Nutrient Solution and Solution pH Influences on Onion Growth and Mineral Content

The effects of hydroponic nutrient solution composition and pH on growth and mineral content of green onions was evaluated. Three onion varieties [Allium cepa L. (‘Deep Purple’ and ‘Purplette’) and A. fistulosum L. (‘Kinka’)] were propagated in three nutrient solutions (Peter's Hydro-Sol, modified Hoagland's, and half-strength modified Hoagland's) at two pH levels (5.8 and 6.5) in a three-by-two factorial design applied in a randomized block with three replications. Seeds were germinated in Cropking's Oasis Horticubes under greenhouse conditions and irrigated with tap water. Once the seedlings reached the flag stage, the plants were placed into hydroponic units within the greenhouse and grown under ambient conditions. Plants were harvested 30 d after transplanting to the hydroponic units. The results indicated nutrient solution, pH, and variety significantly affected several plant physiological variables. Total biomass and edible biomass production was as high for plants grown in half-strength Hoagland's nutrient solution as for those grown in the other solutions. Total biomass was greatest for plants grown at a solution pH of 6.5. ‘Deep Purple’ produced a significantly greater overall total biomass than did ‘Purplette’ or ‘Kinka.’ Hydro-Sol tended to produce onions with highest mineral content. Due to the fact that biomass production was as great in the half-strength Hoagland's as in the more concentrated solution and that a pH of 6.5 produced greater total biomass, the half-strength Hoagland's solution at pH 6.5 was the preferred nutrient solution evaluated in this research. Selection of an appropriate nutrient solution must consider both edible biomass production and mineral content. In the research reported here, the solution that produced the greatest biomass did not produce plant material with the highest mineral content.