2018 ASHS Annual Conference

Greenhouse crop species differ in their effects on root zone pH in soilless and hydroponic production. Objectives of this study were to (1) quantify acidic and basic effects on the root zone pH for eight vegetable and herb species grown in peat-based substrate and hydroponic nutrient solution and (2) determine NH₄⁺:NO₃^- nitrogen ratios expected to have a neutral pH reaction for each species during their vegetative growth phase. In one experiment, plants were grown for 33 days in substrate containing 70% peat:30% perlite by volume and were fertilized with a nutrient solution containing 7.14 mEq∙L^-1 N and NH₄⁺:NO₃^- ratios ranging from 0:100 to 40:60. In the second experiment, the same species were grown for six days in hydroponic nutrient solutions at 7.14 milli-equivalents (mEq)∙L^-1 N with NH₄⁺:NO₃^- ratios ranging from 0:100 to 30:70. Acid and base produced in the root zone and cation versus anion uptake were quantified in both experiments. Species produced from 2.18 mEq of acid (lettuce) to 2.65 mEq of base (arugula) in substrate and from 2.82 mEq (lettuce) to 3.86 mEq (eggplant) in the hydroponic solution. Acid and base per gram of dry mass gain (plant growth) ranged from 0.60 mEq∙g^-1 acid (lettuce) to 1.74 mEq∙g^-1 base (arugula) in substrate and from 1.35 mEq∙g^-1 acid (lettuce) to 1.32 mEq∙g^-1 base (arugula) in solution. Plants produced acid when there was a greater net uptake of cations over anions, and produced base with a net uptake of anions. Overall, arugula had the greatest uptake of anions compared with other species. Percent NH₄⁺-N of total N expected for a neutral pH ranged from 0% (cucumber) to 55% (arugula) in substrate and from 6% (lettuce) to 32% (arugula) in hydroponic solution. Evaluating the effects of vegetable and herb species on root zone acidity and basicity can be used to select a neutral NH₄⁺:NO₃^- fertilizer ratio for substrate and hydroponic production.