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The Dirt on Soilless Growing

By Tamara Scully




A variety of baby lettuce, growing in a deep flow system under T5 fluorescent lights, is just about ready to harvest.
Photos courtesy of Mark Doherty, Aqua Vita Farms, unless otherwise noted.

Soil is a growing medium, but it's not the only one. When planting seeds into trays, a soilless growing mix - traditionally containing some combination of vermiculite, perlite, sphagnum peat moss, and possibly bark or coir - is used to reduce soilborne disease pressures. In other words, the seeds are hydroponically grown.

Hydroponics refers to growing plants without soil. Hydroponic growing media can be liquid, air or an aggregate mix. In addition to the standard soilless potting medium typically used to start seeds, sand, pine bark, rice hulls, sawdust, clay-aggregate "grow rocks," gravel, or even a synthetic polyurethane foam (popular with commercial growers) can be the medium in a hydroponic system.

"One thing I want to be clear about is that soilless is not without media. All hydroponic systems, often referred to as soilless, have some type of media," said Mark Doherty, owner/consultant at Aqua Vita Farms. "Even in an aeroponic system, the roots need something to hold on to."

Plants need to grow their roots in something, and they need the proper nutrients to survive and thrive. In hydroponic systems, nutrients are delivered to the plants in a water-based solution, no matter which growing medium is used. In some systems, the nutrient solution is the growing medium.

Hydroponic growing systems

There are a half-dozen primary types of hydroponic systems. Each delivers the needed nutrients in a slightly different manner. The main systems are: nutrient film technique (NFT), deep flow (or water culture), aeroponic, wick, drip, and ebb and flow (or flood and drain) systems. A nutrient solution is delivered to plants via drip or flow irrigation, or via misting in an aeroponic system. In most systems, the nutrient solution can be reclaimed and recycled.

In the simplest aggregate-medium wick system, plants are rooted in the growing medium, such as perlite or coir, and a wick is used to pull the nutrient solution to the plants from a water reservoir. In an ebb and flow system, the plants are grown in gravel, grow rocks or other solid medium, and the nutrient solution submerges the medium at timed intervals before being drained. In a drip system, the plants are rooted in a solid medium, and the irrigation system delivers the nutrient solution to the individual plants.

There are also a few types of nonaggregate hydroponic systems. In a deep flow system, the plants are supported with their roots hanging into the nutrient solution. The NFT system has a continual film of fluid flowing over the roots of the plants in a gravity flow system. In an aeroponic system, the nutrient solution is misted periodically on the roots, which are in a sealed box, closed off from light.

Growers must monitor many of the same factors in hydroponics as they would when cultivating plants in the field. Water availability, nutrient intake and pH of the medium all remain important factors. There are also pest management strategies to consider.

"Pest management is all about your personal standards, commitment to nature and brand message," Doherty said. "Organic and nonorganic methods work regardless of indoor, outdoor, greenhouse, or hydroponic or soil-based mediums."

Controlled environment agriculture

"Hydroponics allows for a higher yield per acre and a more consistent product," Doherty said. "Combine that with controlled environment agriculture [CEA] and you can get those benefits year-round, regardless of climate."

CEA is done indoors in an environment where heat, light and inputs can be controlled. Hydroponic cultivation often occurs under controlled conditions, but can be done outdoors as well. Plants that are hydroponically grown outdoors will be affected by light, wind, precipitation, temperature, and insect and disease pressures, just like field-grown plants.

Louis Albright, professor emeritus and co-director of Cornell University's CEA program, explained that with the near-optimal conditions provided by CEA, including supplemental light and even carbon dioxide, plant growth is more rapid. Moving production outdoors and into the natural environment negates this advantage.

"We have discussed it [outdoor hydroponics], but not done it, because our goal is consistent production year-round, and the Ithaca, New York, climate is not conducive to that," Albright noted.



A 100-foot-long, double-stacked deep flow hydroponic system.

Hydroponic growing is a large part of CEA, but so is growing in soil. Both systems benefit from the CEA environment. According to Albright, benefits of growing indoors include reduced water use, less environmental contamination from runoff, better biological control outcome, fewer plant stressors, better annual yields, and a consistent working environment.

"These come at a cost, of course," Albright added. "One limitation of soilless growing in CEA is the greater cost - nature provides the growing medium for outdoor growing."

Equipment

Hydroponics, whether indoors or out, isn't as simple as planting a seed in some fertile ground; watering, weeding and feeding it; and keeping pests and diseases at bay. Even outdoors, without the costs associated with erecting and maintaining a greenhouse, hydroponic growing requires a medium, a system to support the plants as they grow, a nutrient solution, a means of providing the solution to plants, a way to monitor and adjust the system, and the tools to put it all together.

For example, a basic outdoor commercial hydroponic strawberry system, designed by the University of Florida, requires perlite as a medium, grow bags, electricity, irrigation system setup and fertilizer.

The University of Colorado offers descriptions of a variety of hydroponic systems, complete with illustrations and basic information on system operations, online at http://bit.ly/SObEyc.

Hydroponic growers can use a trough system, where the trough is filled with the growing medium. Alternatively, beds of growing medium can be established. Grow bags, pots or other containers can also be used, either with drip irrigation or in a system where individual plant containers are placed within the same larger trough in a wick or ebb and flow system.

Plants can also be suspended in the same container, such as in a deep flow system, where plants float on rafts with their roots suspended in a large communal pool of nutrient solution. Some hydroponic systems can be elevated off the ground for ease of use, or can be stacked vertically to optimize production in minimal space.

The choice of nutrient solution can range from premade conventional liquids or powders to homemade mixes. As in field growing, getting the macro and micronutrients correct is important for plant growth and health. Additives such as root healers, mycorrhizal fungi and silicates can enhance the growing environment.

Concerns

Keeping plants healthy in a hydroponic system involves the same basic principles as field growing: providing the proper nutrients, protecting against pests and diseases, establishing and maintaining a healthy growing environment, and bringing the plants to harvest. However, maintaining optimal conditions is more complex when the growing system is not soil-based.

In addition to the many variables of the nutrient solution that must be monitored, the solution must be replaced regularly to maintain the proper nutrient balance.

In hydroponic systems, the pH can fluctuate in various ways: via the water itself, the nutrient solution, and the medium or substrate used. Water pH will fluctuate simply upon exposure to air. Other elements in the water can also cause pH fluctuations.

As plants use the nutrients, the pH level of the nutrient solution will change. A variety of ions are produced as plants absorb nutrients, and the changing level of these ions impacts the overall pH.

The growing medium used can also impact the pH due to its own characteristics and a variety of chemical and biological interactions. Unlike soil, which provides a pH buffer, hydroponic systems are subject to rapid fluctuations in pH due to the ongoing changes that occur as plants utilize nutrients and interact with air and water.

Dissolved oxygen (DO) is another critical factor in hydroponics. Air bubblers, air pumps or air stones can be used to increase the DO levels. Adding hydrogen peroxide to the system is another alternative.

Since water holds dissolved solids as well as gases, the amount of oxygen is impacted by the amount of nutrients in the solution. Without the correct DO levels, plants will not thrive. The level of DO required depends on the type of hydroponic system being used. The amount of root submersion and the flow rate and temperature of the nutrient solution impact the available DO.

The amount of soluble salt, expressed as electrical conductivity of the nutrient solution, needs to be monitored and maintained within acceptable limits for plant growth. Water hardness refers to the concentration of calcium and magnesium, which can naturally be high depending on the water source used. The hardness will impact the amount of salts that need to be included in the nutrient solution and can also impact the availability of other nutrients.



Two-week-old plants, just out of the germination chambers and planted into the deep flow system.

Keep in mind that these factors are constantly variable.

In addition to pH and DO, hydroponic systems are also subject to rapid changes in temperature, lighting and other issues, particularly in CEA systems. For example, a loss of electricity, which prevents oxygenation of the water, can be devastating. The buffer against detrimental system changes is small in hydroponics, and those changes can have dire effects. The response will vary depending on the system, with some being negatively impacted more rapidly. The risk may be greatest in an aeroponic system.

"The small volume of nutrient solution held in the system per plant greatly limits options for buffering against sudden swings of conditions," Albright said.

As a hydroponic operation increases in size, other factors come into play. Considerations include carbon dioxide generators to keep levels optimal under dense planting conditions, odor control, the use of plant stimulant, optimized supplemental lighting and more. The management of these systems becomes more intensive, and more equipment and infrastructure is required.

Future of hydroponics

"Hydroponic growing currently exists as a complementary technique in the world of agriculture," Doherty said. "That being said, however, as the evolution of hydroponics is taking place, we are seeing a great deal of investment in commercial-scale farming ventures using this method."

While greens, tomatoes and strawberries are commonly grown in hydroponic systems, virtually any crop can be grown hydroponically. Even root crops, probably the most difficult for a hydroponic system, are now a possibility.

"I would say that the initial limiting factor would be selecting the right-style system for the crop being grown," Doherty said. "One crop which has proven tricky is root vegetables. However, I have read of people having success growing them in aeroponic systems and flood and drain systems. The most difficult crop will be the one with the least research and historical data."

Hydroponic growing allows for higher yields in less space, doesn't require a land base, and uses less water than traditional soil-based growing. When done as CEA, the controlled environment combined with the benefits of soilless growing is often seen as a solution to many of the issues facing agriculture today, such as climate change, Doherty said.

According to Albright, the benefits of combining CEA and hydroponics include less environmental contamination, erosion and nutrient leaching; no use of animal manures - and therefore less potential for contaminated produce; and more consistent crop growth and greater space efficiency.

"The world of hydroponics and CEA is changing rapidly and rather expansively," Doherty said. "The driver of the emerging hydroponic production is economic viability, and that is a great thing. So once there becomes a profitable business model, then investments will flow in, and we will see these technologies play a larger role in global food production."

The author is a freelance contributor based in New Jersey. Comment or question? Visit http://www.farmingforumsite.com and join in the discussions.