**6. Observation of rare earth element soil distributions in missouri**

#### **6.1 Study area**

Summers are hot and humid with a mean July temperature of 26°C and winter temperatures are mild with a mean January temperature of 2°C. The mean annual precipitation of 1.19 m is seasonally distributed, with greater rainfall in spring. Vegetation is either a mixed hardwood forest or land that has been cleared of vegetation, land-graded, and employed in row-crop agriculture.

#### **6.2 Soils**

The alred series (loamy-skeletal over clayey, siliceous, semiactive, mesic Typic Paleudalfs) consists of very deep, well-drained soils formed in cherty hillslope sediments and the underlying clayey residuum. The Alred has an A-E-Bt-2Bt horizon sequence. The rueter series (loamy-skeletal, siliceous, active, mesic Typic Paleudalfs) consists of very deep, somewhat excessively drained soils formed in colluvium and residuum from cherty limestone. The Rueter has an A-E-Bt-2Bt horizon sequence. The Menfro series (Fine-silty, mixed, superactive, and mesic Typic Hapludalfs) consists of very deep, well-drained soils formed in thick loess deposits on uplands. The Menfro has an A-E-BE-Bt-C horizon sequence. The Kaintuck soil series in Missouri (coarseloamy, siliceous, superactive, nonacid, mesic Typic Udifluvents) are very deep and well-drained floodplain soils formed from loamy alluvium and have an Ap-C horizon sequence.

### **6.3 Protocols**

An aqua regia digestion was employed to obtain a near-total estimation of elemental abundance associated with all but the most recalcitrant soil chemical environments. Aqua regia does not appreciably degrade quartz, albite, orthoclase, anatase, barite, monazite, sphene, chromite, ilmenite, rutile, and cassiterite; however, anorthite and phyllosilicates are partially digested. Homogenized samples (0.75 g) were equilibrated with 0.01 liter of aqua regia (3 mole nitric acid: 1 mole hydrochloric acid) in a 35°C incubator for 24 hours. Samples were shaken, centrifuged, and filtered (0.45 μm), with a known aliquot volume analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Aide and Fasnacht [43] reviewed the application of the aqua regia digestion protocol.
