Smart fertigation: Automated fertilizer application based on plant growth using digital sensors with IoT capability

Liquid organic fertilizers (LOF) are used to supply additional nutrients to crops. However, LOF are expensive to use regularly. In this research, a smart fertigation treatment (S), where LOF was applied based on lettuce plant growth, was compared to a manual treatment (M) in a greenhouse. Digital sensors with IoT capability were installed in S and M treatments. The sensor in S treatment was interfaced to a solid-state relay connected to a pump, to automatically regulate LOF application. Digital sensors in both treatments measured canopy area (CA) of plants once every three days using image analysis and remotely exchanged data. In the M treatment, LOF was applied every third day to plants, whereas the LOF application was regulated by the sensor in S treatment to happen only when CA of plants was lower by 10% than those in the M treatment. Results indicated no differences in fresh weight (33.4 vs. 32.3 g·plant-1) and leaf area (770.2 and 772.6 cm-2) of plants between two treatments. Nutrient concentration in the root zone, measured as electrical conductivity of substrate, was significantly higher (21-69%) in M than S treatment. The number of fertigation events and cost of LOF applied were approximately four times higher in M than S treatment. This resulted in higher fertilizer-use-efficiency in S (1.159 kg ∙ m-2 ∙ $-1) than M (0.168 kg ∙ m-2 ∙ $-1) treatment. These results indicate that the smart fertigation method can optimize LOF application (i.e., reduce wastage and costs) while maintaining crop growth.