Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:6:p3
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 6 (pt 3/3)
Character Range: 781438–783766

removed by adding an aluminium hydroxide suspension (APHA Method 4500-Cl). Alternatively, chloride in the water extract can be determined using an ion-selective electrode or ion-chromatography.

6.6              Organic carbon

    6.6.1         Scope and application
This determination (Rayment & Higginson 1992, p. 29), also known 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, microorganisms, 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 & Higginson 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 & Higginson 1992, p. 32).

For organic carbon analysis in acid sulfate 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.

    6.6.2         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 (%) = Oxidisable organic carbon (%) x 1.3