According to Baldock (2008), four biologically significant types or fractions of SOC exist, viz.:
The four fractions described are composed of different materials with different chemical and physical properties and different decomposition rates. Moreover, the amount of each fraction is highly dependent on management practices; for example, the tillage practices employed. Although crop residues usually break down quickly, the method described herein for POC will include some crop residues as defined by Baldock (2008).
Key functions of POC include improving soil structure and the cycling of soil nutrients. Six et al. (2000) claim POC improves soil aggregation since it can form an organic pore surrounded by clay, silt particles and aggregates. According to Skjemstad et al. (2002), most charcoal-C is retained in the <53 μm fraction but some larger charcoal particles can be retained with the POC fraction. Therefore, sample inspection under a microscope is recommended before analysis for charcoal-C using, for example, the nuclear magnetic resonance approach of Skjemstad et al. (2002). Charcoal in the POC fraction can then be removed as had been done earlier by other workers (e.g. Swain 1973; Tolonen 1986) who both sorted charcoal particles by hand or under a microscope in estimating the soil content of charcoal-C.
The extraction component of this method follows that used by Cambardella and Elliott (1992). Finely ground, air-dry soil is mechanically dispersed overnight in aqueous sodium hexametaphosphate, with subsequent filtering through a 53 μm sieve. The material retained on the sieve is then dried and homogenised using a mortar and pestle before dry combustion (Dumas) for organic C, an approach used by Bouajila and Gallali (2008). Carbonate particles, if present, cause positive interference and should be removed physically or accounted for by separate analysis by Method 19A or 19B.
Calgon solution (5 g/L)
Dissolve 10.0 g of sodium hexametaphosphate [mainly (NaPO4)6; marketed as Calgon] in 2 L of deionised water in a graduated beaker.
Weigh 10.00 g of finely-ground (<0.5 mm; 40°C) air-dry soil into a stoppered plastic container capable of loading on a mechanical shaker (see Note 1). Add 30 mL of 5g/L (NaPO4)6 solution and mechanically shake for 15 h.
Pass the dispersed soil/(NaPO4)6 solution through a 53 μm sieve and, after rinsing several times with deionised water, dry the retained material at 50°C to constant weight, typically overnight. Homogenise quantitatively each sample using a mortar and pestle (if necessary), then determine TOC in the dried residue using the analytical finish described in 6B1 or 6B3. Remove or account for any solid-phase carbonate present in the residue.
Report POC (% C), on an oven-dry basis. Use the air-dry moisture to oven-dry moisture ratio to make the oven-dry conversion. Refer to Method 2A1 for guidance with regard to this soil moisture calculation.
1. Cambardella and Elliott (1992) used a reciprocating shaker. Gentle end-over-end mechanical shaking is preferred, however.