Laboratory Procedures for Soil Organic Matter Analyses
University of Wyoming Soil Resource Lab
Jay Norton
May 8, 2008
QA/QC
Quality assurance: include three no-soil blanks with each run and one sample
duplicate for each 15 samples (treat blanks as soil samples through all
extractions, fumigations and incubations)
Quality control: there is much room for error and in some ways these procedures
can be imprecise; therefore all must be carried out as systematically as possible
with all samples being treated exactly the same as all others. Examples:

field samples placed on ice as soon after disturbance as possible

weigh (to 0.00 g) and record samples in order to assure completeness

record weights & volumes: +/- <0.1 g or mL for 11- and 22-g subsamples,
extractants, H2O additions

times: shaking, fumigating, and incubating for equal time periods

careful labeling with proper marking pens or pencils

samples out of refrigerator as little as possible before extraction,
fumigation, or incubation

“machine-like” LI-COR operation

Double check, initial, and date EVERYTHING
DAY 1 after field sampling
The following procedures describe treatment of field-moist, chilled soil samples within 24
hours after return from field sampling.
1. Print adequate number of ‘weights & sample IDs’ sheet from “Master
spreadsheet for sample setups”;
2. Remove a few at a time from frig and record whole sample weight (tare for bag
weight);
3. 2-mm sieve whole sample under hood;
4. Record weight of >2mm fraction and discard;
5. Weigh out and record weights for setups as follows on master spreadsheet:
a. ~11 g into tin for gravimetric moisture (record tin wght and tin+soil wght;
b. ~11 g into centrifuge tube for K2SO4 extraction (labeled t0 Nmin);
c. ~11 g into 50-ml glass beaker for microbial biomass (labeled MBC);
d. ~22 g into spec cup for incubation (labeled PMCN);
6. Immediately place PMCN and MBC samples in 4ºC frig;
7. Place gravimetric tins in 105ºC oven for 24 hours;
8. Add 30 ml of 0.5MK2SO4 to t0 Nmin tubes; cap and shake on low for 30 minutes
then place in 4ºC frig overnight;
9. Store remaining <2-mm samples in frig for one week, then air dry and store;
except for those to be used for one-time analyses;
DAY 2 after field sampling
1. Filter t0 Nmin samples into spec cups and freeze extracts for analyses of NH4,
NO3, and DOC
2. When moisture content is known, copy Excel “Master spreadsheet for sample
setups” to project file, rename “Master spreadsheet for <project><date>, and
enter dry weights into ‘Calculations 1’ worksheet.
3. Bring PMCN and MBC samples to field capacity mass on spreadsheet by
placing on scale and uniformly wetting with DI water from syringe;
4. MBC Fumigations:
a. under hood, place a few boiling chips and ~15 mL chloroform in 50 mL beaker
b. place moist paper towels in very bottom of dessicator, place chloroform in center
of bottom
c. insert ceramic plate and place MBC subsamples in two layers (use second
desiccator if necessary)
d. grease surface of dessicator with silicon vacuum grease and place lid
e. attach suction until chloroform boils (5-10 minutes)
f.
after rolling boil starts boil for 1 minute
g. shut off dessicator valve and turn off water, record time
h. leave under hood (turned on) for at least 24h
5. PMCN incubations (in Mason Jars with septa inserted into holes in lids)
a. place 2 mL DI H2O into bottom of Mason jars
b. place cup in Mason jar, tighten septum lid, and record time
c. place in dark 25C incubator (or in temperature-controlled lab in boxes)
DAY 3 after field sampling
1. Incubations
a. determine d1 CO2 concentrations in jars with LI-COR
b. vent jars by removing lid, holding above head, inserting vacuum tube for 1
min, moving back and forth 4 times above head, and replacing lid.
c. return to 25C incubator
d. repeat on d7 and d14
e. at d14 terminate and extract in 30 mL 0.5M K2SO4 as for t0 Nmin samples
(DAY 2 no.1)
2. Fumigations
a. terminate fumigations after 24h by thoroughly evacuating chloroform
vapors from dessicator
b. apply vacuum for ~5 min; remove tube from dessicator and let fill with air;
repeat 4 times
c. carefully remove lid and remove samples
d. extract in 30 mL 0.5M K2SO4 as for t0 Nmin samples (DAY 2 no.1)
OTHER TASKS TO BEGIN ASAP
1. fine grind ~10-g subsamples from air-dry 2-mm samples;
2. analyze total C and N on CE1100;
3. analyze for inorganic C;
4. initiate analyses of SOM fractions on air-dry <2-mm subsamples;
5. analyze pH on air-dry <2-mm subsamples;
6. determine bulk density in BD cores from field;
7. initiate particle-size analysis on air-dry <2-mm subsamples;
8. initiate other analyses called for in study plan (e.g., P, K, CEC)