Water Soluble Metals and Soils: Background Information

Wayne P. Robarge

Professor (Emeritus) Soil Physical Chemistry

Department of Crop and Soil Sciences

NC State University, Raleigh, NC Presented at:

Fertilizer & Biostimulant Testing & Analysis

AAPFCO Methods Forum XVIII, Albuquerque, NM Feb. 13 – 15, 2019

Outline • Really brief overview of pertinent (?) soil chemistry

• Chemistry in a porous media (soil)

• Potentially important things we won’t review

• Nailing a foot down – background levels of metals in soil (e.g. Conterminous United States)

• Total concentrations of select trace metals

• Close up example from North Carolina

• Looking at metal associations versus current theory

• Example soil test data from North Carolina

• Trying to assess “soluble/bioavailable” nutrients in soil

• Recommendation(s) on an extraction method

• Do metal soil fractionation schemes provide insight

Chemistry is Chemistry (Soils – chemistry in a porous media)

Ions

in

Soil

Solution

Fertilizer Granule

Clay Minerals

Fe, Al, Mn

Oxides and

Hydrousoxides

Soil Organic

Matter

Plant

Roots

Soil Flora

and Fauna

Leaching

Intermediate

Reaction Products

Why Soil Matters (You cannot fight thermodynamics)

SOM

Complexes

Inner/

Outer

Sphere

Complexes

Solid

Solutions

Co-Precipitates

3-D

Solid

Matrix

Increasing Binding Energy

Do We Know What Controls Metals In Soils?

• pH – definite response (more acid/more soluble) • “adsorption edge” – rapid uptake by soil solids across a unit pH

• Soil organic matter functional groups

• Precipitates (carbonates, phosphates)

• Surface complexation on metal oxides and clay edges • pH dependent, cation & anion interactions possible

• Classic ion exchange? (cation exchange capacity)

• Redox reactions (mobilization/fixation/volatilization)

• Root rhizosphere/bacterial and fungal associations • Release of specific metal dissolution/complexing agents

• Computer chemical speciation models • Usually based on equilibrium thermodynamics

Nailing a Foot Down (Providing some perspective?)

• What is current level of trace metals in soils? • What is normal background level of metals in soils of NC?

• US Geologic Survey (USGS) 2013 Geochemical and Mineralogical Data for Soils of the Conterminous United States Data Series 801 • Low density (1 site per 1600 square kilometers; 4857 sites)

• Three samplings: top 5 cm, A horizon, C horizon

• Six years to complete analysis (2007 – 2013)

• Combination ICP-AES, ICP-MS, Cold-vapor AAS

• Ag to Zn; most elements HNO3/H2O2; also fusion techniques

• Some similar surveys for other countries • Other surveys may not be as comprehensive

Correlations Between Elements USGS 0-5 cm dataset

Vanadium vs Al or Fe; Pearsons p <0.001

Correlations Between Elements USGS 0-5 cm dataset

Copper vs Al or Fe; Pearsons p <0.001

Correlations Between Elements USGS 0-5 cm dataset

Zinc vs Al or Fe; Pearsons p <0.001

Summary Table: 0 – 5 cm depth (Source: USGS 2013 Data Series 801)

Element Count Mean Std. Dev. Minimum Maximum

Units: %

Aluminum 4841 4.59 2.12 0.02 15.3

Iron 4841 2.14 1.39 0.01 13.3

Units: mg/kg

Arsenic 4785 6.5 16.7 0.6 830

Chromium 4841 36.4 89.0 1.0 4120

Copper 4841 17.9 22.1 0.5 996

Lanthanum 4841 26.1 13.0 0.5 239

Molybdenum 4841 1.04 2.10 0.05 75.7

Nickel 4841 17.8 45.2 0.5 1890

Zinc 4841 65.7 176 1.0 11700

Translation to Total Mass/Acre (Assume Bulk Density = 1.35 gcm-3)

(Depth = 20.5 cm or 8 inches)

Element Mean Conc. Mass per Acre

- mg/kg - - lbs/acre -

Arsenic 6.50 16

Chromium 36.4 89

Copper 17.9 44

Lanthanum 26.1 64

Molybdenum 1.04 2.6

Nickel 17.8 44

Zinc 65.7 161

What Controls Cd Bioavailability in Tobacco? (Known Accumulator/NC Soils “Low” in Cadmium - Cd)

(2013 Mass Balance Study At Single Field Near Bath, NC)

Example of Crop Removal/Recycling (2013; Flue-cured tobacco; Bath, NC)

(Harvest: 6,000 plants per acre)

Element

Uptake per

Plant

Total

Crop Removed Recycled

- micrograms - - lbs/acre - - % -

Arsenic 83 0.001 34 66

Copper 5718 0.08 44 56

Nickel 737 0.01 20 80

Zinc 19079 0.25 57 43

- milligrams - - lbs/acre -

Calcium 5408 74 74 26

Note: ~ 53 lbs soil per acre removed at harvest based on Al data

Examples of Plant Uptake Throughout Season (Copper - Cu & Zinc – Zn; Nickel – Ni & Selenium - Se)

Measuring Soluble/Bioavailable Metals in Soil (Application of Soil Test Protocols. E.g. NC)

• Mehlich III Soil Extraction Solution • Concoction of nitric acid, neutral salts, EDTA, HF acid

• NOT designed to extract equilibrium values

• Soil contact time 5.0 minutes (stirred), then filter • Ratio – 2.5 mL of soil to 35 mL Mehlich III

• Results are correlated with yields in field trials and greenhouse studies

• Results commonly reported as index (0 – 100+)

• Adopted by number of states – historical database

• NC Dept. of Agriculture, Agronomic Division • Processes over 300,000+ samples per year

Recommendation on Extraction Method

Soil Fractionation Schemes (Attempt to assign metals across fractions in soil.)

Example Metals Fractionation Scheme (Roberts et al. 2003. In Geochemical and Hydrological Reactivity of Heavy Metals in Soils. Lewis Publishers)

• 1M NH4NO3 – exchangeable/soluble ions

• 1M NH4OAc(pH 6) – weakly complexed

• 0.1M NH3OHCL + NH4OAc – Mn hydroxide metals

• 0.025M NH4-EDTA – metals bound to organic matter

• 0.2 M NH4Oxalate – metals bound to amorphous Fe

• 0.1M Ascorbic acid+Oxalate – metals bound by crystalline Fe

• Conc. HNO3/HCl/HF – dissolve residue

• PROBLEM – fertilizers are NOT soils.

Can/should we try to assess availability?

• Solubility is important (coded into law; e.g. liming products) • Solubility vs. potential reactivity in soil environment?

• Plant is best indicator of bio-availability • Variety of techniques – each with their limitations

• Resurgence in Rhizosphere Research • Better more affordable analytical instrumentation • Genetic profiles now combined with “classic”

investigations • Please don’t forget chemical thermodynamics (computer

models)

• We need to update our current soil test knowledge base! • Genetic changes in crop varieties/higher yield potentials • Changes in fertilizers, amendments and “other additives”