Soil Water Content Sensor Response to Organic Matter Content under Laboratory Conditions.

Ali Fares,Ripendra Awal,Haimanote Bayabil

doi:10.3390/s16081239

Abstract

Studies show that the performance of soil water content monitoring (SWCM) sensors is affected by soil physical and chemical properties. However, the effect of organic matter on SWCM sensor responses remains less understood. Therefore, the objectives of this study are to (i) assess the effect of organic matter on the accuracy and precision of SWCM sensors using a commercially available soil water content monitoring sensor; and (ii) account for the organic matter effect on the sensor’s accuracy. Sand columns with seven rates of oven-dried sawdust (2%, 4%, 6%, 8%, 10%, 12% and 18% v/v, used as an organic matter amendment), thoroughly mixed with quartz sand, and a control without sawdust were prepared by packing quartz sand in two-liter glass containers. Sand was purposely chosen because of the absence of any organic matter or salinity, and also because sand has a relatively low cation exchange capacity that will not interfere with the treatment effect of the current work. Sensor readings (raw counts) were monitored at seven water content levels (0, 0.02, 0.04, 0.08, 0.12, 0.18, 0.24, and 0.30 cm3 cm−3) by uniformly adding the corresponding volumes of deionized water in addition to the oven-dry one. Sensor readings were significantly (p < 0.05) affected by the organic matter level and water content. Sensor readings were strongly correlated with the organic matter level (R2 = 0.92). In addition, the default calibration equation underestimated the water content readings at the lower water content range (<0.05 cm3 cm−3), while it overestimated the water content at the higher water content range (>0.05 cm3 cm−3). A new polynomial calibration equation that uses raw count and organic matter content as covariates improved the accuracy of the sensor (RMSE = 0.01 cm3 cm−3). Overall, findings of this study highlight the need to account for the effect of soil organic matter content to improve the accuracy and precision of the tested sensor under different soils and environmental conditions.

Highlights

Soil moisture measurements at different scales remain challenging due to complexities and often confounding edaphic and topographic factors
Conclusions study is the first scientificexperiment experiment that that reports effects of organic matter on the. This This study is the first scientific reportson onthe the effects of organic matter on the precision and accuracy of an electromagnetic soil water content monitoring sensor
Organic matter had a significant effect (p < 0.05) on the precision and accuracy of the tested sensor, resulting in an underestimation of the soil water content at the dry water content range (0.05 cm3 cm3 ). This confirmed the need for correcting the organic matter effect on the performance of soil water content monitoring (SWCM) sensors

Summary

Introduction

Soil moisture measurements at different scales remain challenging due to complexities and often confounding edaphic and topographic factors. A study by [1] showed that the variability of soil water content at the field scale could be reasonably explained by texture and soil organic carbon (SOC). Based on their study that relied on gravimetric soil moisture measurements and soil organic matter determination using the loss-on-ignition (LOI) method. While the gravimetric method of soil water content measurement is known for its accuracy, its approach is inefficient due to high costs in labor and time. Recent advancements in moisture measurement techniques that use soil electrical properties have overcome most of these challenges [2,3,4], and with the increasing use of in situ and remote sensing techniques (e.g., radiometer, thermal infrared techniques, microwave sensors), soil water content data are becoming readily available at relatively higher spatial and temporal resolutions [5,6].

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