A systematic approach

When starting to produce food crops hydroponically, selecting which system or systems you want to use can be a bit daunting. However, with some knowledge of the most effective systems, you'll be able to make an informed choice of what you want based on what you need. In addition to selecting a system that is well-suited to the crops you will be growing, each system has unique aspects that set them apart.

This is the second article in a multi-part series designed to serve as a primer for hydroponic vegetable crop production. The first article in the February 2016 issue served as an introduction to hydroponic food crop production, while in this article we are going to focus on the most commonly used hydroponic systems for producing food crops in greenhouses and controlled environments.

Hydroponic systems for vining crops

One of the most widely used hydroponic production systems is slab or bag culture (Fig. 1). This system is utilized for vine crop production, such as tomatoes, cucumbers, peppers and eggplants. The slabs or bags that are the hallmark of this system contain a substrate, most frequently rockwool or coconut coir. Though dimensions of the slabs can vary by manufacturer, they generally measure one meter in length. Some slabs are available in different widths to accommodate crop cycles of varying durations; shorter cycles require less substrate and longer crops require more substrate.

When producing vine crops including tomatoes and peppers, it is useful to “steer” the crop between vegetative (non-flowering or non-reproductive) and generative (flowering or reproductive) states. Steering crops is useful for balancing the vegetative growth (foliage) and reproductive growth (flowers and fruits) to maintain the plant’s ability to sustain the fruit load. Drying down or reducing substrate moisture content can promote generative growth, while moister substrates can promote vegetative growth. When selecting your rockwool or coconut coir slabs, pay attention to the characteristics and descriptions of your options. The size, density and arrangement of rockwool fibers and coconut coir particle sizes vary among different products. Substrates that hold more moisture are generally more useful for beginner growers, as moisture management is easier. Alternatively, substrates that can dry down more easily are for more experienced growers who may want to have more ability to steer their crops.

Nutrient solution is provided to the plants with drip stakes attached with spaghetti tubing to pressure-compensating emitters. One or two stakes are provided to each plant, depending on irrigation requirements. Additionally, pressure-compensating emitters are available with different flow rates to provide the required volume of nutrient solution during irrigation events.

A few other items are required for slab systems. Since slits are made in the plastic bags to allow leachate (excess nutrient solution) to drain, you’ll want to collect and channel leachate either to a reservoir (for recirculating systems) or a drain (for open systems). One popular option is to use a metal gutter that the slabs sit on, with leachate running down to collection channels on the side. Another option is to use a large plastic channel, similar to those channels that will be described later for nutrient-film technique systems. Finally, a trellis for the vining crops will be required. Wires run parallel to the crop rows (thus the name “high-wire production”) and synthetic twine that vines are attached to are spooled on plastic or metal hooks attached to the overhead wire.

Dutch or BATO buckets are another type of system commonly employed for vining crops (Fig. 2). The containers are designed to be filled with substrate and, between irrigations, hold a reservoir of nutrient solution at the bottom of the container that can be absorbed by the growing substrate. In order to prevent oversaturation of substrate, a siphoning elbow allows excessive nutrient solution to be siphoned out of the bucket. When placed in rows, the container rests on a drain pipe that carries excess nutrient solution siphoned out of the container. As with slab culture, this system can be an open, drain-to-waste system or a closed, recycling system; either is feasible, though nutrient solution management will differ depending on the method.

Dutch buckets can be filled with different substrates, as long as they provide the right balance of moisture retention and aeration. The most commonly used substrates are coarse perlite or lightweight expanded clay aggregate (LECA). Perlite is the more economical choice. While LECA costs more, it can be sanitized and reused numerous times, though this will require labor. If you do opt for perlite, sometimes the siphoning elbow can get clogged with particles. However, a layer of LECA can be placed at the bottom of the container, covering up the siphoning elbow, and then filled with perlite to prevent perlite from blocking drainage.

Just like the slab system, a trellis and training system will be required for vining crops and the same systems can be employed for Dutch buckets. No gutter is required, since excess nutrient solution is already collected and drained away in the drainage pipe the gutter rests on. While slabs or bags are usually discarded, containers do need to be sanitized between crops, which can be a labor-intensive task for producers.

Hydroponic systems for small, leafy plants

While slabs and Dutch buckets work well for producing vining crops in high-wire systems, smaller leafy crops including lettuces, other greens, and herbs are better-suited for other systems. The nutrient-film technique (NFT) system is a type of recirculating water culture that is widely used for producing leafy crops (Fig. 3). This system utilizes channels or gutters, available in widths from about 4 to 8 inches, depending on the type of crop you want to grow, placed on a slope of 1 to 3 percent. Spaghetti tubes deliver nutrient solution at the top of the channel at a rate of roughly ¼ to ½ gallon per minute, and this nutrient solution then flows down the channel, constantly bathing the roots in nutrient solution. The nutrient solution then drains out of the channel and is collected into a reservoir, where pH and electrical conductivity (EC) are monitored and adjusted. A major advantage of NFT systems is that the channels are placed at heights that are comfortable for people to work with. However, if spaghetti tubes clog or power goes down, plants will not be receiving nutrient solution and can decline very quickly.

For small NFT systems, monitoring and adjusting the nutrient solution may be a manual activity, but meters and peristaltic pumps make these measurements and adjustments automatically. Additional water treatment can include UV or ozone for sanitation. Dissolved oxygen in the nutrient is essential for plant growth, but the movement of water running down the channels in NFT systems can help maintain adequate oxygen concentrations.

Another popular recirculating water-culture system is the deep-flow technique (DFT), sometimes referred to as “raft” or “raceway” culture. This system employs polystyrene rafts that float on top of nutrient solution that is anywhere from 4 to 12 inches deep. Depending on the length of your grow beds, rafts can be planted on one end and harvested at the other; as the rafts are removed after harvesting, the rest of the rafts are moved down the bed, thus the name “raceway.”

While pH, EC, and water pathogens are managed similarly to NFT systems, DFT systems require aeration to maintain adequate dissolved oxygen concentrations. A major advantage to DFT systems is that pumps going down or clogged tubing will not harm crops because they are floating on the nutrient solution.

However, because rafts are usually a foot or so of the ground, they aren’t at a comfortable height for frequent plant handling.

Take-home message

There are a number of different hydroponic systems that are suitable for producing food crops in greenhouses and controlled environments, and the most popular systems were covered here. When evaluating alternative production systems, critically consider how water, nutrients and oxygen will be supplied to the root zone. Which system to use will depend on the crop you are growing, as well as your personal and operational preferences.

Christopher (ccurrey@iastate.edu) is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.