71st SWCS International Annual Conference July 24-27, 2016 Louisville, KY

1. ACTIVE CARBON: EVALUATING
METHODOLOGY OF A
LABORATORY METHOD
SKYE WILLS
MICHEAL ROBOTHAM
CANDISS WILLIAMS
RICH FERGUSON
STEVE MONTEITH

2. ACTIVE CARBON
• Carbon that is oxidized in a dilute permanganate (KMnO4) solution
• Thought to be related to labile or ‘active’ forms of carbon
• Shown to be more sensitive to management than other measures of soil organic carbon
• Referred to more specifically as POX-C (Culman et al., 2012)

3. ACTIVE C IN SOIL SURVEY
• A study was conducted to determine the comparability between laboratory
and field methods (Stiles et al.,2011)
• NRCS-Soil Science Division has a program to measure dynamic soil properties
• Measure soil properties under multiple land use and/or management systems
• Kellogg Soil Survey Laboratory has been working to find consistent
repeatable methods that work for ALL soils
• Irregularities and inconsistencies have been noted by technicians

4. BASIC PROCEDURE
• Kellogg Soil Survey Laboratory
• Air Dry and Grind Soil to <2mm
• Weigh out sample
• Originally 5 g
• Now 2.5 g is recommended (Culman et al., 2012)
• Sample weight is adjusted when invalid results are obtained
• Standard Amount of KMnO4 is added
• A bright magenta color
• Solution and Soil react for 10 min
• An aliquot of solution is removed
• Centrifuge to separate solution from soil solids
• Solution color is assessed with a spectrophotometer (550nm)
• Bleaching is proportional to the amount of active carbon in the sample
• Less color = more active carbon
• A calculation is used to determine the amount of POX-C

6. QUESTIONS
• Is changing the weight of soil used an appropriate way to modify the
procedure?
• Can/should we fine-grind samples (<80um) to get a more representative
sample (especially with high particulate organic matter)
<2mm grind Fine grind (<80um)

7. SOIL WEIGHT
Not Distinguishable
from Blank
Below Detection
Limits

8. KMnO4 C (mg kg−1) = (0.02 mol L−1 − A) x (9000 mg C mol−1) x (0.02 L
solution /sample wt.) x AD/OD

9. GRIND SIZE
Active Carbon Standard Deviation

10. STATISTICAL ANALYSIS
• Analysis of Variance Table
•
• Response: ac
• Df Sum Sq Mean Sq F value Pr(>F)
Grind 1 15815809 15815809 15.066 0.0001198 ***
wt_class 4 99874453 24968613 23.784 < 2.2e-16 ***
Grind:wt_class 4 3636652 909163 0.866 0.4842380
Residuals 438 459814361 1049804
Statistically Significant
Grind Size YES
Weight Class YES
Interaction
(Grind x Weight Class) NO

11. OVERVIEW
Weight
Grind 0.25 0.5 1.0 2.5 5.0
Fine Grind 2171 1703 1329 933 617
2mm Grind 1629 1137 899 684 521

12. PRACTICAL IMPLICATIONS
• This technique does not appear to be a reliable laboratory method.
• Some still find it informative for less formal uses (demonstrations, assessments)
• Only one weight and grind should be used in each project
• Currently, 2.5g is recommended
• If an alternate prep is required – use it for the entire project
How can you make this
determination before you start the
analysis?
Sample
Weight
Percent Valid
Results
0.25 38%
0.50 58%
1.00 79%
2.50 84%
5.00 59%
Estimate Organic
Carbon Content
(generally)

13. PRACTICAL GUIDANCE
• color saturation and darkness (low value) are
indicative of high active carbon
• Background color of red, yellow or grey is not
important
• many small aggregate in the 2mm ground
sample
• many fine plant and root segments
Things that indicate high Active Carbon

14. WHAT WEIGHT SHOULD YOU USE?
Valid results for sample sizes < 2.5g

15. WHAT WEIGHT SHOULD YOU USE?
Valid results for sample sizes > 0.5g

16. All had valid results for sample sizes at 2.5g

17. PROJECTS THAT COMPARE VERY DIFFERENT
SYSTEMS
Pasture - Relatively low levels Native Longleaf Pine – High levels
Tifton Soil Series (Georgia) Surface Samples
Both of these samples have valid results at 1g

18. SUMMARY
• As soil health indicators become more quantitative there is a desire to develop rigorous
laboratory and field based tests.
• Active Carbon (POX-C) analysis is dependent upon sample amount and preparation
• Measured values increase as the sample amount and grind size decrease
• Further work is needed to understand what this method is measuring
• Values are only appropriate for comparing similar soils
• When used for field assessments – one standard weight should be used
• This method will not be relevant for some soils and situations
• NSSC is investigating the interpretation of active carbon comparisons across soil orders and
land-use systems

19. QUESTIONS?