2002-07-08_GENERAL DOCUMENTS - M2002004 ‎(3)‎

• Permit No: M2002004
• IBM Index Class Name: GENERAL DOCUMENTS
• Doc Date: 7/8/2002
• Doc Name: ATTACHMENT, PART 2
• From: HABITAT MGMT
• To: DRMS
• Media Type: D
• Archive: No

192
SMITH, REDENTE, and HOOPER
total C and inorganic _C - an - subtraction of inorganic C from total C, (2)
analysis of total C after destruction of inorganic C, and (3) reduction of
Cr by organic C compounds and subsequent determination of unre-
duced Cr by oxidation- reduction titration with Fe or by colormetric
techniques. Each method has inherent problems (Table 1). But the first
method may yield the most accurate results. However, method three is most
amenable to routine analysis of soils (L. Sommers, Colorado State Univer-
sity, Ft. Collins, personal communication).
The dichromate method is `a modification of the technique described by
Mebius (50). Heating of'the chromic acid -soil mixture oxidizes organic C to
CO . The amount reduced Cr2O, then can be quantified. Mebius and
Nelson "arid _ Sommers recommended using N- phenylanthranilic acid during
titration (quantification of reduced Cr20, - ) because it produces a sharp,
clean endpoint (50, . 38). Nelson and Sommers (57) recommended heating
the chromic acid -soil mixture'=for30 minutes at 150 °C in Folin -Wu nonpro-
tein N placed in an aluminum block on a hot plate. Quantitative re-
sults provided by this technique represent the best combination of digestion
reagents, heating procedure, and titration reagents of the modern dichro-
mate determination methods. The technique uses soil- comprising organic
material that has been dried and passed through a 2 -mm sieve. This tech-
nique assumes that C in soil organic matter has an average valence of zero
and an equivalent weight of 3 gm per equivalent when reacted with Cr20, - .
Several researchers have recommended loss -on- ignition as a method of
direct estimation of organic matter (35, 58). This method gives quantitative
oxidation of organic matter. But inorganic constituents of the soil, largely
hydrated aluminosilicates, lose structural water and carbonate minerals lose
CO2 upon heating at high temperatures. If an ignition temperature of about
430 °C is used (27) for 24 hours, the cumbersome pretreatment of soil with
HCl or HF to eliminate carbonates and subsequent correctional calcula-
tions are unnecessary_ Although direct estimation of organic matter content
using the loss -on- ignition technique at reduced ignition temperature
(430°C) may be a promising method to determine organic matter content,
Nelson and Sommers (58) believe it cannot be recommended at this time
because of insufficient evaluation of its usefulness and applicability.
Humus is the single largest constituent of soil organic matter. Analytical
methods for determining humus content involve extracting humus sub-
stances from soil using dilute NaOH (O.IN or O.5N) in a N atmosphere.
After extraction, analytical methods can be used to study humus: nonde-
gradative and degradative (70). These methods are lengthy and not applica-
ble to reclamation planning.
In summary, because of the difficulty in directly estimating or calculating
organic matter present in soil, a more appropriate procedure is to determine
and express organic C content as a measure of organic matter. Organic C
can be accurately and - precisely measured by the modified Mebius tech-
SOIL ORGANIC MATTER
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