Irrigation scheduling of tomatoes (Lycopersicon esculentum Mill.) and cucumbers (Cucumis sativus L.) grown hydroponically in coir

[摘要] ENGLISH ABSTRACT: The use of capacitance water sensors for the scheduling of irrigation for hydroponic tomato andcucumber crops grown in coir was investigated in a series of laboratory and glasshouse experimentsin the Free State province of South Africa.Laboratory experiments in a climate controlled chamber were conducted to accurately calibrateECH2O capacitance sensors, models EC-10 and EC-20, in coir with an improved calibrationprocedure. Water content predictions by the coir-specific calibration and manufacturer's calibrationequations were compared to actual water content measured from mass loss of the coir sample. Themanufacturer's calibration equation indicated a poor accuracy of prediction, which mostlyunderestimated the volumetric water content, compared to the near perfect prediction of thecoir-specific calibration of individual sensors. A rapid calibration procedure for EC-10 and EC-20sensors was proposed to reduce the calibration time of the sensors and promote their commercialuse for irrigation management in coir. The accuracy of prediction by the rapid calibration procedurefor the plant available water content range was high for both EC-10 and EC-20 sensors and allowedfor the compensation for variation between sensors.Glasshouse studies aimed to characterise the water retention and ability of coir to supply water togreenhouse tomato and cucumber crops through the continuous monitoring of medium watercontent in small and large growing bags with the EC-10 and EC-20 capacitance sensors during adrying cycle, compared to well-watered plants. Stages of crop water stress were identified and,based only on the plant's response to the drying cycle, it was suggested that water depletion can beallowed to the point of mild water stress for both greenhouse tomato and cucumber crops, which canbe detected by soil water sensors. In a second series of glasshouse experiments, the identified stagesof crop water stress were used to determine and apply depletion levels in coir and compare thisirrigation strategy to a well-watered treatment for greenhouse cucumber and tomato plants, withregard to the water balance components, yield and water use efficiency for different bag sizes.Results indicated that irrigation was successfully managed to the pre-determined water depletionlevels for cucumber and tomato plants in coir, through the use of in situ calibrated capacitancesensors. For both crops the depletion of water varied between bag sizes, indicating that various bagsizes require different irrigation management strategies. Scheduling to the highest pre-determined by 124 L m-2 in the small and 240 L m-2 in the large bags forcucumbers and 427 L m-2 in the small and 487 L m-2 in the large bags for tomato plants, comparedto the well-watered treatments. Yields achieved by the greenhouse tomato plants in the largegrowing bags and cucumber plants in the small and large bags were maintained or improved whenscheduled to the highest depletion level (approximately 60% available water content) compared tothe well-watered treatment. The combination of reduced irrigation and improved or maintainedyields resulted in improved water use efficiencies (based on irrigation and transpiration) for thehighest depletion level compared to the well-watered treatments. In all glasshouse experiments thewell-watered treatment resulted in luxury water use by the plants.Finally, a study was conducted in order to compare crop water stress of greenhouse cucumber andtomato plants under luxury water supply and cyclic water deficit conditions. The comparison wasbased on the transpiration ratio and yield, while the use of capacitance sensors was evaluated forirrigation scheduling in coir for both crops. Transpiration data indicated that cucumber and tomatoplants subjected to luxury water supply experience water stress earlier than plants subjected tocyclic water deficit conditions, irrespective of bag size. Results also indicated that irrigationscheduling according to water depletion levels in small bags is not yet recommended for greenhousetomato and cucumber plants grown in coir, until further research is conducted. Scheduling to waterdepletion levels in large bags is, however, justified by the improved or maintained yields of thegreenhouse cucumber and tomato plants. The estimated depletion levels for large bags beyondwhich yield are reduced was at 85% for tomatoes and 70% for cucumbers.In conclusion, the results clearly indicated that the use of capacitance sensors in large growing bagsimproves irrigation management of hydroponic cucumbers and tomatoes in coir by eliminatingover-irrigation and improving water use efficiency. More research is needed before a conclusion canbe made regarding irrigation scheduling with capacitance sensors in small growing bags.

[发布机构] Stellenbosch University