I have never heard the term 'soil organic matter structure'. There is term 'soil structure' which refers to relative proportion of sand, silt and clay in the soil. The stability of soil structure is improved by organic matter of the soil.
Soil structure refers to the shape and size of aggregates. The aggregates are made up of both the mineral (sand, silt, and clay) and organic fractions of the soil. Aggregates are generally held together by root exudates, fungal hyphae, and other organic material. Soil structure can generally be classified as being single grain, granular, blocky, platy, columnar, or massive.
If you want to describe the proportions of sand, silt, and clay in a soil, that is soil texture.
I regret for defining the soil structure as soil texture. Soil structure refers to the arrangement of soil particles in aggregates. Organic matter is helpful in improving the soil structure.
If you refer to chemical structure of SOM you have different methods, chemical and physical ones. For example, you can burn the SOM & measure the heat emission; according the pics of heat emissions you can infer the structure of the humic substances. Another way is following traditional chemical processes, using different basic solvents, pH, complexing power, etc. Please down load some of my papers dealing with.
Dear Michael, In the same light as Dr. Juan, i think it is clearer to talk about "forms of soil organic matter". Briefly, soil organic matter is very heterogeneous and for you to have an understanding of the various constituents, it will be important to consider SOM as a complex mixture consisting of (1)Unaltered materials, which include fresh debris and non-transformed components of older debris (2)Transformed products, or humus, bearing no morphological resemblance to the structures from which they were derived (3) interaction between SOM and other mineral (inorganic) soil constituents such as clay minerals.
For more on this, try obtaining the book entitled "CHEMISTRY OF SOIL ORGANIC MATTER" by KYOICHI KUMADA. Developments in Soil Science 17, Japan Scientific society Press.
Combinations of spectroscopic techniques are used to characterise the structure of soil organics. NMR is often used but it requires removal of Fe compounds which typically means extraction of the organic phase under alkaline, anoxic conditions and further purification before measurement. Papers by Ashlea Doolette et al. may be at the leading edge. EDAX, SIMS, IR and synchrotron spectroscopy allow less intrusive pre-analysis treatment.
Soil organic matter is amorphous in nature, therefore, no perfect lattice structure can be assigned to it. The soil organic matter is consisting of macromolecules of varying molecular weights. Electron microscopic studies (SEM) done with the extracted components of soil organic matter (Fulvic and humic acids) indicate that these have flexible physical appearance which change with equilibrium pH and ionic strength. Spectroscopic studies like colour ratio (E4/E6), IR spectrocopy and 13C-NMR can reveal about the dominance of some chemical moieties in these natural acids. The elucidation chemical structure is also possible by studying the chemical or microbial degradation products of these acids.
There are lots of techniques to evaluate SOM, chemical, physical, spectroscopy methods... Depends on the specific processes or characteristic you want to access.
Here a review of many of this techniques and theirs applicability.