Research investigations have provided some valuable insight on potential applications and limitations with regard to using resistivity methods for agricultural purposes. There are clear indications that resistivity methods can be a valuable tool for assessing salinity conditions in farm fields (Rhoades et al., 1976, 1990). Kravchenko et al. (2002) combined topographic information and ECa data (obtained with a Veris 3100 Soil EC Mapping System) to map soil drainage classes. Studies have also focused on determining the relationship between various soil properties and the apparent soil electrical conductivity (oa, ECa) as measured with galvanic contact resistivity methods. Using conventional resistivity equipment at a location near Quebec City, Canada, Banton et al. (1997) found that ECa was moderately correlated with soil texture (% sand, % silt, and % clay) and organic matter, but not with porosity, bulk density, or hydraulic conductivity. Johnson et al. (2001) found a positive correlation at their Colorado test site between ECa (measured with a Veris 3100 Soil EC Mapping System) and bulk density, percentage clay, laboratory-measured soil electrical conductivity, and pH; but a negative correlation between ECa and total and particulate organic matter, total carbon, total nitrogen, extractable phosphorous, microbial biomass carbon, microbial biomass nitrogen, potentially mineralizable nitrogen, and surface residue mass. These two studies (Banton et al., 1997; Johnson et al., 2001) imply that soil properties can interact in a complex manner to affect the ECa measured with resistivity methods.
The apparent resistivity (or apparent electrical conductivity) of a soil can also be measured with electromagnetic induction methods. Studies carried out using electromagnetic induction methods have shown the feasibility of using ECa for estimating herbicide partition coefficients (Jaynes et al., 1995), determining clay-pan depth (Doolittle et al., 1994), and monitoring soil nutrient buildup from manure applications (Eigenberg and Nienaber 1998). Consequently, with the proper equipment, it is probable that resistivity methods could also be employed to estimate herbicide partition coefficients, determine clay-pan depth, and monitor soil nutrient buildup.
A rather interesting example regarding an agricultural application of resistivity methods involved using a constant separation traversing resistivity survey to provide insight at a field research facility on the soil salinity impact due to different drainage water management practices. Figure 5.16 shows the field research site setup composed of four test plots. No buried drainage pipe network was installed at the C1 test plot. Test plot C2 contained a buried drainage pipe network, but this drainage pipe network was used only to remove excess water from the soil. Test plots S1 and S2 were subirri-gated during the growing season. Subirrigation can be described as the addition of water to a buried drainage pipe network for the purpose of irrigating crops through the root zone.
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