Mapping cation exchange capacity using a Veris-3100 instrument and invVERIS modelling software
Graphical abstract
Introduction
The necessity for the development of cost-effective soil mapping methods to enable precision agriculture techniques has long been realized. One of the most important soil properties which need to be mapped is the cation exchange capacity (CEC - cmol(+)/kg). This is because it is a measure of the total capacity of the soil to hold exchangeable cations, which is an indication of nutrient availability. It also indicates how well buffered soil pH might be and therefore it influences soil's reaction to fertilizers and other ameliorants (Hazelton and Murphy, 2016). In addition, the CEC is an index of the shrink-swell potential and therefore structural resilience to tillage (Triantafilis et al., 2009). For example, small CEC (i.e. < 20 cmol(+)/kg) indicates poor shrink-swell potential, whereas intermediate CEC (20–40 cmol(+)/kg) and large CEC (> 40 cmol(+)kg) suggest moderate and good shrink–swell potential, respectively (McKenzie, 1998).
However, the traditional way of measuring CEC is a costly and time-consuming process because laboratory analysis requires determination of major cations through a leaching process (Holmgren et al., 1977, Rayment and Higginson, 1992). Nevertheless, and in order to use the limited CEC data, various studies have shown that soil CEC can be mapped at the field level using classical geostatistical methods (Castrignanò et al., 2000, Asadzadeh et al., 2012). But the major disadvantage of using geostatistical analysis is that a large number of samples are required to map the target soil property, which also needs to be spatially correlated and strongly variable (Webster and Oliver, 1992, Jung et al., 2006). Another way is to make use of an easier to measure and cheaper to acquire soil property. This was the approach of Rashidi and Seilsepour (2008), who developed LR relationship to model CEC from organic carbon.
More recently, the use of ancillary data is being preferred to soil data, because much larger amounts of this type of data can be acquired much more quickly and cheaply. This can improve the accuracy and minimise the bias of prediction as well as provide a finer resolution map of the target soil property. One of the most commonly used sources of ancillary data is the measurement of the apparent electrical conductivity (ECa - mS/m). This is because this type of data is a function of type and concentration of ions in the soil solution, as well as the amount and type of clays, water content and soil temperature (McNeill, 1980).
Bishop and McBratney (2001) were one of the first to determine an environmental correlation between CEC and ECa; but they also found that terrain attributes, aerial photography, crop yield data and bare soil LANDSAT TM were useful ancillary data. In doing this they were able to develop a multiple linear regression (MLR) to map topsoil CEC (0–0.15 m). Similarly, Triantafilis et al. (2009) used a hierarchical spatial regression model to map the average profile (0–2 m) CEC using proximally sensed EM38 and EM31 ECa along with a digitized remote sensed image (Red, Green and Blue bands) and trend surface (Easting and Northing) data. Sudduth et al. (2005) used EM38 and Veris-3100 instruments and observed high and more persistent correlation between CEC and ECa data acquired across various fields. However, in all the above cases the information regarding the spatial variation of CEC with depth was not investigated given these examples either looked at only one depth increment or average CEC in the soil profile.
In order to map soil properties with depth, several authors have attempted to do this by establishing LR or MLR relationships between ECa and various depth increments. However, model errors can lead to loss of accuracy in prediction (Huang et al., 2015). More recently, various studies have shown how ECa can be inverted to make 2-d and 3-d calibration relationships between estimates of the true electrical conductivity (σ - mS/m) and various soil properties, including exchangeable sodium percentage (Huang et al., 2014), salinity (Zare et al., 2015), soil moisture (Huang et al., 2016, Huang et al., 2017a) and organic matter (Huang et al., 2017b). In this paper our interest is in seeing if we can invert Veris-3100 ECa data acquired from shallow (0–0.3 m) and deep (0–0.9 m) arrays, and then to use the estimates of σ at various depth increments to make a LR with CEC measured at the same depths. We validate the LR using a cross-validation technique for different calibration points considering their individual depth sections one at a time and the accuracy, bias and Lin's concordance attained by this approach are discussed.
Section snippets
Study area
The research was conducted at a farm located in the proximity of Alvarado, about 16 km east from Badajoz, south-western Spain. The farm is called “El Carrascal” and its area is around 16 ha. The study site is divided into two separate hedgerow olive orchards, including a rectangular shaped field which is located to the southwest of a larger square shaped field (Fig. 1a). The topography is dominated by gentle hills. In the substrate, limestones predominate over intrusive acidic rocks. According to
Preliminary ECa data analysis
Table 1 shows the summary statistics of ECa measured by the Veris-3100. The mean shallow (0–0.3 m) ECa was 15.4 mS/m with a minimum of 3.2 mS/m and maximum of 58.8 mS/m. The median was slightly smaller (11.9), with the shallow ECa slightly positively skewed (1.2) with a coefficient of variation (CV) of 64.2%. In comparison, the deep (0–0.9 m) ECa had a larger mean (26.7 mS/m) with a minimum of 12.5 and maximum of 66 mS/m. The median deep ECa was again slightly smaller (24.0 mS/m) than the mean with the
Conclusions
A Veris-3100 instrument was used across 47 parallel transects to collect shallow and deep ECa data across a study field in south-western Spain. Using soil samples acquired across 40 locations and various depth increments, including topsoil (0–0.3 m), subsurface (0.3–0.6 m) and subsoil (0.6–0.9 m), to successfully establish a calibration between measured CEC (cmol(+)/kg) and the estimates of true electrical conductivity (σ - mS/m). The latter generated using a quasi-three-dimensional algorithm
Acknowledgments
This research was co-financed by Junta de Extremadura and European Regional Development Fund (ERDF) through GR15050 (Research Group TIC008). The authors are grateful to Carlos Campillo, from CICYTEX (Junta de Extremadura), for providing soil data. Dr. John Triantafilis acknowledges the Organisation for Economic Cooperation and Development for providing him with a Co-operative Research Programme Fellowship to visit Universidad de Extramedura. We acknowledge funding support from two separate
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