Schedule B3 - COAG Standing Council on Environment and Water

7.5.3 Interferences
Water-soluble colour in the soil may mask the colour change at the endpoint of the titration. If
this occurs, the colour can be removed by adding an aluminium hydroxide suspension (APHA
Method 4500-Cl). Alternatively, chloride in the water extract can be determined using an ionselective
electrode or ion-chromatography.
7.6 Organic carbon
7.6.1 Scope and application
This determination (Rayment et al. 1992, p. 29), known also as the Walkley & Black method,
measures the oxidisable organic carbon content of soils and may also be used to estimate their
total organic carbon (TOC) content.
Soil organic carbon comprises a variety of carbonaceous materials including humus, plant and
animal residues, micro-organisms, coal, charcoal and graphite. It does not include carbonate
minerals such as calcite or dolomite. Australian soils generally contain less than 5% organic
carbon, with higher levels common in surface soils (Rayment et al. 1992, p. 29 and p. 32).
The first method listed in Rayment gives poor recoveries of carbonised materials such as
graphite, coal, coke and similar coal derivatives. If such materials make up the bulk of the
carbon in the sample or if the total organic carbon content is required, an alternative method
which makes use of an external heat source is recommended (Rayment et al. 1992, p. 32).
For acid sulphate soils, consult the Australian standard for the Analysis of acid sulfate soil - dried
samples - methods of test - introduction and definitions, symbols and acronyms, (AS 4969.0-2008) for
relevant definitions and recommended analytical procedures.
7.6.2 Interferences
Overestimation of organic carbon may occur due to large amounts of chloride or metallic or
ferrous iron in the sample. Underestimation may result when large amounts of higher oxides of
manganese are present. These interferences are common in Australian soils. The potential
interferences should be taken into account particularly when analysing some types of poorly
aerated soils.
Since the first method recovers variable proportions of organic carbon actually present in a soil
sample (recoveries typically in the range of 65 - 85%), a correction factor is usually needed. In
the absence of a specific correction factor for the soil being tested, a correction factor of 1.3 is
commonly used such that:
Total organic carbon (per cent) = Oxidisable organic carbon (per cent) x 1.3.
<strong>Schedule</strong> <strong>B3</strong> - Guideline on laboratory analysis of potentially contaminated soils 25