High Nutrient Concentrations of Hydroponic Solution Can Improve Growth and Nutrient Uptake of Spinach (Spinacia oleracea L.) Grown in Acidic Nutrient Solution

Abstract

Hydroponic leafy green production offers high productivity and quality of crops but requires good management of pH and electrical conductivity (EC) to optimize the nutrient uptake. Nutrient solution pH is typically managed between 5.5 and 6.5, whereas lowering pH to more acidic range (e.g., <5.0) can potentially mitigate problematic waterborne diseases. Plant response to low pH is species specific and generally involves direct effect of increased hydronium ions and indirect effects of pH-dependent factors, such as low cations availability. To develop a new hydroponic nutrient management strategy, ‘Corvair’ spinach plants were grown under pH 4.0, 4.5, 5.0, and 5.5 of a hydroponic nutrient solution using a deep-water culture system in a growth chamber. Spinach shoot and root mass after 19 to 20 days declined with lowering pH. At the lowest pH of 4.0, plants displayed stunted overall growth and severely inhibited root development. Plant growth and morphology at pH 4.5 or 5.0 were normal but small, suggesting that growth reduction at these pH was likely a result of reduced nutrient uptake. Plant tissue analyses showed decreased N, P, K, Mg, S, Cu, Fe, Mn, and Zn concentration as pH decreased. When the strength of nutrient solution was increased three times at a low pH 4.5 to improve the overall nutrient availability, spinach shoot and root fresh weight with high nutrient concentrations (EC 3.4 dS·m−1) significantly improved but was still lower than those in the control (pH 5.5 and EC 1.4 dS·m−1), respectively. Plant tissue analysis showed that lowering pH to 4.5 significantly reduced tissue concentrations of P, K, Ca, Mg, S, Cu, Mn, and Zn compared with those in the control. Under low pH and increased EC treatment (pH 4.5 and EC 3.4 dS·m−1), all dry leaf nutrient concentrations were similar or higher than those of the control, except Mg and Zn, which showed a lower concentration than the control with a weak significance (P < 0.06). This suggests that additional optimization of nutrient formula might further improve the spinach growth at low pH. Together, our results will help to develop a new and low-cost nutrient management methodology to produce leafy greens hydroponically.