Electrical Geophysics for Agronomic Soil Characterization

The combination of soil characteristics is the main factor affecting yields. Soil properties are often distributed unevenly over the field area. This leads to heterogeneous soil characteristics (moisture, salinity, acidity, texture, total fertility) in different parts of the field. The absence of accurate soil maps increases fertilizer application costs and irrigation water consumption, resulting in reduced yields and profits. The first studies on applying resistivity methods to soil moisture measurement are known from the 1930s. In the 1960s, the same methods were increasingly being applied in the USA and Europe to determine the salinity of soils Allred et al. [1].

Work on the electrophysical characterization of soils has been carried out in Russia since the 1990s under Prof. Pozdnyakov of Moscow State University Pozdnyakov [2]. He used the resistivity method and obtained typical VES curves for different soil types. VES is not the best methodology for investigating large areas due to the low measurement speed. Modern geophysical electromagnetic survey equipment can be successfully used for the zoning of agricultural soils.

In July 2021, fieldwork was carried out in the Rostov region. AEMP-14 induction electromagnetic frequency sounding equipment was used, mounted on plastic skids 3m long, the height of the device above the ground surface 20cm. The skids were towed by vehicle and tractor, generator of the device forward, the distance from the generator to the tow was not less than 2m (Figure 1).

Figure 1: AEMP-14 on skids.

Point datasets with resistivity values were exported with iiSystem (Version 4.04) and plotted on maps in the ArcGIS Pro (Version 2.8) environment, using the Kriging plugin (3D Analyst Tools). As a result of the research, maps of the apparent resistivity distribution were plotted, zoning was carried out, and eight sampling points were identified (Figure 2).

Figure 2:Resistivity map at 61 kHz with sampling points, Rostov region.

The results of the laboratory tests of the samples were compared with the apparent ground resistivity values. Correlation indices were revealed. It was shown that the moisture content in the soil at a depth of 50-100cm has the most significant influence on ground resistivity at frequencies 9-11kHz, and acidity of the upper part (20-30cm) of the section influences resistivity at frequencies 60-100kHz. Similar studies were carried out in November 2021 in the USA (Montrose, Iowa). A post-harvest cornfield was surveyed using an AEMP-14 on a skid towed by an ATV. A resistivity map with soil map element boundaries is shown in Figure 3.

Figure 3: Apparent resistivity map, Montrose, Iowa.

The data obtained show that the soil characteristics of agricultural soils can be effectively derived from the electrophysical characteristics measured with modern electrical survey equipment.

A limitation of this method is its application to soils with high resistivity. This will cause high deviations between the apparent resistivity calculated from the induction equipment’s signal and the true resistivity values.

Further development of the method, according to the authors, consists of the combination of induction equipment and the Land Mapper portable resistivity method equipment. This technique can be effectively used to measure the resistivity at various spacings, plotting the VES curve. Thus, it is possible to achieve calibration of the apparent resistivity using Land Mapper data by applying it at points selected from induction electromagnetic mapping results.

This research and sensor development was supported by EU HORIZON 2020 research and innovation program, PARSEC Consortium, under grant agreement #824478 “Crop Predictions Take Flight – Linking Genomics and Geophysics”.

In addition, geo-analytical services development was supported by grant no. 43950/1312/3128 “Green attack identification with the use of multi and hyperspectral data” financed by Internal Grant Agency FFWS CULS in Prague.

1. Allred B, Daniels JJ, Ehsani MR (2008) Handbook of Agricultural Geophysics. In: Allred B, Daniels JJ, et al. (Eds.), (1^st edn), CRC Press Boca Raton, Florida, USA.
2. Pozdnyakov AI (2001) Field electrophysics of soils. Nauka/Interperiodika, Moscow.