Response of lettuce grown in potting soil semi-solidified with agar

The aim of the study was to develop a soil-based cultivation method to grow lettuce indoors for 28 days without the need for irrigation or fertilization, and to investigate their growth and physiological responses in this novel growing substrate. Agar (15 g L^− 1) was added to a soil mixture, which comprised potting soil, perlite, fertilizer, and water, to semi-solidify it into a substrate block. Lettuce seeds were sown in the semi-solidified substrate, which was prepared in a custom-made, enclosed container with planting holes in the lid. Three different soil-to-water (SW) ratio (v:v) treatments were used to prepare the soil substrate: SW1:1; SW1:2; SW2:1, and three fertilizer treatments were tested: no fertilizer (control); urea; NPK (15:3:3) fertilizer. Results showed that the semi-solidified soil substrates can be used to successfully grow lettuce for 28 days, however, the extent of their growth varied depending on the soil-water ratio and fertilizer treatment. Lettuce plants grown in the NPK-enriched SW2:1 substrate had significantly higher number of leaves, leaf area, and dry weight compared to all the other treatments. In contrast, the lettuce plants grown without additional fertilizers exhibited poor growth regardless of the soil-water ratio used. In the growing substrates supplemented with urea, the SW1:2 ratio treatment produced lettuces with the lowest leaf number, leaf area, leaf fresh weight, and dry weight. Overall, the chlorophyll a/b ratio and Fv/Fm value of lettuces grown in substrate fertilized with NPK were found to be higher than all the other treatments. The key feature of this cultivation method was the use of agar to semi-solidify the waterlogged soil mixture and transform it into a growing substrate that provided the necessary water and nutrients for plant growth over a 28-day period. The growing substrate developed in this study, which is the first of its kind, can be considered as a new soil-based method of growing lettuce indoors.