Irrigate for success

When people think of hydroponic crop production, irrigation is not the first thing that comes to mind. After all, the hydroponic production systems take care of that, right? Well, sort of. When it comes to recirculating water culture systems such as nutrient-film technique (NFT) and deep-water culture (DWC) or deep-flow technique (DFT) systems, the continually circulating nutrient solution really does take care of keeping crops irrigated. But what about those hydroponic systems that use a larger volume of substrate compared to NFT and DWC/DFT systems, such as the slabs on gutters or Dutch buckets used to grow fruiting vine crops, or the substrate-filled troughs and containers used to produce strawberries? These systems do not have continually recirculating nutrient solutions and, as a result, irrigation must be managed.

When hydroponic crops are irrigated, the nutrient solution provided to plants has not only water, but mineral nutrients, too. Beyond simply providing mineral nutrients, irrigation strategies can also be used for managing the total nutrient concentration, or electrical conductivity (EC), available for plants in the rootzone. Just like recirculating water culture systems measure and manage for nutrient solution EC, producers using slabs or Dutch buckets must do the same for their substrate and make the necessary corrections to keep crops growing optimally.

The best way to monitor irrigation for substrate systems is to measure the pH, EC and volume of nutrient solution provided to plants. Use the same parameters for leachate. To monitor the input, place one or more drippers into a container (add extra drippers in your lines for this purpose so none are removed from plants). Take the volume of nutrient solution collected in the container over the course of the day and divide it by the number of drippers in the collection container to get the volume of nutrient solution per dripper. Now, depending on the number of drippers and plants in each slab or Dutch bucket, the volume of solution per plant or per slab or bucket can be calculated. A similar process can be used to monitor leachate. Place a sufficiently sized collection container under the drain for a line of slabs Dutch buckets. The volume of leachate collected can be divided by the number of slabs or buckets, drippers or plants in the line that is draining into the container (using the same unit you did for the input). Now, using the volume in and volume out per unit, the leaching fraction, or % leaching, can be calculated by dividing the amount provided by the amount leached.

The EC of the leachate is a good representation of the root zone for slab and bucket systems. Comparing the recommended EC values for the crops being grown to the EC of leachate can guide irrigation strategies. If the EC is within recommended ranges, continue to target the leaching fractions you are getting. If the EC exceeds the high end of the recommended range, increase the leaching fraction to remove additional salts. This may be done by increasing the frequency or irrigation events, or the duration of irrigation events. If the EC is below the low end of the recommended range, reduce leaching fraction to accumulate more nutrients in the root zone. This can be done by reducing the duration of irrigation events, though frequencies may need to be increased to maintain adequate substrate moisture content. Whenever irrigation strategies are altered to change EC, keep in mind how those changes will affect substrate moisture — another important factor for hydroponic producers.

The amount of water available to plants also affects growth and development of hydroponic crops. For example, for tomato and pepper, water content can affect their productivity and “steer” plants. For fruiting vine crops, plants exist on the spectrum between vegetative and generative. The vegetative phase, as the name implies, favors production of foliage growth; the generative or reproductive phase favors flower production. Once crops are up and in production, they do not exist exclusively in one phase or another. Instead, tomato and pepper plants require a combination of vegetative and reproductive growth in order to maintain productivity — enough flower production to maintain target yields complimented by adequate foliage production to support developing marketable fruits.

But steering between vegetative and generative is useful to compensate for differences between cultivars or growing seasons. For example, some fruiting vine crop cultivars are naturally more generative or vegetative, and the ability to steer them in a different direction to balance out production is needed. Or, sunscald on peppers can be a problem during the high light and heat of summer, and steering pepper plants in a more vegetative direction to develop more canopy to shield fruits from radiant energy may be needed. Generally speaking, growing more wet — with a higher substrate volumetric water content — steers plants in a more vegetative state.

Alternatively, growing drier, with a lower substrate volumetric water content, steers plants in a more generative direction. Like managing EC, irrigation frequency and duration can both be used to alter substrate volumetric water content. Also, like EC, keep in mind how changing irrigation strategies to change substrate moisture will affect other parameters such as EC.

While there is ample automation in the hydroponic and controlled environment crop production, irrigating slabs and Dutch buckets are not something to put on auto-pilot. Actively monitor irrigation volume, leachate chemical properties and leaching fractions in your production systems to better manage crop culture. Remember that no irrigation decision occurs in a vacuum, and consider how any changes made to alter one root zone condition will affect another.

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