ISHS Acta Horticulturae 633: XXVI International Horticultural Congress: Protected Cultivation 2002: In Search of Structures, Systems and Plant Materials for Sustainable Greenhouse Production
TIME DOMAIN REFLECTOMETRY ACCURATELY MONITORS AND CONTROLS IRRIGATION WATER APPLICATIONS IN SOILLESS SUBSTRATES |
| Authors: | J.D. Murray, J.D. Lea-Cox, D.S. Ross |
| Keywords: | stomatal conductance, volumetric water content, easily-available water, matric potential, time domain reflectometry |
| DOI: | 10.17660/ActaHortic.2004.633.8 |
| Abstract:
Most horticultural soilless substrates have a small range of easily-available water (EAW) for optimum plant growth, and therefore require frequent irrigations. Most soilless substrates generally hold EAW in the range from 0 to -10 KPa matric potential (ψm), with most plant-available water in the range 0 to -5 KPa. Knowledge of the volumetric water (Wv) content of a substrate can be used to precisely define irrigation applications, if an accurate method for sensing Wv is available. Until now, no technology has existed to do this in soilless substrates with any degree of precision. Accurate monitoring and control of irrigation water should retain nutrients in the root zone and maximize plant growth, while minimizing leaching volumes. Time Domain Reflectometry (TDR) has been shown to accurately measure Wv in a few soilless substrates, but there is no information on the variability of such data or how substrate Wv and plant water use are correlated. A range of soilless substrates, including Pro-Mix ‘BX’, a commercial pine-bark mix, a commercial hardwood-bark mix, medium-grade perlite, rockwool, and sieved sand were studied in a range of experiments designed to test these assumptions, using various column (container) heights. Mean TDR coefficients of variation ranged from 0.8% to 7.9% over all substrate and column heights, proving that TDR can precisely measure water contents in soilless substrates under most conditions. Water-release and TDR curves for each substrate indicate that Wv is primarily determined by substrate characteristics, but also by container height. Experiments with azalea in growth chamber and greenhouse experiments showed that a TDR-monitored and controlled irrigation system precisely controlled cyclic irrigation events, with initiation at -10KPa and termination of irrigation at –1 KPa. TDR sensors need to be placed vertically in the rootzone, especially with drip emitters. Water and nutrient leaching volumes were significantly reduced compared to an irrigation method based on container weight. | |
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