Yield and Nutritional Quality of Water Spinach (Ipomoea aquatica) as Influenced by Hydroponic Nutrient Solutions with Different pH Adjustments

It is critical to identify effective buffer chemicals which are capable of regulating the pH of nutrient solution to a desirable level for best hydroponic production of crops. Greenhouse experiments were conducted to examine the pH dynamics of nutrient solutions amended with different inorganic acids during hydroponic production and the yield and nutritional quality of the resulting crops. A typical nutrient solution (pH 8.2) was adjusted to a pH of 5.60.2 with 1 M HNO3, 1 M H3PO4, 1 M H2SO4, and a 3:1:1 (v/v/v) mixture of all three acids, respectively. Water spinach (Ipomoea aquatica Forsk) was hydroponically grown for 25 d in the unadjusted control and the pH-adjusted nutrient solutions. The different treatments were monitored for pH changes of the nutrient solution, and measured for shoot yield and nutritional quality of the grown water spinach. It showed that the solution pH adjustments introduced additional anions but did not significantly increase the electrical conductivity (EC). The HNO3-H3PO4-H2SO4 mixture was able to achieve an optimal solution pH ranging from 5.5 to 6.5, while any of the acids only failed to maintain the solution pH within optimal range for 48 h after each adjustment. The shoot fresh weight, dry weight, and height of water spinach grown in the mixed acids-treated solution were the greatest among the five treatments. Relative to the control, the acid mixture treatment also increased the vitamin C, soluble sugar and crude protein contents in plants. Thus, the mixed 3:1:1 (v/v/v) HNO3-H3PO4-H2SO4 is recommended for regulating the pH of nutrient solution in hydroponic production of leafy greens.