ORGANIC CONTENT
SA04
Last Revised:
June 2008
1. PURPOSE
This exercise compares two methods for determining the organic content of soils.
2. REAGENTS & EQUIPMENT
2.1 soil
2.2 0.167M K2Cr2O7
2.3 concentrated sulfuric acid
2.4 concentrated phosphoric acid
2.5 Visible spectrophotometer
2.6 sodium diphenylamine sulfonate indicator
2.7 0.1 M iron(II) ammonium sulfate
3. PROCEDURE
3A. Colorimetric method
3.1 Weigh accurately 1-4 g of two different soil samples in duplicate. Use the lower weight for soils
high in organic matter and the larger weight for those low in organic matter, such as sandy soils.
3.2 Place each soil sample in a 250 mL Erlenmeyer flask.
3.3 Add exactly 10 mL of 0.167M K2Cr2O7 solution from the burette.
3.4 While gently swirling (keep mouth of flask directed away from you and anyone else nearby),
add 20 mL of concentrated H2SO4 (from a burette).
3.5 After 10 minutes, SLOWLY add 100 mL of distilled water using a measuring cylinder. Mix the
suspension by swirling.
3.6 Vacuum filter the suspension through a medium or fine filter paper. The filtrate must be
absolutely free from sediment. If it is not, filter again.
3.7 Fill a glass cuvette about 2/3 full with the filtrate.
3.8 Determine the absorbance of the sample in a visible spectrophotometer at 610 nm.
3B. Titration method
3.9 Obtain a sample of soil (consult teacher for advice on sample mass between 0.2-5.0g) and weigh
it to the nearest mg into a conical flask (500 mL) (see Note 4.2).
3.10 Add 10 mL of standard dichromate solution (0.167M), from a burette.
3.11 Carefully add 20 mL conc. sulfuric acid by measuring cylinder and swirl the flask gently for 1
minute. Caution: this will generate a lot of heat!
3.12 Allow the sample to stand for 30 minutes whilst oxidation occurs.
During this time perform the standardisation of iron (II) ammonium sulfate titrant (Part 3C).
3.13 Carefully add 200 mL distilled water, then 10 mL of conc. phosphoric acid.
3.14 Add 1 mL of sodium diphenylamine sulfonate indicator, then shake the mixture thoroughly. If
the indicator is absorbed by the soil add a further 1 mL.
3.15 Add the titrant from a burette in 0.5 mL increments, swirling the flask after each addition.
3.16 Continue the additions until the indicator changes from blue to green.
3.17 Add a further 0.5 mL of standard dichromate solution, which should change the colour back to
blue.
3.18 Add the titrant dropwise until the indicator just changes colour again.
3.19 Record the endpoint volume to the nearest 0.05mL.
Soil Analysis Practical Manual
SA04
3C. Standardisation of iron (II) ammonium sulfate
3.20 Add 10 mL of dichromate solution into a 500 mL conical flask.
3.21 Carefully add 20 mL of sulfuric acid (conc.) by measuring cylinder. Caution: this will generate
a lot of heat!
3.22 Carefully add 200 mL of distilled water, then 10 mL conc. phosphoric acid, and finally 1mL of
sodium diphenylamine sulfonate indicator (in the order described), then shake the mixture
thoroughly.
3.23 Titrate as per steps 3.15-3.18.
4. NOTES
4.1 For best results, a new standard curve should be developed for each new set of reagents, or the
old curve should be verified at several points.
4.2 In the titrimetric method, the sample size should be enough to reduce 5-8 mL of standard
dichromate solution.
5. REPORT
Calculations
Colorimetric method
•
plot a calibration graph using the standard values supplied in Table 1
•
determine the percent organic matter
•
divide this value by the sample weight to obtain the correct percentage of organic matter
TABLE 1 Typical values for organic matter standards (see Note 4.1)
%organic matter
Absorbance
0
1
2
3
4
5
6
7
8
9
0.01
0.11
0.17
0.21
0.27
0.33
0.39
0.43
0.47
0.51
Titrimetric method
•
use the formula below to calculate the %organic matter
%organic matter =
7.035
v1 

1 −

sample mass (g) 
v2 
where v1 is the endpoint in the soil titration, and v2 the endpoint volume in the standardisation.
Questions
1.
Compare the results from the two methods. Which do you believe to be more accurate? Why?
2.
Explain the effect the addition of a large quantity of highly carbonaceous material would have
on nitrogen availability in soil in the short run.
Explain why soils of warmer climates generally contain less organic matter than soils of cooler
3.
climates having the same amount of precipitation?
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