Submitted to: International Symposium and Workshop on Time Domain Reflectometry for Innovative Soils Applications
Publication Type: Proceedings
Publication Acceptance Date: 9/20/2006
Publication Date: 9/20/2006
Citation: Logsdon, S.D., Hornbuckle, B.K. 2006. Soil moisture probes for a dispersive soil [CD-ROM]. International Symposium and Workshop on Time Domain Reflectometry for Innovative Soils Applications. Purdue University. West Lafayette, IN. Interpretive Summary: Measurements of soil water content help us understand how water moves in the field and how much water plants take up. This study examined commercial soil water sensors in a soil with a lot of high-charge clays. The sensor with a larger measurement volume and lower cost was favored. This study showed how to obtain a calibration for a specific site. This information is useful to scientists and to those who advise how to get useful information from soil water sensors.
Technical Abstract: Soils high in smectite clays have presented difficulties when using dielectric soil moisture sensors in the field. The purpose of this study was to determine which soil moisture sensor provided reliable data on these soils after site-specific calibration. The CS616 water content reflectometer was compared in the field and laboratory with the ML2X theta probe and the Hydra probe. The soil was Canisteo clay loam (Fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquoll). The CS616 was installed horizontally in the field at 1.5 and 4.5 cm depths (one each depth), and the ML2X and Hydra probes were installed horizontally centered at 3 cm depth (four each probe). Gravimetric soil moisture samples were taken on five dates. Then all the probes were installed in repacked soil columns or cores for laboratory analysis. Additional measurements were made with a twelve wire probe using a vector network analyzer in similar packed cores. Data from the CS616 was theoretically converted to apparent permittivity, and data from the ML2X was empirically converted to apparent permittivity using the manufacturer's equation. The Hydra probe program estimates real and imaginary parts of permittivity. The measurement volume was larger for the CS616 than for the other probes because of the longer electrodes and because of the two-electrode (parallel) configuration. This provided greater consistency between the measurements for CS616. However, the larger volume may have allowed some energy to extend beyond the soil surface for the shallow probe in the field. The permittivity values correlated with water content and among probes, but the values were not identical among probes or with that determined from vector network analyzer. Temperature correction improved field soil water patterns. Lower cost and larger measurement volume favored the CS616 over the other probes.