Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2005
Publication Date: 12/29/2005
Citation: Allred, B.J., Ehsani, R., Saraswat, D. 2005. The impact of temperature and shallow hydrologic conditions versus soil properties on near-surface electromagnetic induction based electrical conductivity measurements. Transactions of the ASAE. 48(6):2123-2135. Interpretive Summary: Precision agriculture (PA) is a growing trend among crop producers. PA depends largely upon being able to sense spatial differences in soil electrical conductivity on-the-go. Because soil temperature and water content vary over time, it is important to know their effect on the soil electrical conductivity as measured by the electromagnetic induction methods used for PA. This research determined that cold conditions, and water content, and water table depth each reduce the soil electrical conductivity. While the magnitude of the electrical conductivity values changed with temperature and water content, the patterns within an area remain consistent over time. This is important to information for farmers, crop consultants, and equipment developers and manufacturers.
Technical Abstract: Precision agriculture is a growing trend, allowing just the right amount of fertilizer, soil amendments, pesticides, herbicides, and even tillage effort to be applied to different areas of the field, thereby optimizing crop yields while reducing input costs. Field crop yield variations are often strongly correlated with spatial soil fertility patterns. The intrinsic fertility of a soil is itself affected by various soil profile properties, such as salinity, organic matter content, cation exchange capacity, grain size distribution, clay mineralogy, claypan/fragipan depth, etc. Apparent soil electrical conductivity (ECa), mapped in situ with geophysical methods such as electromagnetic induction or resistivity, can potentially be used to gauge spatial changes in soil fertility, since the measured ECa is influenced by these very same soil properties. However, temperature and shallow hydrologic conditions can also potentially impact ECa, and therefore, need to be considered in addition to the soil property effects. The effects on ECa due to temperature and shallow hydrologic conditions versus soil properties were evaluated on the basis of 88 ECa mapping surveys conducted over an interval of two years at a test plot ideally suited for this type of research. ECa was measured using the electromagnetic induction (EMI) method at primary field frequencies of 8190 Hz, 14610 Hz, and 20010 Hz. Results were similar at all three frequencies, therefore statistical analysis focused largely on the 14610 Hz data. With respect to temperature conditions, both the air above and soil directly beneath the surface were monitored. The two shallow hydrologic conditions that were assessed included the near-surface soil volumetric moisture content and the water table depth. Based on the average test plot ECa, the EMI surveys and their results can be separated into four groups from different time periods within the two year investigation. The complete record indicates that air temperatures at or below 12 OC and/or soil temperatures at or below 8 OC produce substantially lower values of the measured ECa. Correlation analysis within each of the four data groups shows that on a consistent basis the average test plot ECa (14610 Hz) is most strongly affected by near-surface volumetric moisture content (rMC-GRP-AVG = 0.73), followed by water table depth (rWTD-GRP-AVG = -0.42). with soil temperature next (rST-GRP-AVG = 0.14), and finally, air temperature (rAT-GRP-AVG = -0.10). Correlation analysis of ECa (14610 Hz) maps from 20 EMI surveys (five from each group) conducted under a range of temperature and shallow hydrologic conditions produced rMap values that overall averaged 0.62 and had a standard deviation of 0.17, providing evidence that spatial ECa patterns remain relatively consistent over time. Consequently, these map correlation results are a strong implication that lateral changes in soil properties are what govern the spatial ECa response. However, soil sampling on a scale similar to the EMI ECa measurement volume may be needed to determine which soil properties have the greatest influence on the EMI ECa response.