grow-Tech Inc., (888) 805-8916; firstname.lastname@example.org; http://grow-tech.com
David Kuack is a freelance technical writer in Fort Worth, Texas; email@example.com.