Sodium Iodide and Sodium Polytungstate Solutions for Recovering Free Light Fraction Soil Organic Matter.

The light fraction (LF) of organic matter is composed of partially decomposed plant residues, microbial residues, seed, and spores, and has been shown to be a sensitive indicator to short-term changes in soil management systems. Light fraction is recovered by density-based procedures where sodium iodide (NaI) and sodium polytungstate (SPT) [Na₆(H₂W₁₂O₄₀)] are the chemicals commonly used to prepare the density solutions. The objective of this research is to compare the efficiency of similar density solutions of NaI and SPT in recovering free-LF (FLF). Organic C and N are major elements of interest in the FLF and the concentrations recovered with each procedure were compared. Soil samples were collected at 0-15 cm depth. In each sample, LF was recovered in triplicate using both NaI and SPT solutions, both adjusted to densities of 1.6 and 1.8 g cm^-3. Soil samples were gently shaken with either NaI or SPT adjusted to 1.6 or 1.8 g cm^-3, then centrifuged for 15 minutes at 5000 rpm. The supernatant was filtered through 20 µm nylon filters to obtain the FLF. At the same density (1.6 or 1.8 g cm^-3), C-FLF contents were not statistically different using either NaI or SPT solution. The C-FLF content ranged between 518.3 and 1241.7 mg kg^-1. Increasing the density within a specific solution (NaI or SPT) resulted in significantly higher C-FLF recovery. Nitrogen-FLF presented the same trends as observed with C-FLF regarding solution and density used to extract FLF. An exception, at density of 1.8 g cm^-3, the NaI solution recovered significantly more N-FLF than the SPT solution. The N-FLF content ranged between 25.1 and 78.0 mg kg^-1. These data indicate that NaI and SPT have basically the same efficiency in recovering FLF, and the NaI solution is much less expensive.