The relationship between apparent soil electrical conductivity and soil properties 1

Precision agriculture that is based on the physical and chemical properties of soil requires a dense sampling to evaluate spatial variability in the field. This dense sampling is often expensive and time consuming. One technique to reduce the number of samples is to define management zones based on information that is collected in the field. Some researchers have demonstrated the importance of the electrical properties of soil in defining management zones. Thus, the objective of this study was to evaluate the relationship between the apparent soil electrical conductivity and soil properties in mountainous areas of coffee production. The electrical conductivity of soil was evaluated at soil depths ranging from 0.00-0.20 m (EC20) and 0.00-0.40 m (EC40) using a portable meter. The mean values of EC20 and EC40 were 1.80 mS m-1 and 1.22 mS m-1, respectively. Both EC20 and EC40 exhibited comparatively low correlations with the soil properties, whereas higher correlations were obtained for measurements of remaining phosphorus, wherein values of 0.427 and 0.465, respectively, were obtained.


INTRODUCTION
Coffee production in different countries has been transformed into a specialized field.Consumers demand a high quality product, and coffee producers are looking for ways to reduce costs and add value to their product through production systems that provide special attributes to their coffee.Precision agriculture is a technique that may be used to attain these goals.By practicing precision agriculture, it is possible to identify areas that will give higher yields and that have the potential to produce higher quality coffee.Precision agriculture can also be used to optimize the use of inputs in the production system and make business more profitable.
The use of precision agriculture that is based on the chemical and the physical properties of the soil requires a dense sampling to determine spatial variability in the field (SILVA et al., 2010).A decreased density of sampling can lead to errors in estimating the spatial variability of soil nutrients and, consequently, errors in recommendation.
One technique for reducing the number of samples is to define management zones (MORAL et al., 2010;XIN-ZHONG et al., 2009).The field information frequently used for defining management zones is based on a digital elevation model, maps of soil fertility that have been generated from systematic sampling in the field and yield maps that have been obtained for more than one season.Recently, some researchers (CORWIN;LESCH, 2003;CORWIN et al., 2003;CORWIN;LESCH, 2005a;CORWIN;LESCH, 2005b;CORWIN et al., 2006;LESCH et al., 2005;MORARI et al., 2009;SAEY et al., 2009;YAN et al., 2007a;YAN et al., 2007b;YAN et al., 2007c;YAN et al., 2008) have demonstrated that soil electrical parameters in the field can be used to explain variability in the physical and chemical properties of the soil.Additionally, these parameters are reliable and easy to measure.Thus, the objective of this research was to evaluate the relationship between the apparent soil electrical conductivity and soil properties in a coffee production system that is located in a mountainous area.

MATERIAL AND METHODS
The data for apparent soil electrical conductivity were measured on a farm located in Araponga (20°42'33" S, 42°34'17" W, average altitude of 913 m a.s.l.), Minas Gerais, Brazil .This farm has an area of 86 hectares and is planted with the Coffea arabica L. species.The average altitude is 904 m, the area is predominantly mountainous, and the soil is classified as Typic Hapludox (SOIL SURVEY STAFF, 2006).According to the Brazilian soil classification the soil is classified as Latossolo Vermelho Amarelo.
The resistivity method was used to measure the apparent soil electrical conductivity.In this method, four equally spaced electrodes are placed in contact with the soil surface.An electric current is applied between the two external electrodes and the electrical potential difference is measured in the inner electrodes, as shown in Figura 1.This configuration is commonly referred to as the Matrix Wenner configuration (CORWIN;HEDRICKX, 2002;CORWIN;LESCH, 2003).
A portable sensor, model ERM-02, manufactured by Landviser, was used to characterize the apparent soil electrical conductivity by the resistivity method.Two electrode spacing configurations were used: one with 0.20 m (EC20) spacing and another with 0.40 m (EC40) spacing.
Figura 1 -A diagram of the four-electrode resistance meter with two current electrodes (C1 and C2) and two potential electrodes (P1 and P2) that was used in this study (CORWIN;LESH, 2003) The relationship between apparent soil electrical conductivity and soil properties The apparent soil electrical conductivity and the soil properties were determined in a 20.20-ha area.The EC20 and EC40 were measured at 141 points in the experimental area.The soil was sampled in the canopy projection of culture and depth of 0-0.20 meters in an irregular mesh with 141 points (SILVA et al., 2010).Soil samples were collected at each point in order to determine the soil texture and chemical properties.At each sampling point, three single samples were collected in a radius of 1 m around the point where the EC20 and EC40 were measured.Soil samples were analyzed in order to obtain the soil texture composition (clay, silt, and sand), electrical conductivity in the extract of soil to water ratio (1:5), moisture, and soil fertility (pH; potential acidity; P, K, Ca 2+ , Mg 2+ , Al 3+ , and H-Al levels; sum of basis; effective cation exchange capacity (CEC t); cation exchange capacity at pH 7 (CEC T); basis saturation; aluminum saturation; remaining phosphorous; and Zn, Fe, Mn, Cu, and organic matter).The relationship between the apparent electrical conductivity and the soil properties was determined by calculating the Pearson's correlation coefficient.In order to complement the characterization of the soil, a water retention curve was determined using the Extractor Richards method (RICHARDS, 1965).For this test, 15 out of 141 sampling points were randomly selected to perform the measurements.
As can be observed in Figura 2, each sampling point was georeferenced using a DGPS Promark 3, which was manufactured by Magellan.For differential correction, data from the Brazilian Network for Continuous Monitoring (RBMC) were used.GNSS solution software (developed by Magellan) was used to process the data.

RESULTS AND DISCUSSION
The results of the descriptive statistics for EC20, EC40, and the some soil properties are shown in Table 1.Therein, it can be observed that the EC20 exhibited a coefficient of variation that was greater than that for EC40, which indicates a greater variability in EC20 values due to variations in soil properties near the surface.
The average values of EC20 and EC40 were 1.80 and 1.22 mS m -1 , respectively.These values are relatively low when compared to the apparent electrical conductivity Despite the low values of electrical conductivity, the average apparent soil electrical conductivity values were well within the range of the electrical conductivity of soil extract (1:5) that were determined in the laboratory, which confirm the results from other studies (CORWIN; LESCH, 2003;CORWIN;LESCH, 2005b;MORAL et al., 2010;YAN et al., 2007c).These observed low values may have resulted from a low cation exchange capacity of the soil, as reported by Corwin and Lesch (2005b).The cation exchange capacity of soil is related to the amount and type of clay and organic matter content in the soil.Although the soil has high clay content, probably, the clay itself has a low quality.Moreover, as shown in Table 1, the measured soil has a low organic matter content, which influences reflects a low cation exchange capacity and, consequently, low electrical conductivity.
The relationship between apparent soil electrical conductivity and soil properties The minimum and maximum values of conductivity were 0.40 and 5.24 mS m -1 , respectively.This amplitude is less than that obtained by Machado et al. (2006), who recorded minimum and maximum conductivities of 1.90 and 13.70 mS m -1 , respectively.Aimrun et al. (2007) obtained minimum and the maximum values of 0.90 and 64.10 mS m -1 , whereas Faulin and Molin (2006) obtained values of apparent soil electrical conductivity ranging from 0.60 to 16,6 mS m -1 .These differences in amplitude may not relate to soil moisture conditions.As shown in Table 2, for the 15 water retention curves that were obtained in this study, it was found that the soil during the electrical conductivity measurements was at or near field capacity.Thus, the amplitude of variation was low, probably due to the low cation exchange capacity, as shown in Table 1.
The ranking of the Pearson's correlation coefficients between EC20, EC40, and the soil properties are presented in Table 3.The results in Table 3 indicate that EC20 and EC40 strongly correlate to each other (0.896), which agree with result obtained by Vitharana et al. (2008) in a similar study.EC40 exhibited higher correlation coefficients with most soil properties in comparison to EC20.This may be due to major changes in the physical and the chemical characteristics that occur near the soil's surface, which increase the oscillation of EC20 and reduce the coefficient of correlation.
In general, EC20 and EC40 had low correlations with soil properties.The highest correlation coefficients for EC20 and EC40 were found for remaining phosphorus, which had values of 0.427 and 0.465, respectively.Moreover, EC20 and EC40 did not show significant correlations with the physical properties of soil, except for the silt content.
The recommended phosphorus dosage for crops can be obtained as a function of the remaining phosphorus and the clay content.In soils that have been classified as Typic Hapludox (SOIL SURVEY STAFF, 2006), higher clay and oxide contents correlate with lower remaining phosphorus contents due to a higher phosphorous adsorption capacity of the soil.If silt and sand were exclusively composed of primary minerals, soil with a high content of these two types of particles would be expected to have a low phosphorous adsorption capacity (DONOGEMMA et al., 2008); however, Typic Hapludox soils have silt-sized microaggregates that are formed by the cementing action of Fe and Al 3+ oxides.Thus, the apparent soil electrical conductivity becomes an alternative for the estimation of the available phosphorus.
The soil moisture content was not correlated with the apparent soil electrical conductivity, which is probably due to the low coefficient of variation for soil moisture.While studying the influence of soil moisture on the apparent soil electrical conductivity, Faulin and Molin (2006) concluded that when the amplitudes of variation in clay and moisture content were high, higher correlation coefficients between electrical conductivity with soil moisture and clay content were found.The authors worked in two different areas for two consecutive years and obtained coefficients of variation for moisture content that ranged from 7.30 to 30.51% and for clay content that ranged from 15.86 to 42.47%, respectively.The coefficients of variation for the clay and soil moisture The relationship between apparent soil electrical conductivity and soil properties content that were obtained in the present study are also considered to be low, with values of 9.14 and 12.40%, respectively.These findings explain why there was no relationship between the moisture content and the soil electrical conductivity.

CONCLUSIONS
1.The average values of apparent soil electrical conductivity were low compared to values observed for saline soils and soils with high values of cation exchange capacity; 2. The apparent soil electrical conductivity using electrode distances of 0.00 to 0.20 m and 0.00 to 0.40 m exhibited strongly correlate to each other; 3. The highest correlation coefficient was found between the apparent soil electrical conductivity and the remaining phosphorus; 4. There was no significant correlation between the apparent soil electrical conductivity and the clay content; 5. The soil moisture content was not correlated with the apparent soil electrical conductivity.

Figura 2 -
Figura 2 -Location of the apparent soil electrical conductivity sampling points in the Brauna Farm

Table 1 -
Summary statistics of the soil data

Table 2 -
Soil retention curve

Table 3 -
Ranking of the Pearson's correlation coefficient between the soil properties and apparent soil electrical conductivities for 0 to 0.20-m and 0 to 0.40-m soil layers