Doctoral Degrees (Agronomy)

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Browsing Doctoral Degrees (Agronomy) by browse.metadata.advisor "Van Rensburg, L. D."

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    Irrigation scheduling of tomatoes (Lycopersicon esculentum Mill.) and cucumbers (Cucumis sativus L.) grown hydroponically in coir
    (Stellenbosch : University of Stellenbosch, 2009-12) Van der Westhuizen, Rykie Jacoba; Agenbag, G. A.; Van Rensburg, L. D.; Deckers, Ir. S.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: The use of capacitance water sensors for the scheduling of irrigation for hydroponic tomato and cucumber crops grown in coir was investigated in a series of laboratory and glasshouse experiments in the Free State province of South Africa. Laboratory experiments in a climate controlled chamber were conducted to accurately calibrate ECH2O capacitance sensors, models EC-10 and EC-20, in coir with an improved calibration procedure. Water content predictions by the coir-specific calibration and manufacturer’s calibration equations were compared to actual water content measured from mass loss of the coir sample. The manufacturer’s calibration equation indicated a poor accuracy of prediction, which mostly underestimated the volumetric water content, compared to the near perfect prediction of the coir-specific calibration of individual sensors. A rapid calibration procedure for EC-10 and EC-20 sensors was proposed to reduce the calibration time of the sensors and promote their commercial use for irrigation management in coir. The accuracy of prediction by the rapid calibration procedure for the plant available water content range was high for both EC-10 and EC-20 sensors and allowed for the compensation for variation between sensors. Glasshouse studies aimed to characterise the water retention and ability of coir to supply water to greenhouse tomato and cucumber crops through the continuous monitoring of medium water content in small and large growing bags with the EC-10 and EC-20 capacitance sensors during a drying 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 be allowed to the point of mild water stress for both greenhouse tomato and cucumber crops, which can be detected by soil water sensors. In a second series of glasshouse experiments, the identified stages of crop water stress were used to determine and apply depletion levels in coir and compare this irrigation strategy to a well-watered treatment for greenhouse cucumber and tomato plants, with regard 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 depletion levels for cucumber and tomato plants in coir, through the use of in situ calibrated capacitance sensors. For both crops the depletion of water varied between bag sizes, indicating that various bag sizes 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 for cucumbers and 427 L m-2 in the small and 487 L m-2 in the large bags for tomato plants, compared to the well-watered treatments. Yields achieved by the greenhouse tomato plants in the large growing bags and cucumber plants in the small and large bags were maintained or improved when scheduled to the highest depletion level (approximately 60% available water content) compared to the well-watered treatment. The combination of reduced irrigation and improved or maintained yields resulted in improved water use efficiencies (based on irrigation and transpiration) for the highest depletion level compared to the well-watered treatments. In all glasshouse experiments the well-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 and tomato plants under luxury water supply and cyclic water deficit conditions. The comparison was based on the transpiration ratio and yield, while the use of capacitance sensors was evaluated for irrigation scheduling in coir for both crops. Transpiration data indicated that cucumber and tomato plants subjected to luxury water supply experience water stress earlier than plants subjected to cyclic water deficit conditions, irrespective of bag size. Results also indicated that irrigation scheduling according to water depletion levels in small bags is not yet recommended for greenhouse tomato and cucumber plants grown in coir, until further research is conducted. Scheduling to water depletion levels in large bags is, however, justified by the improved or maintained yields of the greenhouse cucumber and tomato plants. The estimated depletion levels for large bags beyond which 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 bags improves irrigation management of hydroponic cucumbers and tomatoes in coir by eliminating over-irrigation and improving water use efficiency. More research is needed before a conclusion can be made regarding irrigation scheduling with capacitance sensors in small growing bags.