Microwave dielectric properties of horticultural peat products

The microwave dielectric properties of horticultural peat and compost peat were measured with a HP85107C network analyser in conjunction with a HP85070B open-ended coaxial dielectric probe for the frequency range 0.5–20 GHz. Loose samples had volumetric water contents ranging from 0.11 to 0.24 Mg m-3. For analysis, samples were compressed to 1.0× (no compression), 1.2×, 1.5× and 2.0× original bulk density, giving volumetric water contents ranging from 0.11 to 0.48 Mg m-3. The raw relative permittivity data exhibited a high degree of variability, but rank-based removal of outlier measurements helped to improve their coherence with volumetric water content. The difference between horticultural peat and compost peat was insignificant in terms of both the dielectric constant and the loss factor. The results suggest that relative permittivity data after the removal of outliers can be of sufficiently high quality for sensing applications in the horticultural peat industry such as dedicated water content monitoring, nutrient management, and foreign body detection systems for health and safety purposes, given the low-precision requirements that are appropriate for horticultural and compost peat as high-volume, low-value and non-critical commodities.