Edible crop species differ in their pH effect in hydroponics

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).