Zucchini plants (Cucurbita pepo L. cultivar Afrodite) were grown during a summer–fall season in closed-soilless systems using a mixture peat–pumice to evaluate the effects of irrigation system (drip irrigation and subirrigation) and nutrient solution concentration (half = 1 dS·m⁻¹ and full = 2 dS·m⁻¹) in terms of substrate electrical conductivity (EC_s) using the dilution 1:1.5 media:water method, growth, yield, leaf mineral composition, fruit quality, and mineral solution composition. At the end of the cultural cycle, the highest EC_s at the upper (0 to 7.5 cm) and lower (7.5 to 15.0 cm) layers were recorded with subirrigation using a full nutrient solution concentration. The highest plant growth, yield, and leaf macroelements concentration (nitrogen, phosphorus, and potassium) were recorded in both irrigation systems using a full-strength solution, followed by drip irrigation, and finally by subirrigation treatment using a half-strength nutrient solution. Fruit yield, fruit mineral composition (phosphorus, potassium, calcium, and magnesium), and leaf macroelements concentration (nitrogen, phosphorus, potassium, and magnesium) were substantially reduced when the concentrations of macronutrients in the feed solution were lowered to 50% of control. Using half-nutrient solution concentration, the marketable yield reduction was more pronounced with subirrigation (58%) than with drip irrigation (42%). The variation of the macronutrient and EC in the solution during the growing cycle was less pronounced in the subirrigation than with the drip irrigation system, which represents an important aspect for the simplification of the closed-loop management of the nutrient solution.