Fig. 1 is an example of a common problem during hydroponic production of vegetable, leafy greens, and herb crops. Several leafy greens and herb seedling plugs were grown with separate fertilizer tanks. All tanks started with the same fertilizer recipe, complete with all essential nutrients. Nitrogen was supplied mostly (90%) as nitrate, with 10% of nitrogen as ammonium. After a few weeks, the arugula was stunted with the new leaves turning yellow and chlorotic (Fig. 1), which are typical symptoms of iron deficiency. Other crop species, which included lettuce, oregano and spinach, had no deficiency symptoms. The electrical conductivity (EC) of the nutrient solution was 1.8 mS/cm for all crops, indicating adequate nutrient levels. However, the nutrient solution-pH in systems containing arugula was 7.0, whereas the solution-pH for lettuce, spinach, and basil was 5.7 to 6.0. There were no signs of root disease.

This article explains why only the arugula raised pH and developed nutrient deficiency, even though all crops received the same fertilizer and growing conditions. By understanding this process, growers can adjust the fertilizer program, stabilizing pH and preventing plant loss.

Why care about pH?

The pH of a hydroponic solution influences the solubility and availability of nutrients for root uptake, especially iron and other micronutrients such as manganese and boron. A high pH decreases iron solubility and uptake by roots, which can result in deficiency symptoms as seen in arugula (Fig. 1). For most crops, keeping pH within a target range of 5.6 and 6.2 ensures adequate nutrient solubility and availability for plant uptake.

Fertilizer and plant species affect root zone pH

To understand why root zone pH sometimes drifts too high or low, research conducted at the University of Florida evaluated species of vegetables, leafy greens, and herbs grown in a soilless peat substrate or a hydroponic nutrient solution. We grew the plants using three different fertilizers that varied in their proportion of ammonium to nitrate nitrogen. The acid (lowers pH) or base (raises pH) produced by plant roots, and the resulting change in pH was measured over several weeks.

Table 1 summarizes our research results. Across plant species, there was increasing acidity (and pH drop) as the proportion of nitrogen provided as ammonium increased from 0% to 40%, and the corresponding nitrogen provided as nitrate decreased from 100% to 60%.

Plant species differed in their tendency to raise or lower pH, even when supplied with the same fertilizer (Table 1). Arugula had the most basic effect, and cucumber was the most acidic.

Why is ammonium more acidic than nitrate?

When you mix a fertilizer in your hydroponic solution, the amount of ammonium nitrogen (NH₄⁺) or nitrate nitrogen (NO₃^-) has little immediate effect on pH. The main pH effect of these nitrogen forms happens over time, after plant roots or microbes interact with fertilizer nutrients.

Plant roots take up “cations,” which are fertilizer nutrients with a positive charge such as ammonium NH₄⁺, potassium K⁺, calcium Ca²⁺, and magnesium Mg²⁺. Plants also take up “anions,” which are nutrients with a negative charge such as nitrate NO₃^-, phosphate H₂PO₄^-, and sulfate SO₄^2-.

When roots take up cations such as NH₄⁺ the plant becomes more positively charged than the substrate. Plants balance the charge difference between the roots and substrate by exuding an acid (H⁺) which drops the root zone pH (Fig. 2A).

Uptake of anions such as NO₃^- causes the plant to become negatively charged. Plants balance this charge by releasing base into the substrate (hydroxyl OH^- or bicarbonate HCO₃^-) which raises pH (Fig. 2B). Plants can also take up equal amounts of cations and anions, which results in no pH effect (Fig. 2C).

Nitrogen is especially important to root zone pH because (a) roots can take up nitrogen as a cation (ammonium NH₄⁺) or anion (nitrate NO₃^-) and (b) plants require more nitrogen than any other fertilizer nutrient. Increasing ammonium in the fertilizer causes plants to take up more cations and produce more acid. Ammonium is a stronger acid than nitrate is a base, in part because nitrifying bacteria present in the substrate convert ammonium to nitrate through a process called nitrification, which produces acid (Fig. 2D). Many plants also favor ammonium uptake over nitrate.

As an example of a commonly used fertilizer salt, potassium nitrate has 100% of nitrogen (N) in the nitrate form, so has a basic effect. In contrast, ammonium nitrate is 50% ammonium-N and 50% nitrate-N. Because ammonium is a stronger acid than nitrate is a base, ammonium nitrate is strongly acidic. Increasing the amount of nitrate sources in your fertilizer recipe relative to ammonium sources makes the solution more basic. You can increase acidity by increasing ammonium and reducing nitrate.

Why did arugula tend to raise root zone pH?

Plant species differ in their uptake of total cations and anions. Arugula tends to take up more anions, which produces base, and raises root zone pH. Cucumber tends to take up more cations, produces acid, and lowers pH. A “neutral” fertilizer for arugula contains about 35% of the total nitrogen as ammonium and the remainder (65%) as nitrate to stabilize pH. Other crops typically require less ammonium (10 to 30%) for a stable pH.

The other big pH factor in hydroponic solutions is alkalinity (primarily from bicarbonates dissolved in the source water). Alkalinity can be reduced with mineral acids or reverse osmosis, and varies depending on your water source. The neutral ammonium:nitrate ratios above assume reverse osmosis water (zero alkalinity). As alkalinity increases, the amount of ammonium or mineral acid required increases. Get a laboratory test of your irrigation water alkalinity. Check the University of New Hampshire Alk Calc (bit.ly/1NFIcAN) for recommendations for mineral acids needed to reduce alkalinity, and bring initial solution pH to around 6.

What can we do about our yellow arugula?

Our hydroponic arugula ran into nutritional problems because the roots of this species produced base and raised root zone pH to a level where iron was not soluble and available for uptake. Basil, lettuce, and cucumber were given the same fertilizer, but these species tend to be more acidic and did not push pH outside of an acceptable range.

There are several options to manage pH in a hydroponic solution summarized in Table 2. Keys to success are a combination of understanding crop effects on solution pH, the effect of ammonium versus nitrate on solution pH and other nutrients such as calcium, pH monitoring both inline and with a manual meter, and options to add acid or base to the irrigation solution.

Acknowledgements: Thanks to funding support and collaboration from industry partners of the Floriculture Research Alliance (floriculturealliance.org) as well as the USDA Floriculture and Nursery Research Initiative and the Gene and Barbara Batson Scholarship.

Ryan is an Associate Extension Professor at the University of New Hampshire (ryan.dickson@unh.edu). Paul is a Professor and Extension Specialist at the University of Florida (pfisher@ufl.edu).

lant protection of greenhouse-grown vegetable crops such as tomatoes, peppers and cucumbers involves implementing a variety of strategies in order to alleviate problems with insect and mite pests, including the use of insecticides or miticides. However, the “first line of defense” in dealing with insect and mite pests (e.g., aphids, mites, thrips and whiteflies) in vegetable production systems is non-chemical plant protection strategies that include: scouting, sanitation, trapping, and cultural and biological methods.

1. Scouting

2. Sanitation

3. Trapping

Yellow sticky tape can be placed among a vegetable crop in order to mass-trap or capture large numbers of insect pests including: adult aphids, thrips and whiteflies. Yellow sticky tape is positioned in rows hung vertically within the greenhouse (Fig. 6). In addition, yellow sticky tape can be placed near openings (e.g., side walls), which may capture adult insects as they enter the greenhouse from outdoors (Fig. 7).

4. Cultural methods

Vegetable crops that are overfertilized, especially with nitrogen-based fertilizers, tend to be more susceptible to aphids and spider mites. Over-fertilization may change plant quality thus making plants a better food source for insect and mite pests, which will enhance development, growth, and reproduction. Over-fertilizing vegetable crops results in higher levels of amino acids, thus leading to an increase in feeding by sucking insect and mite pests (those with piercing-sucking mouthparts). Therefore, only provide enough fertility for plant growth, based on information from laboratories associated with growing medium/soil samples.

5. Biological methods

Biological control involves releasing natural enemies (parasitoids and predators) to regulate insect or mite pest populations. Natural enemies are commercially available for use against aphids, thrips, and mites (Fig. 8). Greenhouse producers must release natural enemies early in the cropping production cycle, and before insect and/or mite damage is noticeable (Fig. 9). When considering the use of biological control, it is important to correctly identify the pest in question so that the appropriate natural enemy can be purchased and released. Plant type may negatively affect the effectiveness of natural enemies. For example, the hairs (trichomes) on the leaves of certain plants including cucumber may inhibit the ability of predatory mites (e.g., Phytoseiulus persimilis) and parasitoids (e.g., Encarsia formosa) to regulate twospotted spider mite and whitefly populations.

In conclusion, the implementation of non-chemical plant protection strategies such as scouting, sanitation, trapping, and cultural and biological methods is important in the production of greenhouse-grown vegetable crops in order to reduce or alleviate problems associated with insect and mite pests.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits. rcloyd@ksu.edu or 785-532-4750

Photo courtesy of Gilbertie's Herbs and Garden Center

When a company has been in business as long as Gilbertie’s Herbs and Garden Center, it’s because they’ve been able to identify and capitalize on each breaking wave. Started in 1922, Gilbertie’s was originally a cut flower grower. Located in Westport, Connecticut, Gilbertie’s was the only game in town when it came to producing locally grown cut flowers. Back then, cut flowers were trucked in to New York City to feed a large market in the city.

“They were doing cut flowers when there was no air freight,” says owner Sal Gilbertie, whose father started the business. “Then when air freight came in after World War II the cut flower business went down the tubes in the Northeast and they went into vegetables and herbs.”

Sal Gilbertie took over the business in 1959 after his dad passed away and has been doing well ever since, offering potted and fresh cut herbs for restauran ts and retail outlets. Gilbertie’s has gained notoriety in the past few years with visits from Martha Stewart and Chef Plum for an episode of Edible Nutmeg On The Road. Gilbertie has also authored multiple books, including one on cooking with microgreens.

“When the ecology boom started in the '60s everybody started jumping at herbs,” says Gilbertie. “For many years we were the only game in town. If you wanted herbs, you had to go to Gilbertie’s.”

For several years they sold potted herbs to garden centers. Gilbertie said they had up to 800 accounts on the East Coast. And then a wave broke again in 2000 and garden centers faced tough times.

“In 2000, when retail garden centers started going out of business, we started to diversify” says Gilbertie. “That’s how we got into the microgreens and cut herbs.” These days they still sell potted herbs and cut herbs, all USDA certified organic, to restaurants, Whole Foods Market, and various other markets in this population dense region.

Growing under glass in soil

Just about everything at Gilbertie’s is grown under glass in 27 greenhouses sitting on four acres of land. It’s all grown in the ground, which Gilbertie insists ensures a tasty and nutritious product. Compost and organic fertilizers and natural pesticides are used exclusively to comply with organic certification regulations.

“We take care of bug problems (e.g,, thrips, whitefly and aphids) with beneficial insects and also preventative methods,” says Gilbertie. “We have a lot of thrip screens on the outside of houses.”

Fertilization comes from different OMRI approved products, including Sustane and Azomite. He says they also use liquid fertilizers in their feeding program, including Nature’s Source 3-1-1.

Fungus problems in the houses rarely appear because they take preventative measures, particularly in their watering program.

“We watch our watering, we don’t water anything in the afternoon,” says Gilbertie. “We water in the morning so the plants can dry out.” He adds that other steps can be taken to avoid fungus problems, including not crowding plants too much and providing good light and air flow with the use of horizontal fans.

Gilbertie says they grow some of their product in the field. However, he says it’s more difficult to achieve USDA's Good Agricultural Practices (GAP) certification in the field where birds are flying overhead.

“To get GAP certification you would have to wash everything and we can’t afford to do that,” he says.“We’re 100 percent organic and GAP certified. We’re a few steps ahead of most guys except for the big players in the West and Southwest.”

The daylight needed to grow crops under glass on the herb farm is supplemented, when necessary, with sodium lights.

“We use lights because we can get so much more production,” says Gilbertie.

He says despite the higher day-to-day cost of using sodium lights, in his opinion it’s still a better choice than LED lighting.

“From the tests the Dutch have done, you get as much production out of sodium as LED,” he says. “Sure, you save on electricity (with LED), but it takes seven years to get your money back.”

Gilbertie also disagrees with those who use hydroponics to grow cut herbs and microgreens.

“You don’t have the same nutrient value or flavor in hydro

Offering the common and not so common

Gilbertie’s grows the herbs you might expect — basil, thyme, oregano, rosemary and parsley — but also the hard to find herbs that draw eccentric cooks, gardeners and chefs to Gilbertie’s.

“We also grow some oddball stuff,” he admits. “We grow the oddball stuff just because of our reputation and people will come from all over to get it because no one else has it.” One of the more obscure herbs they offer is nasturtium flowers. It’s an herb you could order from the West Coast — but it would be wilted and tasteless by the time it was trucked cross country.

“The demand changes constantly; we have to sell the stuff we can make a profit on,” says Gilbertie. “We can’t be competitive with stuff coming out of Mexico or the far West, it’s impossible to be competitive with that. We focus on organic and local.”

When you have a great product and a great location it’s hard not to be successful. Gilbertie’s has been riding a big wave since 1922.

“It you take a compass and put it where we are and draw a circle you’re talking about 40 million people, you can’t go wrong,” says Gilbertie. “Half of that is in the Atlantic and we’re still in a good spot.”

From their location near the ocean, Gilbertie’s can keep an eye on that next big wave, whatever that may be.

Neil is a horticulturist and freelance writer based in Sault Ste. Marie, Mich.

Photo courtesy of Greener Scenes Aquaponics

Worldwide and in North America, both the aquaculture and greenhouse industries are growing, and for good reason. Water availability is a global concern, world population is rising and high-quality protein is in high demand. The effects of climate change are also being felt on fields around the world, and food safety issues related to field agriculture and livestock farming abound.

Combining aquaculture and hydroponics would therefore seem to make sense. The practice of aquaponics involves feeding fish and using the water containing their wastes as a plant nutrient solution. The water is then recycled back to the fish through (often very simple) water treatment systems. Beyond the obvious efficiencies, fish offer the most efficient feed conversion of any livestock — as little as 1.5 pounds of feed per pound gained. In addition, various sources report that aquaponic plant production uses only about three percent of the water needed for field-grown crops and enables plant growth as much as three times faster.

What’s more, aquaponics systems can be built anywhere using a wide variety of designs, including vertically-integrated. Some systems have the plants growing in floating beds in the fish tanks, but un-coupled systems (interconnected but separate greenhouse and aquaculture operations) such as the one at Superior Fresh in Northfield, Wisconsin, are gaining interest. Right now, Superior Fresh is perfecting its production processes. But by 2018, the firm aims to annually harvest 120,000 pounds of salmon, 40,000 pounds of trout and 1.8 million pounds of greens. The greenhouse is about three acres with a water tank of 850,000 gallons. The company has stated that any fish waste that cannot be handled by the greenhouse plants will be applied to fields or composted for application later, with only a small amount of water recycled back to the fish.

Feasibility of aquaponics

Measuring the feasibility of aquaponics is not an easy thing to do. Dr. David Love (a scientist at the Johns Hopkins Center for a Livable Future in Baltimore) and his colleagues took a stab at it in 2015, publishing a survey of 257 commercial aquaponics operations (81 percent in the U.S., and the rest in 22 other countries) in the journal Aquaculture (see sidebar for details). The results indicate an overall economic advantage in putting the focus on plant production. “Plants reach point of harvest sooner than fish,” Love explains. “With fish, we found that you either lose money or break even. This is also supported by a 2015 USDA Southern Regional Aquaculture Center publication called ‘Economics of Aquaponics’ by economist Carole Engle, who has studied the topic extensively.”

In concert with this finding, Aqua Greens in Toronto, Ontario (which was not surveyed; it began operation in December 2015) has delayed fish harvest. Since the company began, it has delivered 12,000 plants (greens, basil, etc.) a month to upscale downtown restaurants and resorts north of the city, but is scaling back its restaurant count and is building a retail base to make deliveries simpler. Owners Craig Petten and Pablo Alvarez decided on which greens to produce by asking chefs what they wanted, and their roster now includes basil, lettuce and a kale blend. “We’ve spent the last two years on a learning curve to perfect all aspects of greenhouse operation,” says Petten. “We are now looking into markets for the tilapia, and also have to make sure processing will be done correctly by a third party. It may be that we will produce smaller dinner plate-sized fish. Right now, we have the same fish we’ve had since the start and they will be harvested at some point and replaced with our first market production run.”

Before opening, the pair of entrepreneurs enrolled in a three-year ‘Sustainable Energy Building Technology’ program at a local community college and completed planning of an aquaponics operation as their final project. Afterwards, they studied aquaponics ventures in three U.S. cities, took more courses, and settled on a vertical version of a design developed at the University of the Virgin Islands.

They’re using high-pressure sodium lights (HPS), which Petten and Alvarez say are economical and do a good job meeting the needs of their non-flowering greens. However, they are trialing LEDs right now as they will be using these in the expansion. “Work on that will start by the end of the year,” says Petten. “We are selling all the plants we produce and the demand is there for more.”

Greener Scenes Aquaponics in Brazil, Indiana is another operation waiting to market its fish. In April 2017, the venture started selling salad mix greens to a local store called Baesler’s Market. “Our current projections are 300 pounds of mix per week and no fish for at least the next year,” says CEO Zachariah Chambers. “It’s a no-brainer. Lettuce has a very quick turn-around and it’s profitable. By July, when we have more plants growing, the entire business will be self-sustaining just based on income from the greens.”

Other success factors

While the survey by Love and his colleagues found only 31 percent of surveyed commercial aquaponics operations were profitable during the prior 12 months, those who sold aquaponics equipment and/or consulting services were the most profitable. “Whether it’s doing this or offering courses or integrating agritourism,” Love says, “it can have a big benefit on the bottom line.”

Noa Fisheries near Toronto partly illustrates this. While the operation produces fry and fingerlings of two tilapia varieties for aquaponics hobbyists and commercial facilities, it markets no plants of its own — the only aquaponics systems onsite are solely for training purposes. And although it’s not a commercial aquaponics facility, the owners have offered a three-day aquaponics course over a dozen times since they started the business in 2010 and market an online version.

Love and his team also found that most aquaponics producers they surveyed tended towards smaller scale due to limited investment, limited space and other factors. However, Love does not wish “to point-blank advise all those planning an aquaponics operation to invest large amounts of money and have a huge venture.” He notes that “there is a lot wrapped up in the decision of how big to go, and the business plan must be carefully developed.”

One aquaponics venture in Quebec, near Montreal, is going big, but construction is on hold for the moment. The story starts with Marc Laberge, who from 2005 to 2014, owned and operated M.L Aquaponics in the vicinity, producing a very popular locally-branded Boston lettuce and smoked trout fillets. Laberge has spent much of the two years since involved in getting another similar operation — of over 30 times the size — off the ground. The greenhouse and the fish tanks are in place, but further work is now on hold until an upcoming clarification vote this summer about organic certification of plants under the Canadian Aquaculture Organic Standard (CAOS). “Organic certification is worth pursuing in order to secure demand,” notes Laberge. “We need to have every plant we produce sell. We can’t store our greens. It’s not that we have to charge much of a premium because our system will be extremely efficient, but having the organic label means being in demand.”

The valued-added trout fillets are also important to the venture’s future success. Laberge will produce both hot-smoked and cold-smoked trout, the former having a flaky texture and the latter the traditional well-known raw texture. (Tilapia production is not presently allowed in Quebec due to fears they will enter natural waterways and outcompete native species.) Smoked trout is worth 50 percent more than fresh trout, but Laberge says economies of scale are extremely important to make the extra work and equipment worthwhile. Indeed, circling back to the idea that bigger is better, Laberge believes economies of scale are critical for overall aquaponics success. “System design is very important, but you need excellent personnel with experience with water chemistry and so on, and if you are too small,” he explains, “you won’t attract good people and you can’t pay them what they are worth anyway. You get what you pay for.”

Chambers also advises putting together a strong team. “Have core expertise in fish, plants, operations, finance and marketing,” he says, “and plan for delays. While we kept our budget in check, buildout and ramp-up took about nine months longer than anticipated, resulting in pay going out the door with no revenue.”

On the whole, Chambers also believes everyone involved must have strength of character and a fundamental conviction in the vision. “The light is always at the end of the tunnel but the tunnel just keeps getting longer as you hit challenges and obstacles,” he says. “And when tomorrow happens, you need to have a team, the team that doesn’t give up.” He adds, “Starting a business is hard, especially when it requires infrastructure and multiple levels of expertise. Aquaponics takes all of the challenges of hydroponics and aquaculture and elevates the difficulty. Our collective expertise is aquaponics, and without it, we would fail.”

Treena is an Ontario-based national and international award-winning writer who focuses on science, technology and industry issues.