Southwest Environment

Stories written by University of Arizona students

Department of Soil, Water and Environmental Science
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Hydroponics system grows crops with less water, no soil

By Brandon Merrill
U of A greenhouse

Photo by Brandon Merrill
The dull exterior of this greenhouse at the corner of Roger Road and Campbell Avenue belies its lush interior.

On the corner of Campbell Avenue and Roger Road stands a building made from plastic covering a tenth of an acre. This dull-looking building, one of many at the site, hides a small oasis inside. Rows of lush, ripe tomatoes drip from their vines. Golden sweet peppers glisten in the sun and cucumber plants spread their beachball-sized leaves toward the sun.

Hydroponics allows farmers to adapt to any situation, whether it’s Antarctica’s frozen tundra, Saudi Arabia’s windswept and barren deserts, southern Arizona’s Sonoran Desert, or even a space station. The University of Arizona’s Controlled Environment Agriculture Center has built a prototype system – called the Moon Base Greenhouse – that could potentially be used on the moon.tomatoes in greenhouse

Photo by Brandon Merrill
Inside the greenhouse, Asher Baltzell harvests ripe tomatoes.

“We can control everything a plant needs using a hydroponic system and artificial lighting. That is what we are trying to prove with the lunar system we have,” said UA Assistant Professor Patricia Rorabaugh, a plant scientist who does research and outreach at CEAC.

Essentially, hydroponic plants grow in water, with material such as the fibers from coconut shells or Styrofoam holding the plants in place. The roots float in water that provides all the nutrition the plants need. This water works double-time by both quenching the plants’ thirst and feeding them. And, if well managed, hydroponics can actually conserve water. See related story.

Plant Science student Asher Baltzell, who works in the greenhouse, explained that neither the bags nor the tank are ever 100 percent full of water. Just like people, plants need oxygen, and if there is too much water the plants will drown. Plants can “breathe” enough oxygen from that empty space and still get all the essential water.

A controlled environment

Like all greenhouses, those featuring hydroponic systems can be climate-controlled. The target is to keep the greenhouse around 75 degrees Fahrenheit in the day, and 65 at night.

Another greenhouse on the site goes well beyond the usual monitoring and controlling of greenhouse variables, such as temperature, humidity, and light intensity. In CEAC researcher Murat Kacira’s high-tech greenhouse, sensors can detect plant stress and initiate an appropriate response to meet their specific needs.

“What I like to do is control the environment, control the resources that plants need,” said Kacira, who conducts research in the greenhouse. “This way, we can improve the resource use efficiency. That's what I do with the use of advanced sensors.”

But what makes hydroponics worth the trouble? Hydroponics is a technology that fits perfectly in the desert, where water is limited and the intense heat prevents many plants from growing here.

A water-saving system

“With a floating lettuce hydroponic system, you're not going to lose any water except for the water that was transpired,” Rorabaugh said. The water that is transpired exits through the leaves as water vapor. One purpose it serves is cooling the plant, and in that way it can be roughly compared to how people sweat to keep cool.

A hydroponic lettuce system could use only 10 percent of the water needed compared to field-grown lettuce, Rorabaugh explained.

Arizona uses about 70 percent of its water for agriculture. Theoretically, about 90 percent of all that water could be saved if every farm converted to hydroponics. The problem is the cost – in money, energy, and nutrients.

It takes a lot of money to build just the greenhouse. Rorabaugh noted that the initial cost is about $100,000 dollars to build even a one-tenth-acre greenhouse, such as the one depicted above. Any commercial greenhouse will require at least an acre, and be significantly more expensive. The sticker shock is high enough that many farmers will be wary of adopting this technology unless they can be guaranteed the investment will pay off.

Unfortunately, a hydroponic greenhouse is even more expensive because of the energy and resources required. All the water-mixed nutrients need to be in large concentrations to feed the plants, and power is required to pump that water, to run the fans, and to run the sensors.

High-priced crops are chosen to counterbalance these expenses. Tomatoes and specialty crops like yellow peppers and basil and other herbs can deliver profits that support such a greenhouse. Corn, wheat, any grain crops are examples of a crop better suited for an open field.

Inside the hydroponic greenhouses, the plants are growing very close together. Not only does this lend itself to looking more lush and oasis-like, it saves land because the plants need less space to grow. The benefit to a farmer is significant – more food can be grown on less space. The combination of high-priced crops and more food in a smaller area help soothe the pain from the initial cost of the greenhouse with the potential for higher profit. Unfortunately, cost isn’t the only concern for farmers.

Mining for nutrients

In the search for a more sustainable and “green” agriculture, hydroponics is one of many choices. However, the large concentration of nutrients required becomes an issue. These minerals have to come from somewhere – usually a mine.

Mining all these minerals is a sticky issue for hydroponics systems to be considered sustainable. Any mine will, after all, be depleted.

This doesn’t mean that hydroponics isn’t a sustainable practice, as Kacira will point out.

“With sustainability we're kind of concerned about how we use the resources and see how we can replenish them and make it available for future generations,” he said. “So if your system is capable of doing that, or improve the resource use efficiency, I think those systems are more environmentally friendly and sustainable compared to other systems.”

In hopes of solving the fertilizer issue, UA graduate student Stacy Tollefson is combining organic farming with hydroponics. The idea is that organic fertilizers are a renewable resource, potentially replacing the need for mining. This could make it sustainable.

By using a fertilizer made from “compost tea,” Tollefson is growing lettuce in a floating system. In an experiment, she created several treatments: one which receives the traditional inorganic fertilizer, and two treatments that use different amounts of the compost tea.

hydroponic lettucePhoto by Brandon Merrill
Stacy Tollefson raises the Styrofoam boards that hold the hydroponic lettuce, revealing the healthy white roots tapping into water containing “compost tea.”

While explaining the research, Tollefson raised the lettuce to show the water beneath. The traditional system had clear water with no indication that anything was in it, even though it was packed with nutrients. In contrast, the compost-tea looked more like coffee than water.

This organic fertilizer clearly contained more than just water. And that is exactly what excites Tollefson about the organic fertilizer. Traditionally, the fertilizers used just mined minerals that were added to the water. However, their organic counterparts have whole microbial communities in addition to the minerals, she explained.

“Organic fertilizers have microbes that are vital to preventing pathogens,” Tollefson added.

If research can make organics as viable as traditional fertilizers, then the only major disadvantage for hydroponics is the cost.

Some commercial operators are finding hydroponics a worthwhile investment. Several hours away from Tucson, in Willcox, is the country’s largest hydroponics greenhouse facility, Eurofresh. Sunizona, a smaller family-owned hydroponic company is also located there. Perhaps other hydroponic businesses will decide to take advantage of this growing technology.


Brandon Merrill is a senior in Plant Science and Japanese who hopes to put some of these ideas to work after his graduation in the spring of 2011.


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