Exploring nutrient reduction strategies without yield losses in hydroponic lettuce production

• Lettuce yield remained unaffected under reduced EC and N:K levels in the root zone. • Partial NO₃⁻ substitution with Cl⁻ lowered leaf NO 3 - by 33 % improving food safety. • Lower nutrient levels in the root zone ensured optimal lettuce nutrition. This study investigated the impact of reduced nutrient input in closed-loop hydroponic systems on yield, leaf nitrate (NO 3 ⁻) content, and nutrient uptake in three lettuce cultivars: butterhead, lollo bionda, and lollo rossa. Two greenhouse experiments were conducted using four nutrient solutions (NS) treatments combining two electrical conductivity (EC) levels (2.5 and 2.0 dS m⁻¹) with two nitrogen (N) to potassium(K) ratios (3.28 and 2.28). In the lower N:K treatment, NO₃⁻ was partially substituted by Cl⁻ to maintain ionic balance. Fresh weight (FW) was not significantly affected by either EC or N:K ratio, indicating that yield can be preserved despite a reduction in nutrient input. In contrast, leaf NO 3 ⁻ content was significantly influenced by both factors EC and N:K ratio, with reductions of up to 33 % observed in the low EC/low N:K treatment. Leaf NO 3 ⁻ content ranged from 1431 to 1642 mg NO₃⁻ kg⁻¹ of FW across cultivars, which is well below European Union’s safety limit of 5000 mg kg⁻¹ FW. Nutrient uptake concentrations (UCs) were estimated using both mass-balance and plant tissue analysis methods. Significant discrepancies were found between the two approaches for N, K, Ca, and Fe, in the mass-balance method yielding higher values, suggesting losses of these nutrients from the system. The Cl⁻ UC increased significantly under low N:K conditions. These findings demonstrate that reduced EC and a lower N:K ratio than standard recommended values can result in lower NO 3 ⁻ accumulation and nutrient input without compromising yield, supporting the development of more sustainable fertigation protocols for hydroponic lettuce production.