Isolation and fractionation of organic matter from soils and waters

The “lumpers” view that soil organic matter (SOM) should be studied as it exists in its habitat. Here, we support the aim of the “splitters” who regard it as essential to isolate, to fractionate, and to characterize as far as is feasible, the components of SOM and their reactivities in order to better understand how SOM can be conserved and managed as increasing demands are made on world soils. Sequential exhaustive extractions are described that isolate components in aqueous and organic solvent media on the basis of charge density and polarity differences, and the inclusion of urea (6 M) in the aqueous alkaline media ensures that all of the humic components can be recovered. A facile solvent system, involving dimethyl sulfoxide and concentrated H2SO4 enables humin, the major and most recalcitrant component of SOM, to be isolated from soil, and characterized. The uses of Amberlite XAD-8 (now superceded by Supelite DAX-8) allows humic acids to be recovered free from non-covalent bonded hydrophilic inclusions and XAD-8 (DAX-8) and XAD-4 resins in tandem allows the major hydrophilic components also to be recovered. Other resin combinations are suggested for the isolation and the fractionation of humus and of humic components. A range of fractionation procedures are described with the potential to isolate more specific “families” of humate molecules. Applications of the more refined procedures are time-consuming and tedious and, as yet, the value of the additional informational benefits that these can provide is questionable. We consider that emphasis on fractionation might best be focussed on finding additional information about “families” of molecules that are major contributors to the vital functions of organic matter in soil and water environments. We know that SOM levels can be maintained where intensive cultivation applies if good soil management practices, such as conservational tillage are followed, and additions of appropriate amounts of crop residues and composts are applied, and green manuring is included in rotations. A start has been made to initiate chemical reactions that give rise to a resistance to degradation of key components of SOM.