2018 ASHS Annual Conference

Nutrient solution is constantly recycled by maintaining electrical conductivity (EC) in commercial hydroponic production. In recycled solutions, nutrients taken slowly by plants (ex Ca, Mg, and S) tend to accumulate and affect EC. Thus, for a given EC measured in the solution, recycled solution is relatively low in quality compared to fresh solution. Our hypothesis was that recycled nutrient solution can reduce nutrient availability to roots in Nutrient Film Technique (NFT, a thin film of solution) compared to Deep Water Culture (DWC, roots submerged in nutrient solution), due to higher volume of nutrient solution available to roots for nutrient uptake in DWC than NFT. A second hypothesis was that the portion of root system not exposed to nutrient solution under NFT may experience mild to moderate drought stress and affect crop growth. Lettuce (Lactuca sativa L.) was grown under hydroponic conditions in a greenhouse maintained at 22/20 °C (day/night) temperature and daily light integral of 15 mol·m^-2·d^-1 in three separate experiments. In experiment I, three varieties of lettuce (Black Seeded Simpson, Rex, and Redina) were grown under NFT and DWC using recycled nutrient solution. When data from three varieties were pooled, a significant decrease in shoot dry weight and increase in root dry weight were observed under NFT compared to DWC. These results could be due to nutrient or drought stress or combined effect of both stresses under NFT than DWC. Experiment II grew lettuce (var. Rex) in fresh and recycled solutions, each with NFT and DWC systems. No differences in crop growth between DWC and NFT were observed when fresh solution was used but a lower growth in NFT than DWC was observed when recycled solution was used. This data suggests the decreased plant growth in NFT with recycled solution was likely due to lower nutrient availability. However, plants in NFT compared to DWC did not experience drought stress, as there were no differences between NFT and DWC when fresh solution was used. A third experiment using leaf lettuce (var. Black Seeded Simpson) tested fresh and recycled solution in an NFT system. There was a significant increase in shoot dry weight of plants under fresh than recycled conditions. Tissue nutrient analysis indicated suboptimal concentration of N, P, and K in the recycled compared to the fresh solution treatment. These results confirm that reduced growth in NFT system under recycling conditions is due to low nutrient availability to plants.