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Published online 3 August 2006
Published in Soil Sci Soc Am J 70:1600-1611 (2006)
DOI: 10.2136/sssaj2005.0069
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Nutrient Management & Soil & Plant Analysis

Soil Conductivity as a Measure of Soil and Crop Status—A Four-Year Summary

Roger A. Eigenberga,*, John A. Nienabera, Bryan L. Woodburya and Richard B. Fergusonb

a USDA-ARS, U.S. Meat Animal Research Center, P.O. Box 166, Clay Center, NE 68933
b Univ. of Nebraska, Lincoln, NE

* Corresponding author (eigenberg{at}email.marc.usda.gov)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Animal manure can be an important resource providing soil available N for plant needs, but determining the nutrient availability resulting from such amendments is difficult. A study was conducted to examine changes in electromagnetic induction (EMI) soil conductivity and available N levels during four growing seasons in relation to manure or compost application and use of a green winter cover crop. With simultaneous soil samples, a series of soil conductivity maps of a research cornfield were generated using a global positioning system (GPS) and EMI methods. The Clay Center, NE, site was treated during a 10-yr period with a winter wheat (Secale cereale L.) winter cover crop (+CC) and no-cover crop (–CC). The site was split for subtreatments of manure and compost at rates matching either the P or the N requirements of silage corn (Zea mays L.). Differences between the +CC and –CC treatments for values of NO3–N and water-filled pore space (WFPS), as estimated by apparent electrical conductivity (ECa), were compared for each year. Differences in profile weighted soil conductivity explained 79.5, 98.0, 93.4, and 98.4% of the variability due to NO3–N differences, and only 20.5, 2.0, 6.6, and 1.6% of the variability due to WFPS differences for years 2000, 2001, 2002, and 2003, respectively. Sequential measurement of profile-weighted soil electrical conductivity (ECa) was effective in identifying the dynamic changes in plant-available soil N, as affected by animal manure and anhydrous ammonia fertilizer treatments during four corn growing seasons.

Abbreviations: ATV, all terrain vehicle • +CC, cover crop treatment • –CC, no-cover crop treatment • CN, compost applied to meet nitrogen requirements of crop • CP, compost applied to meet phosphorus requirements of crop • DOY, day of year • EMI, electromagnetic induction • ECa, apparent electrical conductivity • GPS, global positioning system • MN, manure applied to meet nitrogen requirements of crop • MP, manure applied to meet phosphorus requirements of crop • NCK, commercial fertilizer treatment • WFPS, water-filled pore space


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SUSTAINABLE AGRICULTURE requires innovative and practical tools to optimize farm economics, conserve soil organic matter, and minimize negative environmental impacts (Johnson et al., 2003). Electromagnetic induction soil conductivity sensors may provide one such tool. Profile weighted ECa can provide an indirect measure of important soil properties (Davis et al., 1997). The EMI instrument is sensitive to factors that influence soil conductivity, including (i) soil moisture content, (ii) amount and type of salts in solution, and (iii) the amount and type of clays present (Brune and Doolittle, 1990). Electromagnetic techniques are well suited for mapping soil conductivity to depths useful for agriculturalists (McNeill, 1990). Electromagnetic induction soil conductivity has been shown to be a very useful tool in locating seepage from animal waste lagoons (Ranjan et al., 1995); Eigenberg and Nienaber (2003) found values to correlate well with salt levels at an abandoned composting site. Sudduth and Kitchen (1993) used EMI methods to estimate clay pan depth in soil. Electromagnetic methods have been used to map soil salinity hazards (Williams and Baker, 1982; Corwin and Rhoades, 1982). Electrical conductivity methods have been shown to be sensitive to high nutrient levels (Eigenberg et al., 1996, 2000) and have been used to detect ionic concentrations on or near the soil surface resulting from field application of cattle feedlot manure. Electrical conductivity has generally been associated with determining soil salinity; however, it also can serve as a measure of soluble nutrients (Smith and Doran, 1996) for both cations and anions, and is useful in monitoring the mineralization of organic matter in soil (De Neve et al., 2000). Doran et al. (1996) demonstrated the predictive capability of soil conductivity to estimate soil nitrate.

The objective of this work was to determine the utility of EMI methods in evaluating the agronomic effectiveness and environmental consequences of nitrogen fertilization for varying rates of compost, manure, and commercial fertilizer with and without the use of cover crops. Additionally, sequential EMI surveys were examined as a tool in monitoring N cycle dynamics in a corn silage research field.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site
A center-pivot irrigated field ({approx}244 x 244 m) of silage corn located at the U.S. Meat Animal Research Center, Clay Center, NE (40°32' N, 98°09' W, altitude of 609 m), served as a comparison site for various manure and compost application rates for replacement of commercial fertilizer, with the same treatment assigned to field plots for 10 consecutive years. The soil series at this site is a Crete silt loam (fine, smectitic, mesic Pachic Argiustolls), 0–1% slope.

Field Operations on the Research Cornfield
The study was a split-plot design (Fig. 1 ), with four replications of the main plot of +CC vs. –CC (the cover crop was a winter wheat no-till drilled following silage harvest). Subplot application to treatment strips (6.1 m, eight corn rows wide) were made with two manure sources: beef feedlot manure and composted beef feedlot manure. Applications were made each spring according to two strategies: (i) to approximately supply the total crop demand for plant-available N (214 kg NO3–N ha–1 average annual uptake, [Ferguson et al., 2003]), denoted MN and CN for manure and compost, respectively; and (ii) to supply the approximate crop demand of P (42 kg P ha–1 annually, [Ferguson et al., 2003]), denoted MP and CP for manure and compost, respectively. Annual commercial fertilizer treatment (NCK) available N rates were 84 kg N ha–1 during the 4 yr of this study with chlorophyll meter readings (data not shown) taken between V6 and V14 stages. Additional commercial fertilizer was applied as needed; supplemental fertilizer was not applied during the 4 yr of this study. The organic amendment application rate (Table 1) was based on soil analysis of carryover from the previous season, the composition of the manure or compost, the anticipated mineralization rate of the manure (35%) and the compost (25%), as well as the anticipated crop needs.


Figure 1
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  		Fig. 1. Split plot design with four replicates of main plot (cover and no-cover) and subplots (manure and compost applied at either the N or P needs of the silage corn crop, as well as commercial fertilizer). Soil cores were taken in Replicate 2 (+). Treatments are designated MN, manure at crop requirement nitrogen rate; CN, compost at crop requirement nitrogen rate; MP, manure at crop requirement phosphorus rate; CP, compost at crop requirement phosphorus rate; and NCK, a commercial fertilizer check. +CC indicates winter wheat cover and –CC is no-cover.

 

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  		Table 1. Levels of N applied to irrigated cornfield in 2000–2003 and corresponding yields. Total N mineralized the first year assumed to be 35% for manure and 25% for compost.

 
Treatments MP and CP each had sufficient carryover phosphorus after 1998 so that no manure or compost was applied to these treatment strips for the 2000–2003 study. Treatments MN and CN were the only treatments receiving manure or compost application for the study period. The average annual application of total N during the 10 yr of this study were 696 and 711 kg total N ha–1 for MN and CN, respectively, with NCK receiving 84 kg N ha–1 of commercial fertilizer as anhydrous ammonia (Ferguson et al., 2003). Field operations and agronomic events including rainfall and pivot irrigation are shown by date in Table 2.


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  		Table 2. Agronomic schedule for each of the 4 yr of the study. Day of year (DOY) survey times are shown.

 
Equipment
Two different magnetic dipole soil conductivity meters were used in this study: (i) an EM-38, manufactured by Geonics Ltd., Mississauga, ON, Canada (2000 and 2001 seasons); and (ii) a Dualem-2 manufactured by Dualem Inc., Milton, ON, Canada (2002 and 2003 seasons). The EM-38 was operated horizontally and had a response that varied with depth in the soil, yielding a soil conductivity sensitivity that was maximum near the surface and diminished with depth as shown in Fig. 2a (www.dualem.com/gsem.htm; verified 28 Mar. 2006). The Dualem-2 operates in the horizontal and vertical dipole modes simultaneously, but only the horizontal mode is reported in this study, with a corresponding response shown in Fig. 2a. The profile-weighted horizontal response of each instrument is reported in this study, and is designated ECa. The EMI instruments were mounted on a plastic sled and transported through the field, pulled by an all terrain vehicle (ATV), or by hand when the corn became too tall for the ATV. Generally, at a transport speed of 2 to 3 m s–1, about 80 samples across the length of each plot were collected with the EMI instruments for each pass. Each survey was started at the same location, with the instrument pulled through each of the 40-plot treatment and replicate combinations. Surveys were conducted on weekly intervals (when possible) between April and October for all 4 yr.


Figure 2
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  		Fig. 2. (a) Soil conductivity meter response showing normalized sensitivity for Geonics EM-38 and Dualem-2. This figure also indicates depths of thermocouple soil temperature probe locations in relation to normalized sensitivity of the EM-38. (b) Soil conductivity meter normalized cumulative response for both Geonics EM-38 and Dualem-2 (http://www.dualem.com/gsem.htm, verified 23 Mar. 2006). This figure also indicates normalized cumulative response relative to the soil core depth of 0.3 m used for this study; cumulative response at 0.3 m was >50% for the EM-38 and nearly 30% for Dualem-2.

 
A Trimble PRO-XL GPS unit was used to obtain positional data. The EMI instruments were connected to the GPS unit through a small dedicated battery powered microcomputer (Onset Computer, TFX-11). The GPS unit collects and stores positional data and field ECa values.

Soil Temperature Measurements
Soil conductivity responds to soil temperature (McKenzie et al., 1989). An array of thermocouple temperature probes was buried at the cornfield site before the 2002 growing season. Probes were installed at 5 and 15 cm beneath the surface, then at 30-cm intervals to 135 cm, and at 60-cm intervals to 315 cm. The array of soil temperatures was logged on 1-h intervals (Campbell Scientific CR10X with AM16/32 multiplexer, Campbell Scientific, Logan, UT), providing soil thermal profiles throughout the 2002 and 2003 survey seasons. The location of the soil probes are shown (Fig. 2a) in relation to the EMI normalized sensitivity response function of the EM-38. An effective soil temperature for correcting the EMI readings was computed based on the fractional contribution to the Dualem-2 response function at each probe depth through the measured profile. The soil temperature profile data were recorded at the survey time on the day of the survey. The temperature data was used to establish a soil temperature correction (Fig. 3a ) for each survey date following the approach of McKenzie et al. (1989). The soil temperatures demonstrated predictable temperature patterns during the 2002 and 2003 season. Soil temperature profile data were not available for the 2000 and 2001 growing seasons during the time that the Geonics instrument was used. Soil patterns from the 2002 and 2003 season were averaged and corrected for the EM-38 response profile, allowing temperature corrections to be estimated for the Geonics instrument for the 2000 and 2001 season (Fig. 3b).


Figure 3
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  		Fig. 3. (a) Soil temperature correction for Dualem-2 based on soil thermal profile to 315 cm (mean values of 2002 and 2003) developed from Dualem-2 response curves (Fig. 2a). (b) Soil temperature demonstrated predictable temperature patterns (2002 and 2003) that allowed temperature corrections to be estimated for the Geonics instrument response profile based on 2002 and 2003 data.

 
Soil Sampling
Soil cores (1.91 cm [dry tip] or 1.75 cm [wet tip] diam.) were taken throughout the growing season, with a hand probe to a depth of 0 to 30 cm. Replicate 2 was selected for soil cores; comparison of replicate mean ECa values during the 4-yr period indicated that Replicate 2 was representative among the four replicates (Table 3). Cores were taken from the treatments of Replicate 2 at three transverses: (i) midsection of the field, (ii) 46 m to the east of midsection, and (iii) 46 m to the west of midsection (Fig. 1). Two cores were taken at each site for a total of six cores per treatment; one of the cores was taken in the row, with the remaining five from between the rows of corn. The cores were taken on each survey date within 24 h of the ECa surveys, with the cores being weighed and blended. The blended treatment cores were analyzed by a local commercial soil testing laboratory to determine NO3–N and soil moisture content. Soil moisture content was used with core weights (wet) and known core volumes to calculate soil bulk density for the cores. Soil WFPS, which is synonymous with soil relative saturation, was calculated from soil gravimetric water content and the soil bulk density measurements (Smith and Doran, 1996).


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  		Table 3. Comparison of average apparent electrical conductivity (ECa) with standard errors for the four replicates during the 4-yr study.

 
At the end of the growing season, soil was sampled to a depth of 1.5 m (the same three traverses as mentioned above but across all replicates) to determine if NO3–N or P had leached below the root zone (Ferguson et al., 2003).

Data Handling and Processing
Survey data were transferred to a personal computer after each survey, with the stored files converted to ASCII format suitable for input into a contouring and 3-D mapping Surfer program (Golden Software, Inc., Golden, CO). Data reduction was accomplished with a Visual Basic program that extracted and labeled the EMI data from each treatment for statistical analysis.

The ECa surveys of each treatment were formatted to be compatible with statistical software (SAS Institute, 1985). The LSMEANS procedure (SAS Institute, 1985) was used to establish significant differences for treatments. A regression procedure (SAS Institute, 1985) was used with the STB command to generate standard regression estimates for comparing relative contribution of NO3–N and WFPS to ECa variability for differences in +CC and –CC crop treatments. Data being compared are soil constituent data to 30 cm from Replicate 2 and the soil conductivity data from Replicate 2. Comparison assumes the dynamics that occur in the top 30 cm of the sampled soil will have an associated response in the ECa measured by the EMI equipment. The Crete series soils at the research site are described as having a slowly pervious layer within the upper 1 m that keeps the soil wet close to the surface for long periods (http://ortho.ftw.nrcs.usda.gov/osd/dat/C/CRETE.html; verified 28 Mar. 2006). The slowly pervious layer tends to minimize deeper nutrient movement and root development. Woodbury et al. (2004) used time domain reflectometry on the same research field to clarify near-surface soil–crop dynamics of an animal-waste amended soil. Woodbury concluded the majority of electrical conductivity dynamics measured by EMI were dominated by activity in the upper 0.15 cm of soil surface. Profile weighted soil conductivity, as measured by EMI instruments, responds to a large volume of soil beneath the instrument. However, when operated in the horizontal dipole mode (Fig. 2a), the greatest response is to near soil surface conductivity characteristics. The instruments used in this research, an EM-38 and a Dualem 2, have a cumulative response of >50% and near 30%, respectively, to a depth of 30 cm (Fig. 2b). Additionally, the deeper soil volume that is included in the instruments response remains relatively unchanged during the growing season and contributes little to response dynamics. Since the instruments are sensitive to near-surface soil conductivity changes and the majority of ECa dynamics occur in the near-surface region that is included in the 30-cm soil core depth, comparing 30-cm soil core data with the measured ECa response provides a valid comparison of dynamics within the same region of soil.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soils Data
Manure source had little effect on crop yield, nutrient uptake, or soil nutrient accumulation and movement (Ferguson et al., 2003). Cover crop was effective in reducing NO3–N beneath the root zone and increasing it near the surface (Ferguson et al., 2003) based on soil cores taken to 1.5 m from this research field following silage harvest each year. Figure 4 shows the 4-yr summary of soil temperature. The soil temperature data is given for 45 cm (2000–2001) and for 45, 195, and 315 cm (2002–2003) depths on the days of ECa surveys. The temperatures shown were taken at the time of the surveys. Figure 5 shows WFPS and NO3–N values for the four seasons (Replicate 2).


Figure 4
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  		Fig. 4. A four year summary of soil temperatures during the growing season. Soil temperatures shown to 45 cm for 2000 and 2001. Thermocouple grid was installed (2002) to 315 cm with representative measurements shown at 45, 95, and 315 cm.

 

Figure 5
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  		Fig. 5. (a) Soil nitrate measurements taken on apparent electrical conductivity (ECa) survey dates. Comparison is shown for winter wheat cover crop and no-cover for the four year study. All values are taken from Replicate 2. (b) Water-filled pore space (WFPS) measurements on ECa survey dates. Comparison is shown for winter wheat cover crop and no-cover for the 4-yr study. Mean of differences of no-cover minus cover demonstrate a water deficit of cover compared with no-cover for the 4-yr study. All values are taken from Replicate 2.

 
Apparent Electrical Conductivity Maps and Plots
The number of ECa survey maps generated during the growing seasons of 2000, 2001, 2002, and 2003 were 29, 26, 25, and 34, respectively. A representative ECa image of the cornfield is shown in Fig. 6 as generated on 17 June 2002 (day of year [DOY] 168). When the series of images for a growing season are viewed in sequence, the maps illustrate field dynamics with overall ECa values rising uniformly with time (images not shown) early in the season. The treatments associated with manure and compost for the –CC main treatments were distinguished by light stripes (higher ECa values) on the map. The cover crop consistently results in a darker (lower conductivity) region for this portion of the season due to higher nutrient uptake of the cover crop. Subsequent darkening of the overall image occurs in later maps as crop uptake and nutrient transport dominate the image. The image dynamics are more evident in the mean values extracted from the survey data and illustrated in Fig. 7 . These values represent averages for each treatment (approximately 80 readings) across replicates for the –CC treatments for 2002.


Figure 6
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  		Fig. 6. Representative apparent soil conductivity (ECa) map as generated on 17 June 2002. Soil conductivity map is overlaid with treatment symbols.

 

Figure 7
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  		Fig. 7. Apparent electrical conductivity (ECa) by treatment for no winter wheat cover crop. Treatments are designated MN, manure at crop requirement nitrogen rate; CN, compost at crop requirement nitrogen rate; MP, manure at crop requirement phosphorus rate; CP, compost at crop requirement phosphorus rate; and NCK, a commercial fertilizer check.

 
Main Plot—Cover Crop
The effect of the main plot treatment, +CC, is shown in Fig. 8 . Each of the 4 yr shows a pattern of –CC ECa values higher at the start of the season, then converging during midseason, and diverging again after establishment of the cover crop in the fall. Figure 8 indicates convergence (data averaged across subtreatments and replicates) (P > 0.05) of –CC, as compared with +CC. In 2000, –CC ECa was significantly higher than +CC until DOY 171, when the ECa of –CC became mostly lower than +CC through DOY 255 until –CC and +CC diverged (cover crop was drilled on DOY 251 [Table 2]) for the remainder of the surveys. The pattern for 2001 was similar, with –CC higher until DOY 171, where +CC converges with –CC; +CC and –CC remain close until DOY 275 (cover crop was drilled on DOY 253), after which the curves diverge. Again in 2002, –CC begins significantly higher than +CC, continues higher through DOY 196, converges for 54 d, and then diverges again around DOY 282 (cover crop was drilled on DOY 261). Finally, in 2003, –CC is significantly higher than +CC through DOY 149 then converges from DOY 155 through 302 (with the exceptions of DOY 189 and 260). The +CC was planted on DOY 266.


Figure 8
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  		Fig. 8. Apparent electrical conductivity (ECa) for the main treatment of winter wheat cover and no-cover during 2000, 2001, 2002, and 2003. Asterisks indicate survey dates when the mean values of cover and no-cover were not significantly different (P > 0.05).

 
Ferguson et al. (2003) reported that use of a winter cover crop was effective in reducing NO3 levels at depths below 0.5 m since 1998 (soil cores for this study were taken shortly after silage harvest). Nitrate levels from 0.5 m to the surface tend to be higher under the cover crop, indicating that the cover crop is immobilizing NO3–N in the upper portion of the soil profile. The trend toward higher NO3–N is also evident in Fig. 5a after harvest and before establishment of the fall cover crop. Statistical analysis supports an association of ECa and NO3–N (see statistical analysis section). Sequential ECa graphs bolster the view that the immobilized NO3–N is released when the +CC and –CC curves converge. Each of the 4 yr investigated shows a convergence within the crop growing season. The convergence indicates the nitrogen is tied up in the organic matter of the cover crop until microbial activity converts the organic N to inorganic NO3–N. Cover crops demonstrated effectiveness in minimizing levels of available soil NO3–N before and after the growing season.

Subplot—Organic and Commercial Amendments
Treatments MP and CP each had sufficient carryover P after 1998 so that no manure or compost was applied to these treatment strips in subsequent years, including the 2000–2003 study; they became essentially equivalent to the NCK treatment. Treatments MN and CN were the only treatments receiving manure or compost application. Each of the 4 yr showed ECa of MN and CN to be significantly greater than the ECa of NCK through major portions of the growing season (Fig. 9 ).


Figure 9
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  		Fig. 9. Apparent electrical conductivity (ECa) for subtreatments and for no winter wheat cover (–CC). Subtreatments are designated MN, manure at crop requirement nitrogen rate; CN, compost at crop requirement nitrogen rate; MP, manure at crop requirement phosphorus rate; CP, compost at crop requirement phosphorus rate; and NCK, a commercial fertilizer check. An asterisk indicates a survey date when the mean values of manure or compost and the commercial fertilizer treatment were not significantly different (P > 0.05).

 
Organic amendment treatment ECa effects of MN and CN under the –CC main treatment are shown in Fig. 9. Compost and manure were applied on DOY 88 and commercial fertilizer was applied on DOY 154 in 2000 (Table 2); MN and CN were not statistically similar (P ≤ 0.05) to NCK for the entire season of 2000. Compost and manure were applied on DOY 114 in 2001, with commercial fertilizer being applied on DOY 162; convergence of the MN or CN and NCK did not occur until DOY 220 and continued through DOY 255 (except DOY 233); the curves did not converge again in 2001. A similar pattern was observed in 2002, when manure and compost were applied on DOY 101 and commercial fertilizer on DOY 166. The organic amendments (CN or MN) converged with NCK on DOY 189 and remained converged until Day 274, then diverged again for the rest of the season. Manure and compost were applied on DOY 91 in 2003 and commercial fertilizer on DOY 170. There was no convergence between MN or CN and NCK for the entire season.

Patterns observed during the 4 yr of the study for the –CC treatment (Fig. 9) indicate that ECa is able to differentiate between NCK and organic amendments. The ECa patterns of the organic amendments and the commercial fertilizer, while similar, are independent and appear to be driven by seasonal factors. Higher salt or mineral content may explain some of the separation between MN or CN and NCK, but convergence followed by divergence (2001 and 2002) may best be explained by soil and crop dynamic differences between the treatments.

The effects of MN and CN under the +CC main treatment are shown in Fig. 10 . Manure and compost were applied on DOY 88 in 2000 (Table 2); MN or CN converged (P > 0.05) from DOY 96 through DOY 137 (with exception of DOY 131), then remained significantly different until DOY 291 (Fig. 10). Manure and compost were applied on DOY 114 in 2001, with commercial fertilizer being applied on DOY 162; convergence of MN or CN with NCK occurred on DOY 92 through 113 (with the exception of DOY 100), and remained significantly different until convergence on DOY 179, and continued converged until DOY 284 (with the exception of DOY 184, 192, and 268). A different pattern was observed in 2002, when manure and compost were applied on DOY 101 and commercial fertilizer on DOY 166. Manure at the nitrogen rate treatment or CN converged with NCK from DOY 134 through DOY 324 (with the exception of DOY 141 and 168). Manure and compost were applied on DOY 91 in 2003, and commercial fertilizer on DOY 170. The organic amendments of CN or MN converged on DOY 118 through DOY 127, then diverged until DOY 197 and remained converged until DOY 210, and diverged again through the end of the season.


Figure 10
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  		Fig. 10. Apparent electrical conductivity (ECa) for sub treatments and for winter wheat cover crop (+CC). Subtreatments are designated MN, manure at crop requirement nitrogen rate; CN, compost at crop requirement nitrogen rate; MP, manure at crop requirement phosphorus rate; CP, compost at crop requirement phosphorus rate; and NCK, a commercial fertilizer check. An asterisk indicates a survey date when the mean values of manure or compost and the commercial fertilizer treatment were not significantly different (P > 0.05).

 
The patterns observed during the 4 yr of the study for +CC (Fig. 10) indicate that ECa is able to differentiate between a NCK and organic amendments under certain crop and season conditions. The converged MN or CN with NCK treatments early in the season is explainable by opportunistic uptake of NO3–N by the cover crop. The +CC treatment for the 2000, 2001, and 2003 seasons demonstrated a significant separation after the cover crop was killed; this is likely due to a higher mineralization rate under the organic treatment. The pattern difference for 2002 is likely due to low precipitation during the 2002 season (average annual rainfall in South Central Nebraska for the 4-yr study was 523, 541, 373, and 424 mm, for 2000, 2001, 2002, and 2003, respectively [Table 2]). Figure 5b shows that the mean differences of WFPS under the –CC crop compared with +CC was greatest in 2002, supporting moisture levels as a reason for differences observed in year 2002 in Fig. 10. The resulting ECa for the 2002 +CC treatments were likely driven by both NO3–N and moisture content (see the next section for further discussion).

Statistical Analysis
ECa as an Indicator of Biophysical Changes in Plant-Available Nitrogen
A study conducted in 1999 (Eigenberg and Nienaber, 2003) concluded that the mineralization and loss of soluble N from soil can be reasonably estimated from soil electrical conductivity values. This conclusion was based on observed trends in the data, statistical correlations between EMI and soil core data, as well as predicted estimates based on ion concentrations, and is supported by examining the 2000–2003 cornfield data.

Three primary dynamic quantities in a cornfield are the soil temperature, soil moisture, and nutrient levels. The ECa data for 2000–2003 have been corrected (Fig. 3) to the equivalent temperature of 25°C (McKenzie et al., 1989). The remaining dynamics of soil moisture and nutrient levels (NO3–N being a primary contributor) are examined in the sections that follow.

Main Plots—Cover Crop
Cover crop is the main plot of the split plot design. Survey ECa temperature-corrected data were collated with all treatments identified and all survey dates concatenated for each year. Soil data were included in the data file and associated with ECa of Replicate 2. A standardized estimate was generated that partitions the percentage contribution between the values of soil moisture and NO3–N toward the variability of ECa across survey dates; this test was done for temperature-corrected ECa data independently for each of the 4 yr (2000–2003), with results shown in Table 4. The results indicated that for the –CC treatments, ECa accounts for 98, 98, 97, and 66% of the variability due to NO3–N and 2, 2, 3, and 24% of the variability due to WFPS for 2000, 2001, 2002, and 2003, respectively. The +CC treatment indicates ECa accounts for 81, 23, 53, and 29% of the variability due to NO3–N and for 19, 77, 47, and 71% of the variability due to WFPS for 2000, 2001, 2002, and 2003, respectively. Year-to-year variations in the standardized estimate for –CC and +CC treatments may be attributed to the dry years in which the surveys occurred. The average annual precipitation for the South Central Nebraska region is 690 mm; the rainfall for the four year study was 523, 541, 373, and 424 mm. Center-pivot irrigation can replace some of the rainfall during the growing season; however, a dry year, coupled with additional water uptake by the cover crop, can result in moisture becoming limiting. The mean values of WFPS show a trend toward a net deficit of +CC compared with –CC during the 4 yr of the study (Fig. 5b). Below-normal precipitation characterized the 4-yr period and may have influenced the standard estimate values for the +CC treatment where both water and nitrate dynamics contributed to the ECa changes. The 4 yr of data from 2000 through 2003 show that ECa dynamics accounted for a majority of the NO3–N variability, with WFPS accounting for much less of the variability under the –CC treatment. The methodology demonstrated nitrate as a dynamic player in the crop production cycle, and EMI as a viable tool for observing NO3–N dynamics in a crop production system.


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  		Table 4. Relative contribution of NO3–N and WFPS in explaining variability of apparent electrical conductivity (ECa). Standard regression estimates shown for 2000–2003 comparing the relative contribution of NO3–N and water-filled pore space (WFPS) in explaining the variation in ECa (all data from Rep 2). Comparisons are made to temperature-corrected ECa values.

 
The plots of +CC and –CC for ECa, NO3–N and WFPS, show seasonal patterns for all 4 yr (Fig. 5 and 8). The ECa for +CC is lower than –CC early and late in the season; similarly, NO3–N and WFPS also demonstrate seasonal patterns. The plots raise the question of what causes the ECa difference through the season: Was soil moisture or NO3–N the primary contributor to the observed differences? The standardized estimate was used to answer this question, with comparisons made between NO3–N and WFPS differences as contributors to the ECa differences. Soil data and associated temperature-corrected ECa data from Replicate 2 as described in the previous analysis were used to create a differenced dataset. Separate columns were created for differences between –CC and +CC treatments for NO3N, WFPS, and ECa. A standardized estimate was generated that partitions the percentage contribution between the differences in values of soil moisture and differences in values of NO3–N toward the differences of ECa across survey dates; this test was done for temperature-corrected ECa data for 2000–2003 (each year was analyzed independently), with results shown in Table 5. Profile weighted soil conductivity differences of –CC and +CC accounted for 79, 98, 93, and 98% of the variability due to NO3–N differences of –CC and +CC, and only 21, 2, 7, and 2% of the variability due to WFPS differences of –CC and +CC for years 2000, 2001, 2002, and 2003, respectively. The primary contributor to the differences between +CC and –CC ECa values during the growing seasons was nitrate level differences. Two different instruments were used, an EM-38 in 2000–2001 and a Dualem-2 in 2002–2003, yet the profile weighted conductivity differences result in similar findings. Main plot standardized estimates for the 4-yr study support the findings of the 1999 study (Eigenberg and Nienaber, 2003), concluding that the mineralization and loss of available N from soil can be reasonably estimated from soil electrical conductivity values.


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  		Table 5. Relative contribution of NO3–N and water-filled pore space (WFPS) in explaining the variability of apparent electrical conductivity (ECa) due to differences between no-cover and cover crop measurements. Survey date differences of NO3–N, WFPS, and ECa were tabulated and compared using regression estimates for all values of Rep 2. The data were compared for each of the 2000–2003 seasons for temperature-corrected ECa measurements.

 
No-Cover, All Treatments
Table 4 indicates that NO3–N is significant and the primary contributor to the –CC profile weighted soil electrical conductivity plot dynamics for all 4 yr. Plot treatment means distinctive shapes (Fig. 11 ) can be interpreted from the perspective of NO3–N as responsible for key features. Every plot begins with manure or compost being applied early, followed by an ECa value that gradually increases as the season progresses. Planting dates varied from DOY 114 to 137, and did not have an immediate impact on ECa. The crop produces a visible change in ECa approximately 50 d after planting, as the crop achieves about 30 cm height; 30 cm is approximately the V6–V9 stage, a time of increasing NO3–N uptake; Ritchie et al. (1986). The apparent conductivity makes a noticeable downturn that lasts until about the time the corn silks. The downturn in ECa corresponds to the time of maximum nutrient uptake; a time when NO3–N was rapidly being removed from the soil. Between the silk stage and harvest, the ECa curve levels out. The apparent soil conductivity begins a gradual increase as a response to mineralization, until the end of the season.


Figure 11
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  		Fig. 11. Distinctive shapes of the apparent electrical conductivity (ECa) mean values (averaged across all treatments and subtreatments) shown with chronological events for each season. M/C, manure or compost; NCK, commercial fertilizer check.

 
Implications
Four years of soil conductivity surveys established response patterns of a silage cornfield for the organic and inorganic treatments under +CC and –CC conditions; the patterns, for the most part, are explainable by organic N transformation to inorganic N with associated crop uptake. The organic amendments demonstrated higher conductivity through nearly all of the growing seasons for –CC, and for much of the growing season for +CC; organic treatments trended toward convergence and a downturn during the growing season. The ECa patterns of the organic amendments are consistent, with a higher N-mineralization associated with the organic amendments having a higher organic pool of N to be mineralized. Opportunistic uptake by a growing corn crop, with nutrient movement into the soil profile, may explain the convergence and downturn during the growing season. Comparing the cover crop effects on soil conductivity during the 4-yr period revealed low ECa values for +CC at the beginning and end of the growing season; +CC and –CC trended toward convergence at the time of increasing NO3–N uptake. Moreover, when the driving mechanism for the difference between +CC and –CC is analyzed, NO3–N is found to be the major contributor in all 4 yr (Table 5).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
A 4-yr study, which included field measurement of soil ECa, identified the effects of manure, compost, fertilizer, and cover crop on ECa values. Compost and manure applied at the available N application rate of 200+ kg N ha–1 resulted in consistently higher conductivity and available N, when compared with the commercial fertilizer (84 kg N ha–1) and manure and compost at the P rate (which hadn't been applied since 1998). Sequential measurements of profile-weighted soil ECa effectively identified the dynamic changes in available soil N, as affected by animal manure and commercial anhydrous ammonia fertilizer treatments during the corn growing season. The sequential measurements also clearly identified the effectiveness of cover crops in minimizing levels of available soil N before and after the corn-growing season, when soluble N is most subject to loss. Ferguson et al. (2003) reported that use of a winter cover crop was effective in reducing NO3–N levels at depths below 0.5 m. Nitrate levels from 0.5 m to the surface tended to be higher at the end of the growing season under the cover crop, indicating that the cover crop is releasing NO3–N in the upper portion of the soil profile. Sequential ECa graphs indicate that the immobilized NO3–N is released when the +CC and –CC curves converge. This 4-yr study supports the initial findings of a 1999 study that soil conductivity appears to be a reliable indicator of soluble N gains and losses in the soil under study, and may serve as a measure of available N sufficiency for corn early in the growing season, as well as an indicator of NO3–N surplus after harvest when soluble N is prone to loss from leaching and/or runoff.


   ACKNOWLEDGMENTS
 
The authors would like to acknowledge the dedicated effort that was provided by Diane Purcell in development of software applications related to this work, as well as work provided by John Holman in development and construction of electronic interfaces for the EMI equipment and the global positioning satellite system. Additionally, field survey assistance was provided by Todd Boman, John Holman, and Krystal Zimmerman.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.

Received for publication March 11, 2005.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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