As far back as the Hanging Gardens of Babylon, man was known to experiment with growing plants in water rather than soil. The Greek terms “hydro” (water) and “ponein” (work, toil) are the root words of what is known today as hydroponics, a sustainable farming method that offers the benefits of resource efficiency along with increased crop yield.

Under the guidance of worldwide research, hydroponic growing systems continue to evolve. Focused studies examine and document the relationship between nutrients, water and varied growing systems that produce crops grown in recycled solutions rather than soil.

Purdue University’s Dr. Krishna Nemali recently led the webinar “Recycling Nutrient Solution Management in Hydroponic Production.” Participants ranged from commercial growers and school program leaders to home gardeners.

An associate professor of horticulture in Purdue’s Department of Horticulture & Landscape Architecture, Nemali teaches as well as conducts hydroponic research. This webinar looked at macro- and micronutrients and how the timing of solution replacement impacts nutrient uptake and plant growth.

“Crops require 14 essential elements for normal growth and development,” Nemali began. These were categorized as macronutrients, micronutrients and trace elements.

The first three listed in the macro group are a familiar fertilizer ratio used in both commercial production and home gardens: nitrogen (N), phosphorus (P) and potassium (K). Completing the group of six macronutrients are calcium (Ca), magnesium (Mg) and sulfur (S).

Measured micronutrients are iron (Fe), zinc (Zn), manganese (Mn), boron (B) and copper (Cu). Trace elements noted in the research were molybdenum (Mo), nickel (Ni) and chlorine (Cl).

Nemali stressed that just because one category is identified as “macro” and the other “micro” does not negate the importance of the latter group.

Hydroponics research continues to evolve and improve. Photo courtesy of Nemali Lab, Purdue University

Electrical conductivity (EC) is a measure of the total concentration of dissolved elements in the solution. One of the studies conducted at Purdue evaluated the growth rate and finished dry weight of lettuce in two different experimental groups. The control group had its circulating solution replaced daily; the second group used standard recycling methods of hydroponic growing.

“We maintained similar EC in both control and recycled treatments during the experiment,” Nemali said. “EC was adjusted daily in the recycling treatment while a fresh solution was supplied in the control.”

He noted that elements like calcium and sodium can accumulate in the recycled solution system. These have high ionic conductivities and can significantly affect EC values and that levels of some unwanted elements (bicarbonates) would increase with the addition of water to the recycled solution group.

Differences in plants could be seen at early growth stages, about two weeks from the start. The control group plantings outperformed those from the group with traditional continuous recycling. Nemali explained how the research project found that:

• Continuous recycling reduces crop growth in hydroponics

• Lower growth is due to decreased availability and/or uptake of essential elements

• Reverse osmosis water or dumping old solution can reduce the negative effects of recycling in hydroponics

It was also noted that replacement of the solution at two-week intervals, which is more cost-effective than the control group’s protocol, would have a beneficial result as well.

With the rise of world populations and concerns about farmland keeping pace with the resulting increased food demand, hydroponic farming is fast being recognized as a resource for increased global food production. Implementing a growing system that requires a much smaller footprint than traditional farming, green industry innovators are beginning to build successful hydroponic operations multiple stories high in urban areas today.

More information on upcoming hydroponics and greenhouse production webinars with Nemali is available at

by Gail March Yerke