Temperature correction of substrate moisture measurements made in coir in polytunnel-grown strawberries

M.S. Goodchild, K.D. Kühn, C. Nicholl, M.D. Jenkins
The aim of this work is to assess the use of temperature corrected substrate moisture data to improve the relationship between environmental drivers and the measurement of substrate moisture content in high porosity soil-free growing media such as coir. Substrate moisture sensor data collected from strawberry plants grown in coir bags installed in a table-top system under a polytunnel illustrate the impact of temperature on capacitance-based moisture measurements. Substrate moisture measurements made in our coir growing medium demonstrate the negative temperature coefficient of the permittivity of water where diurnal changes in moisture content oppose those of substrate temperature. The diurnal substrate temperature variation was seen to range from 7 to 25°C resulting in a clearly observable temperature effect in substrate moisture content measurements during the 23-day test period. A temperature correction algorithm is presented in this work that has been derived from the complex refractive index model (CRIM) equation, which was easily calibrated for coir and applied to the substrate moisture and temperature data. The diurnal variations seen with the temperature compensated substrate moisture data now align very well with the expected diurnal water demands of the strawberry plants. To further evaluate the relationship between environmental drivers of solar radiation and vapour pressure deficit with substrate moisture, the temperature correction algorithm was programmed within a GP2 data logger and controller (Delta-T Devices Ltd.). The GP2 also was employed to collect solar radiation, air temperature and relative humidity data. The resulting comparison of substrate moisture responses to environmental drivers illustrates an improved correlation with temperature corrected substrate moisture measurements. We conclude that this new temperature correction algorithm addresses the effect of temperature on the relative permittivity of water, which can contribute to observable measurement errors using capacitance-based sensor measurements in high porosity soil-free growing substrates such as coir.
Goodchild, M.S., Kühn, K.D., Nicholl, C. and Jenkins, M.D. (2018). Temperature correction of substrate moisture measurements made in coir in polytunnel-grown strawberries. Acta Hortic. 1197, 147-154
DOI: 10.17660/ActaHortic.2018.1197.20
substrate moisture temperature correction, coconut fibre, complex refractive index model

Acta Horticulturae