Autonomous Irrigation is Enabled by Soil Water Tension Control

An autonomous irrigation controller, based on soil water tension measurement was developed and tested in a tomato irrigation experiment. The experiment was carried out on winter grown tomato cv. Ikram in a polyethylene-covered greenhouse in the southern coastal region of Israel. The three treatments tested included a control based on current irrigation recommendation and two control treatments, -8 kPa and -15 kPa threshold tension irrigation. Soil water tension was monitored every 15 minutes and uploaded to a server for visual inspection and data analysis. Irrigation was by drippers applying a continuous and uniform concentration of fertilizers to all treatments. Data collected included records of irrigation timing and volume of water applied, total and marketable yield, number of fruit clusters, non-marketable fruit (due to split, small, green, rotten, and blossom end rot) and fruit shelf life. At the end of the experiment root systems of plants from each treatment were uncovered, the adjacent soil was sampled for salinity and water content measurement.

The results showed that the autonomous irrigation maintained the pre-set threshold tension steadily. Compared to the control treatment, the threshold tensions of -8 kPa and -15 kPa saved 12% and 24% of water and fertilizers, respectively. Water use efficiency increased from 42.7 kg fruit/m³ of water in the control to 46.8 kg/m³ and 55.6 kg/m³ in the -8 and -15 kPa treatments, respectively. There were no significant differences between treatments in total yield (247 - 257 T/ha), marketable yield (224 - 243 T/ha) and the shelf life of the fruit. Soil salinity was low, without difference between treatments. The salinity profile and root excavation indicated that the main root system was in the 0-45 cm layer. Soil water content in the 0-90 cm depth was highest in the control and lowest in the -15 kPa treatment.