Soil Electrical Conductivity as a Crop Productivity Measure for Claypan Soils

Abstract

Inexpensive and accurate methods for spatially measuring soil properties are needed that enhance interpretation of yield maps and improve planning for site‐specific management. This study was conducted to investigate the relationship of apparent profile soil electrical conductivity (EC a) and grain yield on claypan soils (Udollic Ochraqualfs). Grain yield data were obtained by combine yield monitoring and EC a by a mobile, on‐the‐go electromagnetic (EM) induction meter. Investigations were made on four claypan fields between 1993 and 1997 for a total of 13 site‐years. Crops included five site‐years of corn (Zea mays L.), seven site‐years of soybean [Glycine max (L.) Merr.], and one site‐year of grain sorghum [Sorghum bicolor (L) Moench]. Transformed EC a (l/EC a was regressed to topsoil thickness giving r 2 values > 0.75 for three of the four fields. The relationship between grain yield and EC a was examined for each site‐year in scatter plots. A boundary line using a log‐normal function was fit to the upper edge of data in the scatter plots. A significant relationship between grain yield and EC a (boundary lines with r 2 > 0.25 in nine out of 13 site‐years) was apparent, but climate, crop type, and specific field information was needed to explain the shape of the potential yield by EC a interaction. Boundary line data of each site‐year fell into one of four condition categories: Condition 1–site‐years where yield increased with decreasing EC a; Condition 2–site‐years where yield decreased with decreasing EC a; Condition 3–where yield was less at low and high EC a, values and highest at some mid‐range values of EC a; and Condition 4–site‐years where yield variation was mostly unrelated to EC a. Soil EC a provided a measure of the within‐field soil differences associated with topsoil thickness, which for these claypan soils is a measure of root‐zone suitability for crop growth and yield. Research Question: Grain yield mapping has demonstrated to farmers that much of the yield variability within fields seems to be associated with soil and landscape properties. A basic premise for a successful site‐specific management program is that the causes of grain variability can be identified and quantified, hopefully by using automated sensors or devices. An inexpensive and accurate method for spatially measuring soil properties that explain variability in grain crop production would greatly enhance the interpretation of yield maps and improve planning for site‐specific management. Apparent soil electrical conductivity (EC a) obtained using mobile on‐the‐go sensors has been suggested as one such measure. The objective of this research was to evaluate claypan EC a as a measure of the relative within‐field variability of grain crop production. Literature Summary: Many procedures have been examined for measuring the effects of soil and landscape properties on crop production. Traditional soil surveys give a general understanding of the impact that soil mapping units have on crop productivity. Slope position and landform are topographic features that have been used to explain water and crop productivity relationships. With the advent of yield monitoring systems, averaging multiple years of yield maps has also been suggested as a way of establishing productivity patterns related to soil water. Our initial investigations using EC a showed that this measurement was an estimator of topsoil thickness and thus, claypan soil root‐zone suitability. These soils have a unique hydrology controlled by slow water infiltration through a restrictive clay layer generally located 0.5 to 2.0 ft below the soil surface. We hypothesized four different potential relationships between EC a and productivity for claypan soils as shown in Fig. Potential yield might either increase with decreasing EC a (plot a), decrease with decreasing EC a (plot b), or be best at some mid‐range of EC a (plot c). Yield variation may also be unrelated to EC a (plot d). Study Description: This field‐scale study was conducted on claypan soils (Putnam and Mexico silty clay loam) in central and north‐central Missouri between 1993 and 1997. Grain yield data were obtained by combine yield monitoring and EC a by a mobile, on‐the‐go electromagnetic (EM) induction sensor. Crops included five site‐years of corn, seven site‐years of soybean, and one site‐year of grain sorghum. The relationship of grain yield vs. EC a was examined for each site‐year in a scatter plot. The points along the upper edge of the scatter‐plot data were believed to represent the response of potential yield at given EC a measurements when other factors were nonlimiting with respect to yield. This is referred to as a “boundary line analysis.” Applied Questions: Do EC a measurements help explain variation observed from grain yield mapping? This research showed that EC a on claypan soils did measure within‐field differences associated with properties of the root‐zone. Apparent electrical conductivity was strongly correlated to the depth of topsoil above the claypan soil horizon. We found there were significant relationships between grain yield and EC a, but that climate, crop type, and specific field information were required to help interpret these relationships. The scatter in the data of this study illustrated how EC a alone can not be used to accurately predict crop productivity variation. Many other layers of information are needed (e.g., insects, weeds, diseases, fertility, crop stand, topograhphy) for both yield map interpretation and management planning. How might producers use an understanding of the relationship between spatially‐measured EC a and grain yield for improved site‐specific management? This research pointed to a few specific management options that could be considered for claypan soil fields. Without irrigation, improvement to droughty, high EC a areas (areas with low topsoil thickness) is limited to either management that can increase water infiltration and water conservation (e.g., conservation tillage methods) or planting more drought‐tolerant crops (e.g., soybean or grain sorghum). Management options for areas of low EC a were often associated with areas of excessive water early in the growing season. Improvements to surface and/or subsurface drainage are possible. Subsurface tile lines are uncommon on claypan soils because the claypan itself causes poor internal drainage; but areas of low EC a, where the claypan is deeper, could potentially be suitable for tile drainage. A significant potential use of EC a measurements is identifying sub‐field management zones based on soil water properties. Many other potential yield‐limiting factors will be dependent on soil water. For example, we found in other research that after dividing claypan fields into sub‐fields using EC a and elevation, correlation coefficients between yield and soil test data (e.g., soil‐ test pH, P, K, Mg, and Ca) were greatly improved over correlations calculated on a whole‐field basis. Conclusion: The credibility and future adoption of precision farming strategies will be conditional on producers' ability to measure, interpret, and predict soil and landscape properties that help explain their impact on grain crop production. Use of the boundary line analysis EC a helped to delineate the magnitude of potential yield loss due to less than ideal conditions in the root‐zone. Because EC a measurements were a good estimate of topsoil thickness, EC a may be used to diagnose potential rooting and water‐related problems affecting grain crop production. Four plots that illustrate bow crop grain yield may be related to EC a on claypan soils. image