*3.1. Fe and P: Concentrations, Soil Order, and Vegetation*

## 3.1.1. P

The average total P concentration (referred to simply as "P" here onwards) in the continental crust is ca. 870 ppm P, and there is relatively little variation between bedrock lithologies [77]. The maximum soil P concentration in this compilation was >20,000 ppm (>2 wt%; C horizon of an anthropogenically-modified soil), an order of magnitude greater than the crustal average. Mean P concentrations for each horizon studied were depleted relative to the crustal average: Top 5 cm, A, and C horizons means were 660, 632, and 508 ppm P, respectively (USGS data), and B horizons from the new dataset had a mean of 937 ppm P. P concentrations vary spatially throughout continental U.S. (Supplemental Figure S2), with hotspots in a variety of different geologic, climatic, and biologic provinces. P concentrations varied among soil orders (Figure 1), with the highest concentrations in Inceptisols (Figure 1A), a relatively weakly-developed soil type.

**Figure 1.** P and Fe concentrations in modern soils' B horizons, binned by soil order. Red lines are median values, blue bins are 25th and 75th quartile, and red crosses are beyond the 75th quartile; the horizontal dashed gray line is the crustal average (870 ppm P and 3.5% Fe) [77]. (**A**) P concentrations, showing that younger soils (Entisols, Inceptisols) tend to have higher P than older soils (Alfisols, Ultisols). Oxisols have high ranges of P, but a much smaller bin size than other orders (n = 8). Andisols have high inherited P from volcanic parent materials. (**B**) Fe concentrations, showing consistent Fe concentrations between ca. 1 and 10 wt% with the exception of Oxisols, which are defined by their high rates of Fe oxide accumulation. Bin sizes: Entisols (44), Inceptisols (75), Alfisols (42), Ultisols (6), Oxisols (8), Aridisols (76), Mollisols (60), Andisols (15), Spodosols (9), Gelisols (30), Unknown (39).

The vegetation groups are Barren (lacking vegetation), Altered (i.e., affected by human activity), Forest (no differentiation between deciduous and coniferous), Herbaceous/Grassland, Developed/Cultivated (i.e., occupied or manicured), Shrubland, and Other (mostly including early-succession plants or microbial earths). For USGS samples, protocols dictated that roads, buildings, and industrial sites be avoided [64]. Among vegetation types, there is little variability between P concentrations and vegetation types (Figure 2); while Barren landscapes' B horizons show higher mean P, this is likely due to small bin size (n = 5; Figure 2C). Overall, variability in concentrations in different vegetation types and soil orders was minimal, with most soils being depleted in P relative to the crustal average. Vegetated soils (both natural and anthropogenically-altered) had higher ranges

of P than unvegetated (Barren). There was a weak positive correlation with mean annual precipitation (Supplemental Figure S3A), but the range of P values at a given precipitation amount is typically too large (~1500 ppm) to be of predictive use. This trend could also be explained by related factors, such as vegetation coverage (cover versus bare ground) and weathering intensity, which are associated with precipitation.

**Figure 2.** P concentrations in all horizons, binned by vegetation type. Dashed gray line in all is the crustal average P (870 ppm) [77]. Arrows indicate off-plot values. (**A**) P in the Top 5 cm, where Barren (no vegetation) and Altered soils have the lowest range of P concentrations. (**B**) P in A horizons, with similar distributions to Top 5 cm. (**C**) P in B horizons, with much smaller bin sizes than other horizons. (**D**) P in C horizons. Bin sizes for (**A**,**B**,**D**): Barren (68), Altered (209), Forest (1252), Herbaceous (815), Developed (1568), Shrubland (945). Bin sizes for (**C**): Forest (43), Grassland/Herbaceous (19), Shrubland (50), Barren (5), Cultivated (59), Unknown (74).

3.1.2. Fe

The crustal composition of total Fe (Fetot) is more variable than that of P because while P is sourced primarily by apatite-group minerals, Fetot can be present in a wide range of minerals and lithologies. The Phanerozoic upper continental crust is estimated to have 3.5 wt% Fe [77]. Fetot averages in the soils studied here were 2.1 wt% for Top 5 cm, 1.6 wt% for A horizon, 2.6 wt% for C horizon, and 4 wt% for our dataset (primarily B horizons). Density-normalized Fetot varied slightly by soil order

(Supplemental Table S3), showing the expected trend of modest loss shifting to modest accumulation during the Alfisol-Ultisol transition (Figure 1B), but otherwise was relatively consistent. Oxisols had the highest values and range in Fe, with some variability among the other soil orders but generally within consistent ranges (Figure 1B).

Fetot is generally consistent both within a given horizon and between different vegetation cover types (Figure 3). While most soil orders and most horizons had <3 wt% Fetot, values ranged up to >15% (Figure 3; Supplemental Figure S2). The only notable variability in Fe concentrations among vegetation is in Cultivated soil B horizons (Figure 4C), which are depleted, and 'Other' soil B horizons that are substantially enriched relative to the other vegetation cover groups. 'Other' vegetation contains mostly basalt-parented soils from a limited geographical area (primarily Iceland), so this result is likely an artifact of our sampling. There was no strong correlation between Fetot in a soil and mean annual precipitation (R2: 0.2; Supplemental Figure S3B).

**Figure 3.** Fe concentrations in all horizons, binned by vegetation type. Dashed gray line in all is the crustal average (3.5%) [77]. Arrows indicate off-plot values. (**A**) Fe in the Top 5 cm, where Barren and Altered soils have the lowest maximum Fe concentrations. (**B**) Fe in A horizons, with similar trends to Top 5 cm. (**C**) Fe in B horizons. There were no data for Barren B horizons' Fe. (**D**) Fe in C horizons. Bin sizes for (**A**,**B**,**D**): Barren (68), Altered (209), Forest (1252), Herbaceous (815), Developed (1568), Shrubland (945). Bin sizes for (**C**): Forest (43), Grassland/Herbaceous (19), Shrubland (50), Barren (0), Cultivated (59), Unknown (74).

**Figure 4.** (**A**) Latitudinal distribution of soils used in this study, where smaller bins represent the USGS dataset [64] and the wider bins are our soils. The range of latitudes covered here is 7◦ to 73◦ N, but most of the samples fall between 20 and 50◦ N. (**B**) Latitudinal trends in Chemical Index of Alteration (CIA). (**C**) Latitudinal trends in clay content. (**D**) Correlation between CIA and clay content (R2: 0.05, <0.01, and 0.19 for A, B, and C horizons, respectively). Overall, these results support common assumptions about the relationships between latitude and weathering.

#### *3.2. Fe and P: Latitude, Weathering, Soil Order, and Clay Content*

Latitude, weathering intensity, and clay content should each be associated with each other as well as with Fe (e.g., [24]). Some of the trends described here could be influenced by a latitudinal sampling bias, with most of the samples coming from the mid-latitudes (Supplemental Figure S1; Figure 4A). In the soils analyzed here, maximum weathering (as measured by CIA) decreases as latitude increases (Figure 4B), while clay content (a weathering product) does not show a strong trend with latitude (Figure 4C), with the exception of a weak mid-latitude (35–40◦ N) peak in clay content in some C horizons. Weathering trends between soil orders behave as expected, with CIA values increasing from Entisols to Ultisols/Oxisols (Figure 5). Clay content and weathering show a well-behaved and expected pattern of increase that matches the theoretical understanding of that metric [24,75,78,79], with B horizons specifically showing the highest clay content until the highest CIA values are reached (Figure 4D, green points). Clay content and geochemical data were not available for the Top 5 cm subset of the USGS dataset, so that horizon is excluded from those comparisons.

**Figure 5.** Chemical Index of Alteration (CIA) binned by soil order. CIA increases through the Entisol-Ultisol soil development progression, as expected, and Oxisols have the highest CIA with a median near 100. Aridisols' median CIA falls between Entisols and Inceptisols. Bin sizes: Entisols (44), Inceptisols (75), Alfisols (42), Ultisols (6), Oxisols (8), Aridisols (76), Mollisols (60), Andisols (15), Spodosols (9), Gelisols (30), Unknown (39).

Fetot content increases moderately with higher latitudes (Figure 6A), higher weathering (Figure 6C), and clay content (Figure 6E). P in soils behaves less linearly with respect to these variables: P increases moderately with latitude (Figure 6B), but rather than decreasing with weathering, as might be expected based on soil age-P relationships, average P concentrations in all horizons vary little, though there is a general increase in range at moderate weathering intensities (CIA~40–70) (Figure 6D). P concentrations are weakly negatively correlated with clay content (Figure 6F). Fetot and P showed positive correlations with one another in all horizons, as expected (Supplemental Figure S4).

#### *3.3. Weathering, Clay Content, and Vegetation*

Between vegetation types, there is low variability in clay content, with Forests showing slightly lower values in B horizons than the other groups (Supplemental Figure S5). Forests have the highest weathering in natural (unaltered/not developed) soils, followed by Grasslands and Shrublands (Figure 7).

**Figure 6.** Trends between latitude, weathering, and clay content and total Fe and P concentrations. Solid lines in (**C**,**D**) are running averages for Top 5 (yellow), A (red), B (blue), and C (black) horizons. Solid lines in (**E**,**F**) are linear least-squares regressions for A (red), B (blue) and C (black) horizons. (**A**) Fe in soils by latitude. (**B**) P in soils by latitude. (**C**) Fetot and weathering intensity. Solid lines are the running averages (10-CIA-unit window) for each horizon; colors correspond to data colors. The B horizon average (blue line) is highest at very high CIAs due in part to lower sample density at very high CIA values, especially between 98–100. (**D**) P concentrations and weathering intensity. (**E**) Fe and clay content (R<sup>2</sup> is 0.34 and 0.37 for A and C horizons; 0.01 for B horizons, much smaller dataset; *p* = 0.7 for B horizon, <<0.01 for other horizons). Solid lines are linear least-squares regressions; the B horizon (blue line) is skewed due to high-Fe, low-clay points off plot. (**F**) P and clay content. Solid lines are linear least-squares regressions (R2 is 0.11, 0.08, and 0.01 for A, B, and C horizons, respectively; *p* << 0.01 for all horizons).

**Figure 7.** Chemical Index of Alteration (CIA) binned by vegetation type, for horizons (**A**–**C**) (bulk chemistry data for CIA were not available for the Top 5 cm horizon). (**A**) Altered soils have the highest CIA values overall, and Forest soils have the highest CIA for non-anthropogenically-affected soils, in A horizons. (**B**) Forests have the highest CIA values in B horizons, followed by Grasslands and Shrublands (Barren soils' small bin size, n = 5, precludes it from analysis here). (**C**) Altered soils also have the highest CIA in C horizons, followed by Forests.
