Soils of the Mall in Washington, DC: I. Statistical Summary of Properties

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Abstract
Twenty profiles were examined in each of five transects along the length of the Mall in Washington, DC. The soils were formed in mixed, man‐deposited materials with lithologic discontinuities present in 95% of the profiles. Ninety‐nine percent of the profiles were within fine‐loamy and coarse‐loamy taxonomic families. B horizons were identified in 26% of the profiles based upon colors redder or stronger than those of underlying materials and/or the presence of soil structure, most commonly subangular blocky. Inorganic, manufactured artifacts were present in the particle size control section of 94% of the profiles. The loamy textures were conducive to compaction to high bulk densities of up to 2.03 Mg m−3. Mean bulk density values of 1.61 Mg m−3 for the surface horizon and 1.74 Mg m−3 at 0.3 m were found. The surface horizon had a mean particle density of 2.54 Mg m−3, and the mean particle density at 0.3 m was 2.59 Mg m−3. Pore space was limited, with a mean of 37% at the surface and 33% at 0.3 m. Pore space ranged from 21% to 50%. Data were statistically analyzed according to the position of the horizon from the soil surface. Mean pH of all horizons was 6.2 or higher, and mean pH of the surface horizon was 6.4; however, the pH of 27% of all samples was >7.0. Electrical conductivity was −1 in 80% of all surface horizons. Mean organic matter content was 19.7 g kg−1 at the surface, and tended to decrease with depth, although variation in this trend was observed. Mean cation exchange capacity was 11.2 cmol (NH+4) kg−1 in the surface horizon and tended to decrease with depth. Mean base saturation exceeded 88% in all horizons, with Ca as the dominant cation. Elevated levels of heavy metals were observed, with a mean Pb content of 184 mg kg−1 in the surface horizon. Enrichment of other metals was not as pronounced. Statistically fewer samples were required to estimate the mean for physical properties than for chemical properties. Only one sample was needed to estimate the mean particle density within 10% with a 95% confidence interval, whereas over 7000 samples were required to estimate the mean Ni content of the third horizon with a similar confidence interval.

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