Abstract
ABSTRACT Despite the growing use of the time domain reflectometry (TDR) technique to monitoring ions in the soil solution, there are few studies that provide insight into measurement error. To overcome this lack of information, a methodology, based on the central limit theorem error, was used to quantify the uncertainty associated with using the technique to estimate potassium ion concentration in two soil types. Mathematical models based on electrical conductivity and soil moisture derived from TDR readings were used to estimate potassium concentration, and the results were compared to potassium concentration determined by flame spectrophotometry. It was possible to correct for random and systematic errors associated with TDR readings, significantly increasing the accuracy of the potassium estimation methodology. However, a single TDR reading can lead to an error of up to ± 18.84 mg L-1 K+ in soil solution (0 to 3 dS m-1), with a 95.42% degree of confidence, for a loamy sand soil; and an error of up to ± 12.50 mg L-1 of K+ (0 to 2.5 dS m-1) in soil solution, with a 95.06% degree of confidence, for a sandy clay soil.
Highlights
Time domain reflectometry (TDR) is used to measure the propagation velocity of an electromagnetic pulse along parallel transmission lines through the soil
The theory assumes that dielectric losses (K”) of the electromagnetic pulse, from emission to reception, by TDR are negligible
As implied by Santana et al (2007), the greater the electrical conductivity of the soil solution, the greater the uncertainties associated with the estimate of TDR readings
Summary
Time domain reflectometry (TDR) is used to measure the propagation velocity of an electromagnetic pulse along parallel transmission lines through the soil. This velocity is expressed in terms of a soil dielectric constant (Ka), which is a compound of two parts: real (K’) and imaginary (K”) (Topp et al, 1980). Due to the strong correlation between the dielectric constant of the soil (Ka) and the soil water content (θ), the TDR instrument can be calibrated to determine the soil water content. According to Topp et al (1988), it is possible to simultaneously determine Ka and the apparent soil electrical conductivity (ECa) (solid-liquid-gas phase), because of the proportionality between the energy attenuation of the emitted signal and the soil ECa (Giese & Tiemann, 1975)
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