Abstract
By using different physical foundations and technologies, many probes have been developed for on-site soil salinity appraisal in the last forty years. In order to better understand their respective technical and practical advantages and constraints, comparisons among probes are needed. In this study, three different probes, based on electrical resistance (ER), electromagnetic induction (EMI), and frequency domain reflectometry (FDR), were compared during a field survey carried out in a large salt-threatened agricultural area. Information about the soil bulk electrical conductivity (σb) at different depths was obtained with each of the probes and, additionally, other soil properties were also measured depending on the specifications of each instrument and, moreover, determined in samples. On average, the EMI and FDR techniques could be regarded as equivalent for σb measurement, whereas ER gave higher σb values. Whatever the case, EMI, and also ER, had to be supplemented with information about soil clay, organic matter, and water mass fractions to attain, despite this effort, poor soil salinity estimations by means of multiple linear regression models (R2 < 0.5). On the contrary, FDR needed only probe data to achieve R2 of 0.7, though root mean standard error (RMSE) was still 1.5 dS m−1. The extra measurements and calculations that modern electrical conductivity contact probes integrate, specifically, those based on FDR, remarkably increase their ability for soil salinity appraisal, although there is still room for improvement.
Highlights
It has been estimated that nowadays soil salinity constrains agriculture in 10 to 25% of the irrigated lands globally [1,2,3]
Soil scientists and practitioners have different techniques available for salinity appraisal by means of σb measurements. These techniques can be arranged in order of increasing complexity and age, from first to last: Electrical resistance (ER), electromagnetic induction (EMI), time domain reflectometry (TDR), and frequency domain reflectometry (FDR)
The field comparison made between three classical commercial devices capable of electrical conductivity (EC) detection, each one featuring a different physical foundation, i.e., electrical resistance (ER), EMI, and FDR, revealed several interesting things related to the evolution the technology for field soil salinity appraisal has witnessed in the past forty years
Summary
It has been estimated that nowadays soil salinity constrains agriculture in 10 to 25% of the irrigated lands globally [1,2,3]. In order to cope with this degradation process, it is necessary to count on reliable, and fast, methods of soil salinity appraisal. The procedure of sampling and subsequent laboratory preparation of water-saturated soil-pastes is destructive and, time-consuming and laborious, and very expensive for data-intensive works. For this reason, several probes for the on-site measurement of the EC of the bulk soil, i.e., σb , have been developed during the last forty years in order to estimate the σe from the σb value. The σb values result from the combination of several physicochemical soil properties, which include the soluble salts, and the clay percentage and mineralogy, the water
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