Evaluation of nutrient concentration of lettuce cultivars by different methods of nutrient solution replacement in a floating hydroponic system

Recycle and reuse nutrient solutions of closed hydroponic systems can reduce environmental problems and economic costs. It not only prevents groundwater pollution but also decreases water consumption during the growing season. Most producers discard the nutrient solution every week and use the new solution and do not have a proper strategy for replacing the used nutrient solution. Some vegetable producers analyze nutrient concentration, pH, and electrical conductivity (EC) of nutrient solutions with sensors. But the use of sensors has decreased due to high costs and disruptions in operation. Adjusting the EC of nutrient solution is one of the nutrient management methods under hydroponic conditions. In this method, mineral nutrient absorbed by the plant from nutrient solution is replaced by adding mineral nutrients and the EC of the solution must be kept constant. We tested and compared the new method of nutrient replacement according to plant demand with two previews methods.

This experiment aimed to compare the effect of three nutrient replacement methods (complete replacement, EC based replacement, plant demand-based replacement) on a floating hydroponic system on three lettuce cultivars (Chinese lettuce, red lettuce, and Kazeroon lettuce) in a factorial experiment based on completely randomized design, in 3 replicates. Under complete nutrient solution replacement treatment, the nutrient solution was replaced every 4 days. Under partial replacement according to EC, nutrient solution EC was adjusted at 2.3 ds m-1 by adding predetermined amounts of potassium sulfate, calcium nitrate, magnesium sulfate, potassium dihydrogen phosphate every 48 hours. Microelements were applied to the nutrient solution every 10 days, while under treatment of partial replacement according to plant requirements, the concentration of potassium nitrate was used as a standard solution but the concentrations of calcium nitrate, magnesium sulfate, potassium dihydrogen phosphate concentration in nutrient solution were decreased to half strength. The concentrations of K were measured by a flame photometer (Jenway, model PFP7, UK). Analyses of Ca, Fe, Zn, Mn, and Cu were carried out with an atomic absorption spectrophotometer (GBC Avanta, Australia) and phosphorus concentration was determined using a spectrophotometer. Total nitrogen concentration was measured according to the Kjeldahl method and expressed as a percent of DW.

The results showed that nutrient solution consumption was more than 60% lower in plant nutrition according to plant demand and EC of the nutrient solution compared to the complete replacement method. The concentration of nitrogen, potassium, and calcium in Chinese lettuce decreased under the EC control method. Leaf magnesium concentration increased in all cultivars nourished according to EC control and plant nutrient demand compared to the complete nutrient replacement method. The highest concentrations of micronutrients such as iron, zinc, and manganese were observed in the complete nutrient replacement method in all three lettuce cultivars. The results also showed that the amount of Fe and Mn in the leaf of red lettuce cultivar and zinc concentration in the leaf of Chinese lettuce and Kazeroon decreased significantly under nutrient replacement according to the EC control and plant nutrient demand.