maximize SOM combustion, while minimizing inorganic carbon combustion, are challenging to determine. Both of those variables can substantially affect LOI results (Salehi et al., 2011). Dry combustion using elemental carbon analyzers is now considered the gold standard for SOM measurement. This method is based on thermal oxidation of a soil sample at ~ 1000 °C and determining the quantity of CO2 produced by gas chromatography or infrared analysis. This method is considered the most accurate but is also the most expensive method for determining total carbon. Furthermore, to accurately measure SOC, inorganic carbon must be removed by pre‐treating samples with hydrochloric, phosphoric, or sulfuric acid before combustion. It is also important to point out that dry combustion measures SOC instead of SOM. Thus, a conversion factor is used to convert SOC into estimates of SOM. Historically, the most used conversion factor has been 1.724, which assumes organic matter is composed of 58% of carbon. However, recent studies have shown that this factor is too low for most soils. In a review of previously published data, the median conversion factor value was found to be 1.9 from empirical studies and 2 from more theoretical considerations. We concur that using a factor of 2, based on the assumption that organic matter is 50% carbon, will, in almost all cases, be more accurate than 1.724 (Pribyl, 2010).
Table 3.1 Advantages and disadvantages of the most common methods for measuring soil organic matter.
| Method | Advantages | Disadvantages |
|---|---|---|
| Walkley‐Black | Relatively simple, accurate, and quick | High environmental pollution potential, uses hazardous chemicals |
| Loss‐On‐Ignition | Simple, inexpensive, convenient | Heating temperature and time significantly affect accuracy. It has to be calibrated. |
| Dry combustion by elemental analyzer | Most accurate, quick | Elemental analyzers are expensive to purchase and maintain. |
For this chapter, we chose to present both the LOI and dry combustion methods since they are environmentally safe, simple, and the most widely used methods in commercial laboratories.
Methods for SOC Analysis
Soil Sampling and Preparation
The same general principles that apply to soil sampling for nutrient evaluation apply to soil sampling for SOM determination. Soil samples should be collected to consistent soil depth(s), a consistent number of soil cores collected per composite samples should be maintained, thatch or mulch from the soil surface must be removed prior to sampling, composite samples should be inspected after collection and any obvious pieces of crop residue should be removed. Georeferenced points should also be collected within the field when possible. For all methods described below, soil samples should be dried at 35 to 40 °C in an oven and passed through a 2000‐μm sieve.
Dry Combustion
Apparatus
1 Forceps
2 2000 μm sieve
3 250 μm sieve
4 Aluminum tin capsule
5 Analytical balance with 0.001 precision
6 C/N elemental analyzer
Reagents
1 Concentrated H2SO4 or HCl
2 4N phosphoric acid solution
Procedures
1 Air‐dry or oven‐dry soil at 35 to 40 °C.
2 Carefully remove all plant and animal materials from the soil using forceps.
3 Pass the soils through a 2000‐μm sieve and grind the soil to a powder to pass through a 250‐μm sieve.
4 Proceed to step 11 for soil with pH < 7.00.
Pre‐treatment for soils with inorganic carbon content
1 For soils that have a pH > 7.50
2 Test for the presence of inorganic carbon or carbonate (CaCO3) using concentrated H2SO4 or HCl.
3 Weigh 0.50 mg of the soil into a silver vessel and add 5 drops or (5 mL) of concentrated H2SO4 or HCl.
4 If effervescence is observed, inorganic carbon is present in the soil sample. If inorganic C is not present, go to Step 11.
5 Treat soil containing inorganic C with 0.1 mL of 4N phosphoric acid solution.
6 The treated sample should be allowed to dry for one or two days before analysis. Repeat steps for removal of inorganic C if necessary. Be cautious when handling the tins and do not use Al vessels as they will disintegrate when acid is added.
7 Weigh 50 mg of soil into an aluminum tin capsule using an analytical scale with 0.001 precision.
8 Fold the aluminum tin capsule containing the soil sample and load it on to the analyzer.
9 Weigh a known standard to serve as a check, using one standard every 12 to 20 samples. Standards can either be EDTA, aspartic acid, or soil with a known concentration of C.
* The operation of the analyzer should be operated according to the guidelines of the machine.
Loss‐On‐Ignition (LOI) (Salehi et al., 2011; Schulte and Hopkins, 1996)
Apparatus
1 20 mL porcelain crucibles
2 Forceps
3 2000 μm sieve
4 50 mL beaker
5 Desiccator
6 Muffle
7 Analytical balance
Procedures
1 Air dry or oven‐dry soil at 35 to 40 °C.
2 Carefully remove all plant and animal materials from the soil using forceps.
3 Pass the soil through a 2000 μm sieve.
4 Weigh 10 ( ± 0.05) g of the soil sample in a tared 50 mL beaker and oven‐dry at 105 °C overnight. This process will ensure the removal of all water from the gypsum in gypsiferous soils.
5 Using a desiccator, cool the samples and then record the weight.
6 Transfer
