Field Calibration of PR2 Capacitance Probe in Pullman Clay‐Loam Soil of Southern High Plains

Abstract

Core Ideas A noncalibrated PR2 capacitance probe showed significant deviations from actual soil water content. Calibration improved the accuracy and precision of soil moisture monitoring with the PR2. Calibration was necessary for using the PR2 probe for research‐quality soil water measurements. Multi‐depth capacitance sensors are popular to monitor soil water content for scheduling irrigation thanks to their ease of operation and maintenance. The PR2/6 Profile Probe (Delta‐T Devices) measures soil moisture by using either the manufacturer's built‐in equation or a user‐calibrated equation if the soil is high in clay or organic matter. The objectives were to evaluate the performance of the PR2/6 Profile Probe in a perennial grassland and to develop a site‐specific equation to correct probe measurements in Pullman (fine, mixed, superactive, thermic Torrertic Paleustolls) clay loam soils. Permittivity recorded by the profile probe was regressed on the gravimetrically measured soil volumetric water content (VWC). Parameters were optimized to obtain least RMSEs. The default equation provided by the manufacturer estimated VWC with average RMSE and r2 values of 0.053 m3 m–3 and 0.71, respectively. New calibration coefficients were effective in explaining 91% of the variability in soil VWC measurements and reducing RMSE to 0.017 m3 m–3. The results indicate that site‐specific calibration of the capacitance probe is necessary to attain research‐quality accuracy and precision when applied to grasslands in Pullman clay loam and associated clay loam soils.

Interseeding alfalfa into native grassland for enhanced yield and water use efficiency

“…The subsurface soil contained illuviated clay overlying a secondary CaCO 3 layer (caliche) at the 60‐ to 130‐cm depth (USDA‐NRCS, 2017). The soil was more clayey in three of the four replicates; however, clay contents in greater depths were similar for all replicates (Dhakal et al., 2019a). The average organic matter content was 25.8 ± 3.8 g kg −1 , and the C/N ratio was 12.9 ± 0.6 g kg −1 at the top 0‐ to 20‐cm soil layer when averaged across years.…”

“…The predominant soil of the experimental site was Pullman clay‐loam (fine, mixed, superactive, thermic Torrertic Paleustolls) with average pH values of 7.3 in the single west replicate site and 7.5 in the three east replicates. The average soil bulk density was 1.50 Mg m −3 for the top 60 cm and 1.54 Mg m −3 for 60–100 cm (Dhakal, West, Deb, Kharel, & Ritchie, 2019a). The land was nearly flat (0–1% slope).…”

Row spacing of alfalfa interseeded into native grass pasture influences soil‐plant‐water relations

“…The performance of the default and the calibrated equations was described in Dhakal et al. (). Coefficients in the manufacturer's equation were modified and applied to all soil VWC data.…”

“…The PR2 probe was calibrated in situ to correct the VWC readings given by the manufacturer's default equation. The performance of the default and the calibrated equations was described in Dhakal et al (2019a). Coefficients in the manufacturer's equation were modified and applied to all soil VWC data.…”

Establishment and Stand Development of Alfalfa Interseeded into Native Grass Mixture: Cultivar and Row Spacing Effects

“…The Seven treatments, comprising one grass-only control and a factorial of three alfalfa cultivars ´ two row spacings, were arranged in a randomized complete block design with four replications. The distance range across the four replicates was 1.1 km, which resulted in some difference in soil properties (Dhakal et al, 2019). The size of each plot was 9.1 ´ 18.3 m. Trial areas were mowed to 8 cm the day before planting on 5 Oct. 2015.…”