How Does Redox Status Influence Exchangeable Potassium In Soil?

How Does Redox Status Influence Exchangeable Potassium In Soil? Michael L. Thompson Taslima Stephen Iowa State University 1

Why do we care about exchangeable K? K is required by all plants. A soil may have a lot of K, but only a fraction is available. We d like to predict the effectiveness of applying fertilizer K in raising the stock of available K in a soil. We d like to predict when K will become available during the growing season. 2

1939 AGRONOMY JOURNAL 31:940-953 3

1968 In Kilmer et al. (ed.) The Role of Potassium in Agriculture ASA, CSSA, SSSA, Madison, WI p. 79-108 4

Fools rush in where angels fear to tread. English poet Alexander Pope 1709 5

Sparks, D. L., and P.M. Huang. 1985. Physical chemistry of soil potassium. p. 201-276. In Munson, R. D (ed.). Potassium in agriculture. SSSA, Madison, WI. 6

Sparks, D. L., and P.M. Huang. 1985. Physical chemistry of soil potassium. p. 201-276. In Munson, R. D (ed.). Potassium in agriculture. SSSA, Madison, WI. 7

Slowly exchangeable / available with exceptions Sparks, D. L., and P.M. Huang. 1985. Physical chemistry of soil potassium. p. 201-276. In Munson, R. D (ed.). Potassium in agriculture. SSSA, Madison, WI. 8

Clay micas are more than just mica. (a) Randomly interstratified mica-smectite in the fine clay (<0.2 μm) dispersed from a fine-silty, mixed, mesic Typic Argiudoll in Iowa. (b) Clay mica particle dispersed from a fine, smectitic, Quaternary paleosol in Iowa. Note frayed, vermiculitic edges where monovalent cations such as K or ammonium might be fixed. 10

Microstructure in an undisturbed Mollisol Organic matter Clay quasicrystals Charcoal fragments Bacteria 2 µm 11

K fixation has been studied for decades Vermiculite fixes K Sites in wedges and at frayed edges of micas are selective for K Hydroxy-interlayered vermiculite fixes K 12

Ammonium blocks K fixation The relation of potassium fixation to ammonium fixation. George Stanford and W. H. Pierre. 1946. Soil Sci. Soc. Proc. 11:155-160. 13

Extraction of interlayer NH 4+ is inhibited by high concentrations of K in the extractant. Replaceability of Ammonium Fixed in Clay Minerals as Influenced by Ammonium or Potassium in the Extracting Solution. J. J. Hanway, A. D. Scott and G. Stanford. 1957. Soil Sci. Soc. Am. Proc. 21:29-34. 14

Biotite weathering Darrell Schulze, Purdue University 15

J. Stucki and colleagues: Reduction of Fe in smectite crystals increases layer charge Effects of Reduction and Reoxidation of Structural Iron on the Surface Charge and Dissolution of Dioctahedral Smectites Joseph W. Stucki, D. C. Golden, and Charles B. Roth. Clays and Clay Minerals 32:350-356, 1984. Microbial reduction of structural iron(iii) in smectites. Stucki JW, Komadel P, Wilkinson HT. Soil Sci Soc Am. J. 51:1663-1665, 1987 Swelling and Texture of Iron-Bearing Smectites Reduced by Bacteria. Will P. Gates, Anne-Marie Jaunet, Daniel Tessier, Michael A. Cole, Henry T. Wilkinson and Joseph W. Stucki. 1998. Clays and Clay Minerals 46:487-497, 1998. 16

Dithionite reduction of Fe in two smectites led to increased K fixation (without drying the sample) Iron Oxidation State Effects on Cation Fixation in Smectites Eid M. Khaled and Joseph W. Stucki Soil Sci. Soc. Am. J. 55:550-554 (1991) 17

But it is hard to verify that reduction of Fe is an important factor in soil K fixation. 18

What regulates Fe reduction in layer silicates? Redox potential must be low enough Temperature must be high enough Appropriate bacteria must be present Particle size, basal surface area, and edge area must allow access to electron-rich organics (or other electron shuttles) 19

Mitigating Factors in Soil K Fixation by Redox Processes The soil may not be saturated long enough at a high enough temperature for the redox potential to get low enough for Fe 3+ Fe 2+. As redox potential goes down, ph tends to rise, increasing the CEC and the proportion of nonselective sites for K retention. Abundant fertilizer ammonium under low redox conditions may promote release of K by exchange or it may (at least temporarily) increase fixation of K by shutting down silicate layers. 20

Mitigating Factors in Soil K Fixation by Redox Processes Organic matter associated with soil clay may block access to the selective sites. Organic matter may serve as a source of ammonium via dissimulatory nitrate reduction. Abundant Fe oxides may buffer the Eh so that it does not get low enough for Fe 3+ Fe 2+. When Eh does get low enough, Fe oxides are dissolved, opening up non-selective exchange sites and increasing the CEC. 21

Fools rush in where angels fear to tread. English poet Alexander Pope 1709 22

Our study: K fertility in biomass cropping systems in central Iowa Webster (a fine-loamy, mixed, mesic Typic Argiaquoll) Soil Properties Sand Silt Clay ph OC NH 4 OAc K CBD Fe AmOx Fe -------g kg -1 ------ g kg -1 mg kg -1 mg kg -1 27 39 33 7.0 33 150 1215 948 23

Clay mineral suite Mg - glycerol Dominated by smectite, with vermiculite, clay mica, and kaolinite How much K-fixing capacity is present in the soil? Ca EC / K EC Method (Coffman and Fanning (1974) (following ML Jackson)) K fixation is the difference between the exchange capacity determined with a Ca-saturated sample and that determined with a K saturated sample that was heated to 110 deg C overnight. Organic matter removed using H 2 O 2. Index of vermiculite: Convert to vermiculite percentage assuming 154 cmol(-) / kg of vermiculite cmol kg -1 Ca EC 31 K EC 19 Fixed K 12 Vermiculite 7% (whole soil basis) 24

How much K-fixing capacity is present in the soil? How much new CEC might be created by oxidation of all Fe in the smectitic portion of the clay fraction? Assume that clay fraction is 40% high-charge smectite with the chemical formula (Laird et al., 1991) Ca 0.2 (Si 3.8 Al 0.2 )(Al 1.2 Fe 0.6 Mg 0.2 ) O 10 (OH) 2 [formula mass = 385 g] 25

Will reducing conditions increase the K buffering capacity of the soil? Wang et al. 2004 Soil Sci. Soc. Am. J. 68:654 661 K sorption isotherms 2.5 g soil + 25 ml solution 0.0, 0.5, 1.0, 1.5, 2.0, 3.5, 5.0 mm KNO 3 Shake, then equilibrate 18 h Determine K, Ca, Mg in solution Determine NH 4 OAc exchangeable K at end Plot change in Exch K and Non Exch K vs. conc. ratio Treatments Aerobic, no amendment, no incubation Anaerobic, no amendment, incubation for 10 days Anaerobic, glucose amendment, incubation for 10 days 26

Potassium Exchange Isotherms POTASSIUM BUFFERING CAPACITY NON- EXCHANGEABLE EXCHANGEABLE TOTAL [ΔEX-K] [(K )/(Ca + Mg)1/2] [ΔNEX-K] [(K )/(Ca + Mg)1/2] [ΔK] [(K )/(Ca + Mg)1/2] Non Exch fraction of total PBC Aerobic 23 10 33 0.30 Anaerobic, no glucose 34 20 54 0.37 Anaerobic, glucose 41 25 66 0.38 27

Conclusion Yes, reducing conditions increased the capacity of the soil to buffer added K at both exchangeable and non-exchangeable sites of the soil. We speculate that the increase in nonexchangeable K was due to changes in the oxidation state of Fe and subsequent K fixation in interlayer sites. We speculate that the increase in exchangeable K was due to increases in CEC as Fe oxides dissolved and opened up more clay mineral surfaces. 28

Next Steps Conduct the same experiment with the clay fraction only Conduct the same experiment on the silt fraction Measure redox potential and ph at the end of the equilibration period Measure soluble and fixed NH 4 + at the end of equilibration Vary the length of the equilibration period Pretreat with NH 4 NO 3 or urea before adding K Compare soils from cropping systems plots (annual row crops vs. perennial biomass crops) with respect to N and K dynamics 29

Acknowledgments Individuals David Laird Randy Southard Ali Tabatabai Funding Agronomy Department, ISU College of Agriculture and Life Sciences, ISU Graduate College, ISU Teresita Chua Renuka Mathur Suroso Rahutomo Yili Meng 30

Neoformation of clay mica Illitic dimers Permanent K fixation 32

T. Scott Murrell Why are Soil Test Potassium Levels so Variable over Time in the Corn Belt? 35

Potassium Fixation and Its Significance for California Crop Production Stuart Pettygrove, Toby O Geen, and Randal Southard Better Crops/Vol. 95 (2011, No. 4) 37