MEASURING WATER-CONTENT OF SOIL SUBSTITUTES WITH TIME-DOMAIN REFLECTOMETRY (TDR)

Optimization of irrigation and fertilization regimes in greenhouses an d other controlled environments requires accurate and frequent measure ments of soil-water content. Recent studies on TDR use in gravely soil s and in closed-container studies have indicated a potential use of th is method in horticulture. In this study, TDR calibration curves were determined for tuff (granulated volcanic ash), vermiculite, perlite an d a mix of two composted agricultural wastes (grape marc, separated co w manure). Widely used as horticultural substrates, mixes of these mat erials were tested as well. For all soil substitutes tested, measured calibration results are well described by linear equations throughout tested values of water content that cover the working range in horticu lture. Ledieu's equation, widely used in soils, describes fairly well the measured results for perlite, but underestimates those obtained fo r organic media, vermiculite (because of the presence of bound water) and tuff (probably due to water in occluded pores). The differences ob tained between the measured calibration equations and Ledieu's equatio n indicate that in order to avoid an erroneous irrigation management, calibration is necessary whenever a new soil substitute is used.