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Soybean Root Distribution Related to Claypan Soil Properties and Apparent Soil Electrical Conductivity

^a Dep. of Soil Environmental and Atmospheric Sciences, Agricultural Engineering Building, Univ. of Missouri, Columbia, MO 65211
^b USDA-ARS Cropping Systems and Water Quality Unit, Agricultural Engineering Building, Univ. of Missouri, Columbia, MO 65211
^c Division of Plant Sciences, Agriculture Building, Univ. of Missouri, Columbia, MO 65211
^d Dep. of Soil Environmental and Atmospheric Sciences, Anheuser-Busch Natural Resources Building, Univ. of Missouri, Columbia, MO 65211. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture or the University of Missouri

View larger version (55K): [in this window] [in a new window]   Figure 1. (a) Site 1 aerial photograph of existing research plots running east to west. Overlaid on the plot photograph are the root sampling locations, order-one soil survey, and depth to claypan (DTC) contour map. (b) Site 2 root sampling locations are overlaid on a DTC map. (c) Site 2 order-one soil survey with root sampling locations. * See Table 1 for soil map unit key with taxonomy.

View larger version (27K): [in this window] [in a new window]   Figure 2. (a) Backslope (B) and summit (S) landscape position effects on root length density (RLD) was different for some depths on Site 1. Dot-plots indicate the sample measurements and the shaded boxes represent the 95% confidence interval of the mean. Significant differences between B and S within each soil layer are indicated with an asterisk. (b) Fitted profile models (fifth order polynomials, B r2 = 0.52, S r2 = 0.56; P < 0.001) of RLD show an initial minimum in root density and a secondary maximum and highlight the differences between summit and backslope RLD profile distributions. An exponentially decaying root distribution for ideal soils is plotted for reference (Kiniry et al., 1983).

View larger version (33K): [in this window] [in a new window]   Figure 3. Site 2 root length density (RLD) and average root diameter (ARD) as a function of depth to claypan (DTC) and Veris Model 3100 deep apparent soil electrical conductivity (ECa-dp) by sampled layer. Regression statistics and regression fit included for models with a significant linear or quadratic term (P < 0.05)

View larger version (16K): [in this window] [in a new window]   Figure 4. Clay-maximum translated depth (Dt) profiles of (a) root length density (RLD) for Sites 1 and 2, and (b) average root diameter (ARD) for Site 2. Exploratory local regression models ("loess" function; Cleveland et al., 1992) are plotted to emphasize soil morphology influence. No surface samples are included for either panel. Dt coherently aligns samples taken at any depth from across a wide claypan landscape by using the profile clay maximum as a common measurement origin.

View larger version (41K): [in this window] [in a new window]   Figure 5. Clay-maximum translated depth (Dt) profiles of several key physical and chemical soil properties in claypan soils. These profiles indicate the systematic variation of soil properties relative to the claypan morphology. Exploratory local regression models ("loess" function; Cleveland et al., 1992) are plotted to emphasize soil morphology influence. See Fig. 4 for an explanation of Dt.

View larger version (30K): [in this window] [in a new window]   Figure 6. Clay-maximum translated depth (Dt) profiles of relative soybean root length density (RLD), average root diameter (ARD), and clay concentration, compared to relative values of (a) silt content, cation exchange capacity (CEC), and base saturation (B-Sat) of CEC, and (b) buffer pH (pHs), neutralizeable acidity (NA), Al3+ saturation of CEC (Al-Sat), and P. Soil properties in panel a reflect the influence of soil texture on cation supply and buffering. Soil properties in panel b generally relate to soil acidity and its influence on the distribution of available P. See Fig. 4 for an explanation of Dt.