2025/08/19 by Noah James Langenfeld, Bruce Bugbee · 1 voice
Chemical Engineering · #Analytical Chemistry and Sensors
paper · doi:10.21273/hortsci18771-25
openalex publication_date 2025/08/19 · openalex created_date 2025/10/10 · openalex updated_date 2026/06/26
Low concentrations of ammonium in hydroponic solutions typically improve plant growth and better simulate field conditions, but it is challenging to achieve steady-state levels because ammonium uptake is several orders of magnitude faster than nitrate uptake. Millimolar additions of ammonium cause rapid pH decreases because ammonium uptake is coupled with proton release from roots. We used an automated pH control system to add micromolar concentrations of ammonium several times an hour in combination with nitric acid. This stabilized pH and allowed for the addition of 3% to 13% of the nitrogen as ammonium. The type of ammonium salt was important: ammonium sulfate led to sulfur accumulation in solution and ammonium dihydrogen phosphate led to phosphorus (P) accumulation. Both salts caused intermittent pH decreases from pH 6 to 4. When the pH control solution included ammonium nitrate in a 1:2-M ratio with nitric acid there was no anion accumulation and a stable root zone pH. The resulting micromolar equilibrium concentration of ammonium facilitated repeated cropping of lettuce in the same solution at an electrical conductivity (EC) of 0.4 mS/cm without a decrease in yield. This better simulates field environments where ammonium ions are present at a low, steady concentration in the root zone solution.