In addition, changes in multiple soil properties caused by changes in soil moisture can further affect the actual crop water requirements in localized areas of the field. Especially in arid regions with low salinization, untimely surveys of the spatial distribution of soil moisture can lead to slower crop growth and, more seriously, lower yields [
3]. On the other hand, over-irrigated is more likely to causing the water infiltrate into groundwater and consequently pollutes groundwater quality [
4,
5]. Therefore, real-time and accurate acquisition of spatial information of soil moisture at field-scale is beneficial for timely adjustment of crop irrigation strategies and refinement of irrigation schemes in local areas, which is helpful for promoting crop growth and improving yields.
Usually, field-scale soil-moisture content monitoring methods not only limited by measurement points, but also take a lot of time, which makes it difficult to investigate the moisture content of soil at a certain depth effectively. In addition, traditional field-scale soil moisture monitoring methods (e.g., mass drying method) do provide accurate soil-moisture content data, but often cause damage to the original soil structure. The measuring device is radioactive (e.g., neutron source) [
6], posing a potential threat to the safety of the device itself. All these reasons make it extremely difficult to monitor the soil moisture at a certain depth at a largescale. Therefore, there is an urgent need to adopt an accurate, fast and efficient soil moisture monitoring method that can be applied to larger scales. In recent years, monitoring various soil properties using electromagnetic induction has been intensively studied by a wide range of scholars [
7,
8], such as soil salinity [
9,
10,
11], soil moisture [
12,
13], pH [
14,
15,
16], soil capacity [
17], soil texture [
18], clay content [
19], calcium carbonate [
20], and soil organic carbon [
21,
22,
23]. Electromagnetic induction techniques are commonly used to determine the weighted average of apparent conductivity of soils at a specific depth [
24], and the soil apparent conductivity is a combination of several key properties [
25], including soil-moisture content, soil soluble salt content, clay content, and soil temperature, making the use of electromagnetic induction techniques to obtain. Therefore, it is possible to use electromagnetic induction to obtain moisture content in the soil space [
26]. Among the studies that used electromagnetic induction to obtain soil moisture, Kachanoski et al. (1988) were one of the first researchers who used electromagnetic induction instruments to obtain soil moisture [
27]. Early results showed that the apparent soil conductivity data measured by electromagnetic induction could explain 77–96% of the variability in soil moisture. In addition, linear and second-order polynomial regression models could be developed on this basis. Calmita et al. used soil apparent conductivity data obtained by electromagnetic induction to predict the temporal variability of soil-moisture content with R
2 > 0.5 [
28]. The results demonstrated that although soil pore water is spatially variable, solid particle properties and pore water conductivity remain constant in time. Hanson and Kaita investigated the accuracy of soil-water content obtained from inversion models based on apparent soil conductivity data at three different levels of salt content [
29]. The results showed that the R
2 of the horizontal direction dipole inversion model was 0.81, 0.89, and 0.92, and the R
2 of the vertical direction dipole model was 0.76, 0.94, and 0.95 for the three salt content levels of low, medium and high, respectively. This study was performed by calibrating the EM38-MK2 to the soil-moisture content estimations, which was determined by the neutron method, and each neutron probe tube of 0.3 m was measured using EM38-MK2 throughout the experiment. But this method would cause errors in the apparent conductivity values from EM38-MK2. Previous studies such as Huang and Hossain provided theoretical support for the application of electromagnetic induction technique for soil moisture monitoring in agricultural fields [
30,
31,
32]. However, previous studies still lacked the application in practical irrigation period. In many oasis farmlands in the arid zone of South **njiang, due to the combination of various factors such as local climate, the soil-moisture content in the surface layer is often very low and spatial variability is high, but the soil-moisture content in the bottom layer near the water table is high and spatial variability is low [
33]. This phenomenon will not only allow farmers to misjudge the soil-moisture content at the field scale, but also easily cause over-irrigation. Therefore, it is especially important to obtain accurate soil-moisture content at the field scale, especially at a certain depth. Electromagnetic induction technology helped obtain soil apparent conductivity data non-destructively and over a large area, and showed superiority in inverting soil capacitance, field water-holding capacity and soil-water content to predict the irrigation volume of farmland, refine field-scale irrigation strategies, and provide guidance for timely monitoring and management of soil moisture conditions in local areas.
Take these all into consideration, this study takes typical cotton fields in the arid zone of South **njiang as the research field, and takes advantage of the electromagnetic induction technology to obtain the information of physical and chemical properties of soil profiles quickly and efficiently. The main contents of this study include: