K, Ca, and Mg can form complexes with SOM (Broadbent and Ott, 1957; Schnitzer and Skinner, 1963) and the capacity of soils to store mineral nutrients (i.e., CEC), increases with SOM.
Physical Effects
The physical benefits of SOC relate to the formation and stabilization of soil aggregates. Several studies have reported high correlation between soil aggregation and SOC (Wilson et al., 2009; McVay et al., 2006; Six et al., 2002). Degradation of organic materials by soil organisms leads to the formation of humified materials associated primarily with mucilage that surrounds and binds to clay particles, thus developing and binding the particles into microaggregates (Balesdent et al., 2000). The decomposition of protected SOC can become slow due to the clay barrier, thus promoting soil carbon sequestration. Soil health benefits of greater soil aggregation include less crusting, compaction, and bulk density (Diaz‐Zorita and Grosso, 2000); enhanced soil structure for greater water infiltration and water holding capacity (Hudson, 1994; Emerson, 1995; Gupta and Larson, 1979, Yang et al., 2014); decreased soil erosion (Schertz et al., 1994; Benito and Diaz‐Fierros, 1992); and improved aeration for root growth and microbial activity. As tillage intensity increases, soil microbial activity increases right after tillage and microaggregates are dispersed, thus releasing SOM from protection (Puget et al., 1995, 2000).
Crop Productivity
In addition to supporting biological control of crop pests and disease, SOM contributes to agricultural crop yields in various ways by overcoming negative soil conditions. Adequate amounts of SOM can enhance: (i) release of nutrients from decaying organic materials and thus reduce commercial fertilizer requirements; (ii) soil porosity which increases plant available water retention and aeration for root development; (iii) soil structure which reduces soil erosion potential and increases aggregation; and (iv) storage of other nutrients, including an increase in cation exchange sites for plant nutrient retention.
Measurement of SOC
Recognizing there are multiple methods for measuring SOC, dry combustion, loss‐on‐ignition (LOI) (Schulte and Hopkins, 1996), and the Walkley–Black method (Nelson and Sommers, 1996) were evaluated for SOM are evaluated to examine the advantages and disadvantages of each method (Table 3.1). The traditional method for SOM analysis was wet oxidation in potassium dichromate (K2Cr2O7), better known as the Walkley–Black method. The potassium dichromate solution oxidizes soil organic material through a chemical reaction that generates heat when two volumes of sulfuric acid are mixed with one volume of dichromate. The remaining dichromate is titrated with ferrous sulfate, with the titer being inversely related to the amount of C present in the soil sample (Nelson and Sommers, 1996; Meersmans et al., 2009).
Although Walkley‐Black was the standard method for SOM analysis for decades, its use has diminished due to the high potential for environmental pollution during disposal and exposure of personnel to hazardous chemicals, such as potassium dichromate and sulfuric acid. In contrast, the LOI method is a simple, inexpensive method for SOM estimation that involves the combustion of samples at high temperatures and measuring weight loss after ignition. The ability of LOI to quantify SOM content has been considered reliable, but optimal heating temperatures and duration to 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