Nutrition solutions

We most commonly use the electrical conductivity (EC) of a nutrient solution to measure the concentration of mineral nutrients in it. This measurement doesn’t reflect the concentration of any specific, individual ion; rather, it reflects the total concentration of nutrients.

To maximize yields and appearance, nutrient solutions are maintained at target ECs, which can vary across species. For fruiting vine crops like tomato, ECs of ~2.5 to 3.5 mS/cm are appropriate. For lettuce, producers may target 1.5 to 2.0 mS/cm. In our research at Iowa State University, we found nutrient solution ECs ranging from 0.5 to 4.0 mS/cm didn’t affect yields for herbs including basil, parsley, dill and cilantro.

Culinary herbs are clearly a nutrient-efficient crop. Now, although yields at 0.5 and 4.0 mS/cm were comparable, you may want to avoid either of these extremes. On the high end, it is simply a waste of fertilizer inputs. A lower EC (i.e. 0.5 mS/cm) may result in nutrient imbalances developing more quickly at the lower concentrations.

It is not simply the total concentration of nutrients we are concerned with, but also specific elements. As nutrients are taken up by plants and concentrated fertilizer stock solution is delivered to the nutrient solution to increase the EC back to target concentrations, nutrients are generally not taken up in the same proportions they are added.

Over time, nutrient imbalances can occur, be they deficient or toxic levels. Periodic nutrient solution sampling by a commercial lab is the only way to know the concentrations of nutrient in your solution. As the nutrient ratios become unbalanced, a fraction of the nutrient solution can be exchanged out to try and restore nutrient balances.

There are some nutrients that warrant special attention in culinary herb production. One of the most important is magnesium in basil. Though it can sometimes be mistaken for a micronutrient deficiency (such as iron), magnesium deficiency in basil can be fairly common.

There are two factors that contribute to this. First, basil simply has a higher-than-average requirement for magnesium compared to other species. Second, the majority of hydroponic fertilizers on the market are formulated with higher concentrations of calcium. Since calcium and magnesium have an antagonistic response with respect to plant uptake, the higher concentrations of calcium can suppress magnesium uptake.

In addition to EC, the pH also affects mineral nutrient dynamics, specifically micronutrients. As pH increases, the availability of micronutrients in the nutrient solution decreases. When this occurs, micronutrient deficiency symptoms — most commonly interveinal chlorosis — can occur.

Beyond micronutrient deficiencies reducing marketability, increasing pH much beyond 7.0 to 7.5 can suppress growth and diminish yields. Although an increasing pH has detrimental effects on culinary herb growth, research at The Ohio State University and Iowa State University is demonstrating lower nutrient solution pHs (4.0 to 4.5) don’t cause micronutrient toxicity symptoms or limit growth.

While we are generally maintaining nutrient solution pHs between 5.5 and 6.0, this new research indicates a lower pH may help maintain micronutrient availability and therefore leaf greenness, as well as not limiting yields. Additionally, research at The Ohio State University also shows Pythium is suppressed in nutrient solutions maintained at lower pH levels, providing a cultural practice to prevent the root rot.

Christopher J. Currey is an associate professor in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu