Temporal Variation of Nutrient Ratios Within Hydroponic Nutrient Solutions

Indoor hydroponic farming enables pesticide-free food production with significantly lower water use than conventional agriculture. However, standard practices often involve frequent replacement of nutrient solutions to manage nutrient imbalances and microbial contamination, generating large volumes of nutrient-rich waste with both environmental and economic costs. We investigated the temporal dynamics of nutrient composition under two contrasting nutrient management strategies, focusing on the impact on cost per yield and sustainability on Red Batavia lettuce production. In Treatment 1, half-strength Hoagland's solution was replaced weekly to maintain an electrical conductivity (EC) of 1.0–1.2 mS/cm⁻¹ as per typical industry practice to maintain target nutrient ratios. In Treatment 2, nutrient solution was not replaced, and instead topped up only to maintain sump tank volume, irrespective of EC. Macro and micronutrient concentrations were monitored across both treatments to evaluate EC as a proxy for nutrient availability. While Treatment 1 achieved significantly higher yields (> 20%; p < 0.05), the cost per gram of fresh and dry weight was substantially higher than in Treatment 2, by 63% and 66%, respectively. Nutrient depletion accelerated at later growth stages, particularly in Treatment 2, suggesting increased uptake efficiency under limited availability. Over both treatments, EC was found to be a poor proxy for actual nutrient changes. These findings highlight a trade-off between yield maximisation and nutrient-use efficiency, emphasising the need for more strategic nutrient management. Our results support the advancement of circular, resource-efficient hydroponic systems aligned with long-term sustainability goals. Specifically, the integration of solution reuse, targeted nutrient dosing, and non-disruptive pathogen control could improve environmental performance and economic viability in indoor food production.