Plant Species Effects on Acidity or Basicity of Hydroponic Solutions

The relationship between solution-pH change and plant cation/anion uptake was quantified for three floriculture species (Pelargonium x hortorum, Impatiens walleriana, and Petunia x hybrid). Plants were grown in complete hydroponic nutrient solutions that contained 0%, 10%, or 20% ammonium-N with the remainder of N supplied as nitrate. The solution-pH in each hydroponic vessel replicate was maintained between 5.5 and 6.5 and titrated back to initial solution-pH (6.0) after 7 days using HCl and NaOH. Specific nutrient uptake by plants was determined by depletion from the nutrient solution after 7 days, and net acidity or basicity was determined by additions of acid (HCL) or base (NaOH) during solution-pH correction. Ion uptake and net acid or base addition were converted to milli-equivalents (meq) for comparisons.

There was a close 1:1 relationship between net cation or anion uptake and net acidity or basicity, (meq acidity of the solution = 0.962±0.195*(net meq of cations – anions taken up) + 0.140±0.478; adjusted-R² = 0.739) over all species and solution types, which indicated a strong correlation between solution-pH change and net cation-anion uptake. The largest difference between species occurred in the 0% ammonium solution, where Petunia produced the most base, followed by Impatiens, and then Pelargonium. All three species responded to increased ammonium concentration with increased total cation uptake and solution acidity. However, in the 20% ammonium solution all species produced similar net acid after 7 days. Ammonium uptake was preferred over nitrate, and the ratio of ammonium to nitrate nitrogen supplied in solution greatly influenced solution acidity or basicity. Both Impatiens and Petunia depleted nearly all ammonium within 7 days which probably affected final net acidity or basicity measurements. Understanding species acidity-basicity potentials has practical applications in commercial floriculture for matching fertilizer type to plant species. Additional trials evaluating nutrient uptake over time, over wider ammonium to nitrate ratios, and in soilless substrate are necessary to characterize species effects on solution-pH.