The effect of water quality on lettuce growth in deep-culture hydroponic systems

Hydroponic systems are identified as efficient soilless alternatives to mainstream agriculture. Ensuring proper physical and chemical composition of the nutrient solution is the key to fostering the plant's growth in hydroponic systems. In this study, the effect of overall water quality parameters including electrical conductivity, nutrient levels, and the effects of pH and aeration on the growth of lettuce in a controlled hydroponic environment was investigated. This research aims to identify an effective strategy to enhance resource efficiency, crop yield, and water quality management in hydroponic systems.

Seventy-two lettuce plants were grown hydroponically over a 42-day period in a recirculating deep water culture hydroponic system. The growth rates of each plant were closely observed for changes in leaf count, root lengths, and leaf size. Daily measurements included electrical conductivity, water temperature, total dissolved solids, and pH. Nutrient levels were analyzed for nitrate–nitrogen, ammonia–nitrogen, phosphorus, and chemical oxygen demand in the nutrient solution.

The system averaged a pH of 7.16, with electrical conductivity values ranging from 1680–2010 µS/cm. The peak of the highest dissolved solids was 1010 mg/L, showing a strong positive correlation with leaf count, which surpassed past 900. Nutrient analysis showed NO3-N levels as high as 226 mg/L and COD values varying between 53.7 and 88.3 mg/L. A drop in pH below 6.8 was closely related with slowdown in root growth and increased browning around plant edges. Plants that were exposed to optimal water chemistry showed faster leaf development and more complex roots, with root lengths exceeding 13 cm. These findings emphasized the importance of the water pH, nutrient balance, and oxygenation in hydroponic systems. This research adds strong evidence for the importance of the close management of water quality and its necessary role in hydroponic systems.