Soil Solution Electrical Conductivity Research Articles

Estimating Water Content from Electrical Conductivity Measurements with Short Time‐Domain Reflectometry Probes

Time domain reflectometry (TDR) is a widely used technique for measuring the dielectric constant (Ka) and bulk electrical conductivity (σa) of soil. The Ka measurement can be converted to water content (θ) by means of a (soil specific) calibration. Since the accuracy of the Ka measurement is dependent on the TDR probe length, probes longer than about 0.1 m are preferred. However, shorter probes are desired for many applications. The possible use of the σa measurement of short TDR probes for estimating θ under conditions with constant soil solution electrical conductivity (σw) is investigated and the accuracy of the Ka and σa measurements of two reference TDR probes (0.20 m long) and four miniature probes (0.02 m long) is determined. The standard deviation of the Ka measured by the miniature probes was found to be ten times higher compared to the reference probes. The standard deviation of the σa measured by the miniature probes was only slightly larger compared with the reference probes. A calibration experiment in sand using the reference TDR probes showed that when σw is constant, the θ estimations from Ka and σa measurements have the same accuracy. TDR measurements were taken with the miniature probes in small sand samples. From the Ka–θ and σa–θ relationships determined in the calibration experiment, the Ka and σa measurements of the miniature probes could be converted to θ. The root mean square error (RMSE) of the θ estimated by the Ka measurements was 10 to 20 times higher compared with the reference probe measurement. The RMSEs of the θ estimated by the σa measurements was only two to three times higher compared with the reference probes. The results presented in this study clearly show that the σa measurement made with short TDR probes can give accurate θ estimations under conditions of constant σw

Estimating soil solution electrical conductivity from time domain reflectometry measurements using neural networks

Time domain reflectometry (TDR) is a widely used method for measuring the dielectric constant ( K a) and bulk electrical conductivity ( σ a) in soils. The TDR measured σ a and K a can be used to calculate the soil solution electrical conductivity, σ w. The σ w, in turn, can be related to the concentration of an ionic tracer. Several models of the σ w– σ a– K a relationship can be found in the literature. Most of these models require extensive calibration experiments in order to obtaining best-fit parameters. In this paper, we attempt to model the σ w– σ a– K a relationship using neural networks (NN). We used TDR measured K a and σ a along with five different soil physical parameters (sand, silt, clay, and organic matter content and bulk density) measured in nine different soil types using three different σ w levels in each soil type. In total, 2953 K a and σ a measurements were obtained. The NN estimated σ w was found to have a root mean square error (RMSE) of 0.05–0.13 dS m −1 for the nine different soil types whereas the RMSE of two traditional σ w– σ a– K a models was 0.12–0.87 dS m −1. Furthermore, the traditional models exhibited larger errors for low σ a and K a, whereas the NN estimated σ w did not show any trend in the errors. A sensitivity analysis showed that the NN model was more sensitive to small changes in σ a compared to K a. Of the five soil physical parameters, the silt and clay content affected the σ w– σ a– K a relationship the most. The results presented shows that using NN, the σ w– σ a– K a relationship can be predicted using soil physical parameters without need for elaborate soil specific calibration experiments.

Root Growth and Yield of Differing Alfalfa Rooting Populations under Increasing Salinity and Zero Leaching

ABSTRACTAccumulation of salinity in the root zone can be detrimental to sustained crop production. Irrigation, even with moderately saline water, pushes accumulated salts deeper into the root zone, allowing roots to proliferate in regions of relatively low salinity. Two alfalfa (Medicago sativa L.) subpopulations with low‐ and high‐fibrous rooting characteristics, MnPL‐9‐LF and MnPL‐9‐HF, were used to test the effectiveness of increased rooting on yield when plants were irrigated with saline water but without leaching. Treatments were three levels of heterogeneous root zone salinity predicted by the SOWACH model to represent 10, 20, and 30 yr of irrigation with saline water. Plants were grown for five successive harvests in 10‐cm‐diam., 130‐cm‐deep cylinders. The treatments were constructed with NaCl and gypsum. As soil became depleted to 50% extractable water, irrigation water with an electrical conductivity (EC) of 2.8 dS m−1 was applied. By the fifth harvest, soil solution EC from the top to the bottom of the profile ranged from 3 to 12 dS m−1 for the control and from 3 to 23 dS m−1 for the highest salinity treatment. Root production of the high‐fibrous root type was stimulated more at low and medium salinity than that of the low‐fibrous root type. Across salinity treatments, final root length density (cm root length per cm3 soil volume) was 24% higher for the high‐fibrous root type, and herbage yield of the high‐fibrous root type was 14% higher than that of the low‐fibrous root type. Differential rooting was greatest in the upper half of the root zone. High fibrous rooting in alfalfa is a trait with potential usefulness as a salinity stress avoidance mechanism.

EFFECTS OF THE ST. LUCIE ESTUARY MUCK SEDIMENT APPLICATION ON METAL SOLUBILITY AND pH OF SANDY SOILS: A LABORATORY OBSERVATION 1

Heavy metal contamination from the beneficial re-use of muck sediment in agriculture is a cause for concern. Information is needed about the leachability of metals from land-applied muck sediment and the effects of muck sediment application on soil properties. For this purpose, the water solubility of metals from muck sediments at various pH values and the effects of muck sediment application on metal leachability and pH values of sandy soils were determined by leaching soil samples amended with different rates of muck sediments. The leachates and soil solutions extracted from the muck-amended soils were analyzed for pH, electrical conductivity (EC), and concentrations of heavy metals (Cd, Cr, Cu, Co, Ni, Pb, and Zn). Rapid increases in solubility of heavy metals from the muck at pH levels below 6.0 suggest that muck sediments may be most suitable for application to slightly acid to slightly alkaline soils. Soil solution pH values from soils amended with muck sediments increased with increasing muck rates. Electrical conductivity of soil solution in the muck-amended soils was higher than in soils without muck amendment. The EC values increased with increasing muck rates. Application of muck sediment increased the concentrations of Cd, Co, Ni, and Pb, and decreased the concentrations of Cu and Zn in soil solution. However, the concentrations of Cd (<40 μg L−1), Co (<44 μg L−1), Cr (<30 μg L−1), Cu (<105 μg L−1), Ni (<84 μg L−1), Pb (<128 μg L−1), and Zn (<134 μg L−1) were generally low in the soil solution from muck-amended soils. The salts in muck sediments were easily removed from the amended soils by leaching, suggesting that the added salts from muck sediments may not have long-term effects on plant growth. Relatively high EC values in the first leachates from the muck-amended soils were observed, especially for soils amended with muck at a high rate. However, application of increasing amounts of muck sediments increased soil pH. The pH increased with increasing muck rates, and was correlated positively with Na/Ca, Na, K, Na/(Ca+Mg+K), and Na/(Ca+Mg+K+Na) in the leachates and water soluble Na/Ca, Na, Na/(Ca+Mg+K), and Na/(Ca+Mg+K+Na) in the muck sediments; it was negatively correlated with Ca in leachates and water soluble Ca in the muck sediments. These results suggest that application of muck sediments does not result in a marked change in soil pH at low rates.

A fertility index for submerged rice soils

Soil fertility indices that consider the dynamic nature of submerged soils are needed for assessing the fertility status of wetlands. It is proposed that the soil solution electrical conductivity (EC) determined at 4 weeks after submergence can be used as an index of general fertility of wetland soils. This proposition is based on the finding that the soil solution EC provides a measure of the concentration of nutrient elements mobilized in solution. The results with 15 diverse West African rice soils showed that at 4 weeks after flooding, the soil solution EC was highly significantly correlated (r=0.927,n=15) to the total concentration of macro- and micronutrient elements released in solution. This period (4 weeks after flooding) coincided with the establishment of a dynamic equilibrium between pH and Eh (redox potential) for the 15 soils and the increase in pH was quantitatively (R 2=0.84) associated with the decrease in Eh. The soils had a wide range in solution EC and nutrient concentration in soil solution, indicating a range in the fertility status. The soil solution EC was significantly correlated to organic C and iron extracted by EDTA. It is suggested that soil solution EC at 4 weeks after flooding can be used as an index of the fertility status of wetland soils not affected by salinity.

TIME DOMAIN REFLECTOMETRY FOR MONITORING SOIL NITROGEN DYNAMICS

The study of soil nitrogen transformations, both under laboratory conditions and in situ, requires a large number of time consuming and expensive chemical analyses. In this paper we present a possible alternative to chemical analysis when monitoring N dynamics. Time domain reflectometry (TDR) allows simultaneous monitoring of soil moisture content and bulk soil electrical conductivity (EC). In non-saline soils the nitrate ion plays an important part in the changes of soil solution electrical conductivity over time. We used the strong correlation between NO3--N concentration and EC for on-line monitoring of the mineralization process in two soils with different mineralization potential. Agreement between measured and TDR calculated NO3--N concentrations were good. The observed discrepancies between measured and calculated NO3--N concentrations can be explained by the influence on the EC of other anions, mainly SO42- and HCO3. In a second experiment we used TDR to monitor the immobilisation of native and added NO3--N in soil with addition of glucose. The TDR calculated NO3--N concentrations showed a strong immobilisation of N in the first 2-3 days, followed by a gradual remineralization. From the results presented here it can be concluded that TDR has potential to monitor soil N transformations, especially under controlled environmental conditions such as in laboratory incubation studies.

Using neural networks for calibration of time-domain reflectometry measurements

Time-domain reflectometry (TDR) is an electromagnetic technique for measurements of water and solute transport in soils. The relationship between the TDR-measured dielectric constant (Ka ) and bulk soil electrical conductivity ([sgrave]a) to water content (θW) and solute concentration is difficult to describe physically due to the complex dielectric response of wet soil. This has led to the development of mostly empirical calibration models. In the present study, artificial neural networks (ANNs) are utilized for calculations of θw and soil solution electrical conductivity ([sgrave]w) from TDR-measured Ka and [sgrave]a in sand. The ANN model performance is compared to other existing models. The results show that the ANN performs consistently better than all other models, suggesting the suitability of ANNs for accurate TDR calibrations.

Water and Salt Distribution in a Field Irrigated with Marginal Water under High Water Table Conditions

Development of an impermeable layer at some depth beneath the soil surface and the presence of a high groundwater level are common phenomena in the Yizre'el Valley, Israel. The main objective of this study was to determine the salt and water distributions and salt leaching in a field irrigated by sprinklers, under nonisotropic and high groundwater level conditions. A field experiment was conducted in a cornfield (Zea mays L.) on a Vertisol (Typic Chromoxerets) with subsurface drains. The electrical conductivity (EC) of the irrigation water was 2.5 dS m−1 The variations in water table level, in EC of soil solution and soil saturation paste, and in gravimetric water content along the field were determined at different times. Likewise, corn yield from various sites across the field was determined at the end of the growing season (August). The water table level increased sharply in the winter to 49.5 m, and then decreased continuously in the summer despite the irrigation. The EC increased in the downhill direction, more sharply in the deeper soil layers. In the upper part of the field, the average EC in saturation paste in the 0- to 1.2-m soil layer was 1.1 dS m−1 in March (end of the rainfall season) and 2.1 dS m−1 in August. Conversely, in the lower part of the field, the ECs in March and August were 4.4 and 3.7 dS m−1, respectively. A linear reduction of the corn yield with increasing EC was observed. The relatively low level of the groundwater at the upper part of the field allowed vertical salt leaching by the rainfall. Conversely, the rise of the saline (EC ≈20 dS m−1) groundwater in the lower part of the field in the winter with lateral salt movement increased the soil EC. Decline of the water table in the latter part in the summer allowed vertical leaching of salt by the irrigation water.

Irrigation Water Quality, Soil Amendment, and Crop Effects on Sodium Leaching

Due to prolonged irrigation with water of marginal quality, salination of irrigated soils in some areas of southeastern Montana has led to a need for better understanding of the soil and water management alternatives for irrigators. A study was conducted with Haverson silty clay ( fine-loamy, mixed, calcareous, mesic Ustic Torrifluvent ) to determine the effect of combinations of chemical amendments, crop species, and irrigation water quality on Na + and salt leaching from saltaffected soils. Amendments included CaSO 4 , P-CaSO 4 and MgCl 2 ; also included was a nonamended control treatment. Crops included alfalfa ( Medicago sativa L.) , barley ( Hordeum vulgare L.), sorghum-sudangrass [( Sorghum vulgare 2 Sorghum drumondii ) (sordan)] ,and a noncropped control. All soil columns (0.15 m 2 0.5 m) were irrigated with either high Na + adsorption ratio (SAR adj = 16.6) , high total dissolved solids (TDS = 1647 mg L -1 ) water, or low SAR adj (1.15), low TDS (747 mg L -1 ) water. Drainage volume, electrical conductivity (EC), SAR, Na + of drainage water, and Na + leaching were monitored over three crop cycles. Irrigation with high SAR-high TDS water increased the soil solution EC to approximately 5.5 dS m -1 , but did not decrease crop yields relative to irrigation water having SAR adj and TDS of 0.37 and 747 mg kg -1 , respectively. Magnesium displaced Na + on the exchange complex, but the effects were short-term compared to CaSO 4 or P-CaSO 4 . Amendments increased yields of barley from 14%-27% and alfalfa by 25% but had no effect on sordan. Columns cropped to barley had 28% greater Na + leaching than columns planted to other crops. Noncropped columns accumulated the least net soluble salt and Na + . Results of this study demonstrate that specific crop and amendment combinations can significantly affect the efficiency of saline soil reclamation strategies and impact quality of irrigation return flow.

Estimation of Soil Solution Electrical Conductivity from Bulk Soil Electrical Conductivity in Sandy Soils

Studies of solute transport through soil and attendant environmental impacts are hampered by the lack of methods for continuous monitoring of solute concentration. Measurement of bulk soil electrical conductivity (ECb) using time domain reflectometry (TDR) is a promising technique, but it is indirect, and estimation of solute concentration from such measurements requires a model relating soil solution electrical conductivity (ECw) to ECb Several models of varying complexity exist, but further testing is required to determine their relative merits and applicability. This study was conducted to determine the efficiency of various models that use different methods of estimating the tortuosity factor, Fg, in order to estimate ECw from ECb All models assume that the ratio of ECb/ECw is proportional to soil water content, θ, with Fg as the coefficient of proportionality. In this study, two types of models were compared, those in which Fg is obtained from soil hydraulic properties and those in which Fg is estimated as it is in gas diffusion models, except with θ rather than porosity as the independent variable. Measurements were conducted in a sandy soil, across a range of ECw from 0.10 to 0.56 S m−1 The models in which Fg is obtained from soil hydraulic properties did not perform as expected. The results with the gas diffusion analog models were variable; the most successful of these was based on the 1959 model of Marshall, in which Fg is a power function of θ, θb Optimal results were obtained with , not far from Marshall's suggested value of 0.5 for gas diffusion. We conclude that there is no benefit to the use of soil hydraulic properties in estimating ECw from ECb measurements, at least for sandy soils, where simpler relationships appear to provide superior results.

New geometry factors for hydraulic property-based soil solution electrical conductivity models

Improved methods are needed to intensively measure ionic solutes in soils. The concentration Ci of ionic solutes in soils is directly proportional to soil solution electrical conductivity σw. Time domain reflectometry (TDR) measures both soil water content θ and bulk soil electrical conductivity σa using the same probes. However, physical/conceptual models are required along with TDR measurements in order to use TDR for in situ estimates of Ci. We discuss a modeling approach [Mualem and Friedman, 1991] based on assumed analogy between tortuosity of electrical and hydraulic flow paths in variably saturated soils. We review the model approach, then derive a general expression for a pore geometry factor FG considering flow of electrical current through randomly distributed capillary soil pores. Two FG are derived based on two conceptual considerations of tortuous capillary length. Four water retention models (WRM) are used to describe soil hydraulic properties in the FG. When fitted to the same measured water retention data, the four WRMs provided substantially different magnitudes for FG. The model was then compared in terms of σw estimates using the two new FG in combination with field-measured θ and σa. One of the new FG produced smaller estimated σw than did that proposed in the original model. This is desirable based on our own and several other published comparisons that indicated the original model may overestimate σw in comparison with independent measurements.

Nitrate Concentrations in the Root Zone Estimated Using Time Domain Reflectometry

Improved ability to monitor soil water and ionic chemical distributions in field soils will contribute to better land management opportunities. We evaluated the potential to use time domain reflectometry (TDR) to simultaneously estimate volumetric soil water content (θ), soil solution electrical conductivity (σw), and soil nitrate concentrations in an irrigated peppermint (Mintha piperita L.) field using simple models and calibration methods. Two models and three calibration approaches for estimating σw from TDR were compared to σw obtained from soil cores and soil solution samplers at three depths, and over a wide range in applied KNO3 The applied N rates were well within those commonly used in peppermint production. Soil nitrate concentrations were estimated based on TDR σw predictions and a simple linear calibration of σw vs. nitrate concentration and then compared to independent soil core measurements. Estimates of σw and nitrate concentrations using TDR exhibited similar pattern, magnitude, and variance to those based on direct soil measurements. The Mualem and Friedman (1991) model (MF), which is based on soil water retention measurements, provided σw and nitrate estimates in better agreement with soil cores than did the Rhoades et al. (1976) model. Our results suggest that simple physical–conceptual models combined with laboratory or field parameter estimation methods may be suitable for automated real‐time field monitoring.

Validity of various physiological traits as screening criteria for salt tolerance in barley

Limited knowledge of the physiological basis of the detrimental effects of soil salinity on growth and yield of barley and the consequent lack of suitable screening traits are two reasons for the limited success of plant breeding in saline environments. We assessed the relationships between grain yield, carbon isotope discrimination (Δ), canopy temperature, stomatal conductance, and grain ash content in a set of barley cultivars grown in a soil salinity gradient imposed by a triple-line-source sprinkler system. A linear increase in soil salinity produced significant ( P<0.05) and linear decreases in grain yield and Δ, and increases in midday differential canopy temperature ( δT). For unit increase in soil solution electrical conductivity (ECss), Δ decreased by 0.2‰ and δT increased by 0.3°C. Increasing soil salinity from the control (non-saline) to a highly saline level (ECss=22 dS m −1) decreased leaf stomatal conductance by 65% and increased grain ash content by 11–29%. Grain yield and grain Δ of 34 barley cultivars were significantly ( r=0.58; P<0.01) correlated in non-saline conditions, suggesting that Δ is a useful indicator of yield potential in barley. However, there was no significant correlation between grain yield and grain Δ under highly saline conditions ( r=0.29; P>0.05). A multiple linear regression of Δ, stomatal conductance and grain ash content on grain yield was highly significant ( R 2=0.77, P<0.001) in non-saline conditions, but not in highly saline conditions. We concluded that none of the studied characters would be useful in screening for high yield under saline environments, and that grain yield under salt stress remains the only reliable means of identifying higher salt tolerance in barley.

Texture and Electrical Conductivity Effects on Temperature Dependency in Time Domain Reflectometry

AbstractIf not accounted for, temperature effects on the water content and electrical conductivity measured with time domain reflectometry (TDR) will introduce significant errors. In this study, an automated TDR system was used to quantify the temperature dependence of the apparent dielectric constant and electrical conductivity in wet soils. Sandy, clayey, and organic soils were used to examine the effect of soil texture. The temperature correction factor for water content measurements was predicted, by using different dielectric mixing models, to lie between −0.00253 and −0.00419θ°C‐1, the negative value of the temperature correction factor indicating a decrease in the water content measurements with increasing temperature. These values agreed well with the correction factor measured in sandy soils: −0.00269θ°C‐1. In soils with large specific surface, i.e., clayey and organic soils, however, the temperature dependence was significantly lower. Large specific surface combined with high soil solution electrical conductivity resulted in a positive correction factor, i.e., water content measurements increased with increases in temperature. The temperature dependence of the bulk electrical conductivity was investigated using different soils and soil solutions. The temperature dependence was close to that of the soil solution and was independent of soil texture.

Simulation of a Potential Error in Determining Soil Salinity from Measured Apparent Electrical Conductivity

AbstractA common method for estimating soil salinity is by in situ measurements of the apparent electrical conductivity, ECa, usually by a four‐electrode probe. It is assumed that the ECa can be regarded as taking place in parallel mode by two conductors on a bulk scale: the dissolved (ECb) and the adsorbed (ECs) ions. Therefore, the contribution to the ECa by the soil solution electrolytes, ECb, which serves to assess its salinity, can be deduced by subtracting the estimated ECs from the measured ECa. This assumption is wrong and leads to an evaluation of ECb higher than its real value. This study was conducted to characterize the error in estimating soil solution electrical conductivity, ECw. A simplified model of a randomly diluted and pore‐size‐distribution‐decorated simple cubic lattice serves to describe the pore network of a saturated soil. It is assumed that only within each pore can the electrical conductance be represented by a sum of two conductors: the dissolved and the adsorbed ions, acting in parallel. Using Monte Carlo lattice simulations, it was shown that the error due to the assumption of parallel mode on a bulk scale increases with increasing broadness of the pore‐size distribution, decreasing connectivities, and increasing cation‐exchange capacity. An illustrative example of real soils, typical of irrigated soils, indicated errors of up to 25% for electrolyte concentrations.

Estimating transport parameters in an undisturbed soil column using time domain reflectometry and transfer function theory

Modelling solute transport in the unsaturated zone is a complex problem given the inherent soil heterogeneity. Using time domain reflectometry (TDR) as described in this paper can improve data quality to be used in predictive models. The TDR technique requires appropriate calibration if it is to be used for solute concentration measurements under constant and varying water content. Two ways of determining the parameters in the relationship between bulk soil electrical conductivity, σ a, soil solution electrical conductivity, σ w and volumetric water content, θ, were used. Among the two calibration methods tested here, the one which involved eluting the soil with several pore volumes of water of known electrical conductivity under transient flow conditions in an undisturbed soil column gave a linear σ w− σ a− θ relationship with depth dependent parameters. The other calibration method involved the use of disturbed soil under static conditions and resulted in a curvilinear σ w− σ a− θ relationship. The former calibration was subsequently used to determine solute concentrations in breakthrough experiments. Two solute transport experiments were carried out in the same undisturbed soil column; one for steady-state and one for transient flow condition. Parameters of the convective log normal transfer function (CLT) model were determined using time-integral-normalized resident concentrations. It was shown that the stochastic-convective approach described the experimental data well. Horizontal heterogeneity was more important during the transient experiment, where the topsoil was periodically ponded during a short period, than in the steady-state experiment, where unsaturated conditions were maintained throughout the experiment. This increased heterogeneity during transient flow was also reflected by the fact that preferential solute flow was only observed in the column's outlet under transient conditions.

Soil Solution Electrical Conductivity Measurements under Transient Conditions Using Time Domain Reflectometry

AbstractSeveral studies have shown that time domain reflectometry (TDR) can be used to measure water content and solute concentration during steady‐state experiments with constant soil water content. Few studies have been carried out to identify the relationship between bulk electrical conductivity (σa) and soil solution electrical conductivity (σw) to be used for transient conditions with varying water content. In this study, a single TDR probe was calibrated and found to accurately measure water content and σa. The σa measurements were fitted to σw using several previously published models. The accuracy of the σa and σw measurements increased with σa; however, the error in the σw estimation was &lt;10% for σa higher than 5.7 mS m−1. The calibration was verified in a large sand column; σa measurements were fitted to σw measured in soil solution extracted with suction samplers. The calibration was also used in a transient solute transport experiment. If not accounted for, the temperature effects of the water content measurement introduce significant errors in the σw estimations.

Alkaline sewage sludge solids affect the chemical speciation and bioavailability of Cu and Zn in the rhizosphere soil solution

Perennial ryegrass (Lolium perenne cv. Magella) was grown in a glasshouse experiment in which nylon mesh bags separated the soil into ‘rhizosphere’ and ‘non-rhizosphere’ zones. The soil was mixed with four application rates of an organic fertiliser made by mixing sewage sludge cake with cement kiln dust and composting. Soil solution was obtained by low speed centrifugation and filtration after addition of water (1:1 soil:solution ratio). A resin exchange method was used to distinguish between labile and non-labile forms of dissolved copper (Cu) and zinc (Zn). Soil solution pH, electrical conductivity (EC), absorbance at 360 nm (A360) and Cu and Zn were determined, together with plant (shoot + root) Cu and Zn concentration and uptake.Sludge cake application increased total soluble Cu but decreased total soluble Zn in the soil solution compared to unamended controls and these effects were associated with increased soil solution pH, EC and AMI. Soluble Cu accumulated while soluble Zn was depleted in the rhizosphere compared to the bulk soil. Cu occurred mainly in non-labile form, probably as stable organic complexes, while Zn was mostly in labile form, possibly as free ions and weak complexes. Sludge cake decreased the labile form of each metal and increased the non-labile form in the rhizosphere. It increased plant yield and uptake of Cu and Zn but did not change the plant concentrations of the two metals. The strong influence of the organic fertiliser on plant Cu and Zn uptake by promoting plant growth and by changing the solubility and distribution of the metals in the rhizosphere soil solution is discussed.

Solute Transport under Transient Flow Conditions Estimated Using Time Domain Reflectometry

AbstractImproved methods having high resolution in space and time are needed to monitor transport of soluble chemicals in soils. We used time domain reflectometry (TDR) to measure breakthrough curves (BTCs) for Br− during miscible displacement in repacked soil columns under nonsteady water flow conditions. A simple linear model was used to estimate soil solution electrical conductivity (σw) based on TDR‐measured bulk soil electrical conductivity (σa) and volumetric water content (θ). An efficient calibration method was developed whereby required model parameters were estimated in the same soil volume and under the same flow conditions as were used in subsequent transport experiments. Breakthrough curves measured using TDR were similar to those based on electrical conductivity or Br− concentration in effluent fractions, although peak σw were substantially lower for TDR than for effluent from two of the four soils. Effluent σw BTCs were adequately predicted based on TDR σa and θ for all four soils when parameters of the simple linear model or two alternative models were optimized to the effluent data. Time moments and convection‐dispersion equation (CDE) transport parameters based on time moments of measured BTCs were similar for TDR and effluent analyses. Our results indicate that TDR has potential to monitor the status of representative concentrations of ionic solutes under laboratory and field conditions, without the need to impose steady water flow.

Predicting soil solution chemical attributes from more easily measured soil and soil solution parameters

Abstract An understanding of how soil solution ionic strength (Is) and major cation activities influence crop growth is often limited by the extensive measurements required to characterize ionic composition and subsequent speciation exercises. Easily measured solution and soil attributes need to be identified that can predict these important solution parameters. Soil and soil solution chemical properties of four Ultisols in the Coastal Plain and Piedmont of North Carolina were used to develop models to predict ionic strength and solution cation attributes. GEOCHEM‐PC‐predicted Is was linearly related to electrical conductivity (EC) across soils (r2=0.92), confirming that Is for soil solutions with complex composition can be estimated from their electrical conductivity. Models of the form lnMs=a+blnEC+clnME, or modifications thereof, were developed for predicting solution aluminum (Al), calcium (Ca), magnesium (Mg), and postassium (K) levels (Ms) from a knowledge of EC and either soil exchangeable cation #OPME) or cation saturation (MSATE) attributes. For each cation, total and free solution concentration and activity in absolute and saturation terms were investigated. The best models explained, at most, 68% of the variability associated with total solution Al concentration (Als‐T) or 74% when Als ‐T was expressed as a percent of major solution cations. Greater than 85% of the variability associated with solution Ca and Mg could also be accounted for, but only 67% of the variability associated with solution K could be explained. Including soil pH and interaction terms (MExEC, MExpH, and ECxpH) in models improved the relationship for total Al concentration (R2=0.87) and solution Ca parameters (R2 ≥0.93), but not for solution Mg and K indices. None of the models could account for >30% of the variability associated with free concentration and activity of Al3+, suggesting that the prediction of these parameters for a particular Al species could not be made from a knowledge of soil pH, solution EC, and ME or MSATE data.