The Role of Soil Organic Matter in Potassium Fixation Nathan Smith

Transcription

1 The Role of Soil Organic Matter in Potassium Fixation Nathan Smith Undergraduate Research Assistant in Agronomy Senior in Agronomy and Environmental Science Iowa State University

2 Soil Potassium (K + ) Pools Structural K (in primary minerals, feldspars, micas) applied K (as fertilizer) Exchangeable K (in clays, organic matter) Solution K (non exchangeable) Fixed K (in vermiculite) Plant/Crop K (in biomass, residues) Figure modified after ML Thompson 2014, Sparks and Huang 1985.

3 K Fixation Importance plant uptake Fixed K longer equilibration (weeks seasons) Exchangeable K instantaneous equilibration Solution K applied K Fixed K is not immediately available to plants. Fertilizer recommendations based on exchangeable (available) K levels, but a fraction of applied fertilizer is fixed before uptake by plants. McLean et al, 1979, 1982 found a range of 10 80% of applied fertilizer is fixed in 7 weeks

4 K Fixation Good or Bad? Well, that depends On one hand, K fixation might be viewed as a loss of fertilizer since plants cannot readily access fixed K On the other hand, one study (Schneider et al. 2013) suggests that fixation may be a favorable factor limiting K leaching. Checking/Savings analogy Exchangeable K is the checking account Fixed K is the savings account Exchangeable K Fixed K

5 Fixed K Edge Mechanism Hydrated Cations (Ca 2+, Mg 2+ ) Fixed K Lattice K weathered edge of mica (vermiculite) mica with structural K Figure modified after Nommik and Vahtras, 1982.

6 Potential to Fix K K fixation capacity One method: Vermiculite determination Coffman and Fanning, 1974 Estimate vermiculite to find fixation capacity % Procedure: 1. Determine total CEC using Ca saturation (CaEC) 2. K saturate and promote K fixation by 110 C oven drying 3. Determine KEC by neutral NH 4 OAc displacement 4. AA analysis of extract solutions for Ca and K

7 Photo credit: Tom Schultz ISU COBS (Comparison of Biofuel Systems) Plots of corn, prairie, and corn/soybean rotations to assess a variety of biofuel sources and environmental impacts. Plot ID ph Organic C Clay CEC (by CaEC) gc/kg soil % cmol/kg

8 Experiment: K Fixation Capacity First Treatment Remove organic matter before determination of CaEC and KEC Oxidize OM with H 2 O 2 Second Treatment Do not remove OM Determine CaEC and KEC on whole soil sample Hypothesis: Organic matter may block the cation exchange surface of the clay particles, reducing the overall CEC (and thus fixation capacity) of the soil. Therefore, removing organic matter should increase the K fixation capacity.

9 Results organic matter removed Plot ID CaEC KEC Fixation Capacity % Vermiculite cmol/kg cmol/kg cmol/kg g/100g soil AVG

10 Results organic matter included Plot ID CaEC KEC Fixation Capacity % Vermiculite cmol/kg cmol/kg cmol/kg g/100g soil AVG

11 Results familiarized It is helpful to visualize the fixation capacity in units of fertilizer application rather than charge per soil mass. The amounts of KCl and K 2 O fertilizer that can potentially be fixed are expressed in ton/ac, assuming an acre of soil with a mass of 2 million lbs/ac to a depth of 6in. Organic Matter Included Fixation Capacity g K/kg soil ton KCl / ac ton K 2 O / ac 3.7 cmol kg Organic Matter Removed Fixation Capacity g K/kg soil ton KCl / ac ton K 2 O / ac 1.3 cmol

12 Results comparison Organic Matter Included CaEC KEC Fixation Capacity % Vermiculite Organic Matter Removed CaEC KEC Fixation Capacity % Vermiculite On average, 60% of the K fixation capacity was lost upon removal of organic matter. In addition, for each sample, there was a larger decrease in the CaEC than the KEC after removal of organic matter.

13 Well What happened? A few things to consider The hypothesis was incorrect removal of OM did not increase fixation capacity it reduced it. However, total CEC did decrease, which is expected as OM is a contributor to soil CEC. But the presence of OM should not enhance K fixation, so why did the K fixation capacity decrease as well?

14 Decrease in total CEC The CEC of OM can be roughly estimated as a function of the soil ph by: ph The CEC lost by OM removal in the procedure is of the same magnitude as the predicted CEC of the OM as a function of soil ph. cmol/kg soil Actual Predicted y = x R² = y = x R² = kg OM/kg soil

15 Decrease in Fixation Capacity One possible explanation: Organic matter was removed by oxidation with H 2 O 2 In addition to oxidizing organic carbon, the H 2 O 2 treatment may have oxidized iron in the clay minerals from Fe 2+ to Fe 3+ reducing the negative charge of the clay fraction. Less negatively charged minerals would have a smaller CEC and diminished ability to fix K.

16 Conclusions Presence of organic matter is still believed to affect fixation of K in soils but in what way? Methods used in this study did not consider the effect of H 2 O 2 on structural iron in clay minerals BUT, iron reduction s role in K fixation is currently under investigation by others (Thompson and Stephen 2014), and results suggest iron reduction as a mechanism of increasing CEC and K fixation.

17 Acknowledgements My mentor, Dr. Michael L. Thompson Iowa State University Agronomy Department Individuals in the Soil Micromorphology Lab Dr. Teresita Chua Fritzie Rivas Suroso Rahutomo Llewin Froome Taslima Stephen Yili Meng Ryan Oltmans Funding Agronomy Professional Development Fellowship Agronomy Department, ISU Undergraduate Research Assistantship Office of Student Financial Aid, ISU

18 Citations Coffman CB and DS Fanning Vermiculite determination on whole soils by cation exchange capacity methods. Clays and Clay Minerals 22: McLean EO, TO Oloya, and JL Adams Soil tests to inventory the initially available levels and to assess the fates of added P and K as bases for improved fertilizer recommendations. Commun Soil Sci Plan Analysis 10(3): McLean EO, JL Adams, and RC Hartwig Improved corrective fertilizer recommendations based on a two step alternative usage of soil tests: II. Recovery of soil equilibrated K. Soil Sci Soc Am. J. 46: Nommik H and K Vahtras Retention and fixation of ammonium and ammonia in Soils. In: Nitrogen in Agricultural Soils. Am Soc Agron. pp Schneider A, R Tesileanu, R Charles, and S Sinaj Kinetics of soil potassium sorption desorption and fixation. Commun Soil Sci Plan Analysis 44(1 4): Sparks DL and PM Huang Physical chemistry of soil potassium. In: Munson, RD (ed.), Potassium in Agriculture. Soil Sci Soc Am, Madinson, USA. pp Thompson ML and T Stephen Exchangeable K why is it so darn complicated? In: Proceedings of the Soil Sci Soc Am Conference; 2014 Nov 2 5; Long Beach, CA.

19 Questions?