Dear Annangi,

Humic substances are currently classified by their different characteristics, such as functional groups, relative density, and solubility in different solvents. For instance, humic acid and fulvic acid as you mentioned are separated by their different solubility in alkali. So the humus are a group of mixed composites with complicated structures and functional groups. Their features depend on the C/N components of parental materials, forming duration, and the micro-environamental parameters. I hope my short answer would help your understanding.

Best,

Ping

I agree with Ping, that humic substances are definitely composite or mixed substances.

Here you can see some materials for students from Natural University of Wroclaw (Breslau):  http://karnet.up.wroc.pl/~weber/humic.htm#start

Even from this we can conclude -that they are mixture, because these processes are runninig all the time (if the soil is not frozen or completely dry), so I would expect, that the mixture of all these substances - not only final, but also intermediate products (in different proportions) is present in the soil.

I think that the oldest and perhaps still most complete book was written by Konnova in sixties. She was a Soviet scientis, but rhis book was certaily translated into English:

https://books.google.pl/books/about/Soil_organic_matter.html?id=ZPvwAAAAMAAJ&redir_esc=y

Regards,

Michał

Dear colleagues,humic substances may have origin from different organic materials undergone decomposition under different environments over time,the resistant component of undecomposed organic material may have acquired the status of humus by synthesis reactions and also interactions with soil mineral phase.We normally group the humic materials into fulvic acid ,humic acid and humin. We also differentiate them based on molecular weight,color,solubility,susceptibility to decomposition ,residence time in soil and also chemical formula.Even shapes and structures were given to humic acid and fulvic acid using very sophisticated techniques like NMR spectroscopy,X-ray absorption near-edge structure spectroscopy,electrospray ionization-mass spectrometry etc.So the main point of concern is that the humic substances acquired some sort of homogeneity in chemical structure,form and properties.So their functions may also differ(some functions may be similar like bonding with cations/holding cations in exchangeable form). Intermediate products may differ but relatively final products may acquire some sort of homogeniety and behave like similar supramolecules or structures.I solicit further comments on these issues.The following reference may be interesting to followers and viewers.

Molecular Structure in Soil Humic Substances:The new view by R.Sutton and G.Sposito Environm.Sci.Technol.,39,(23)pp.9009-9015.

http://pubs.acs.org/doi/abs/10.1021/es050778q

Sir,

HA and FA are composite substances

Humic substances has its basic fundamental structure. However they are differentiated into fulvic acid, humic acid and humin based on the degree of complexation, molecular weight, solubility, functional group etc. Also the structure and composition vary accordingly to material from which it is formed, duration, environmental factors, etc. Hence these compounds are considered as composites when observed at different stages of decomposition and under different conditions. In a particular micro-environmental condition, the final products may attain homogenity with similar structure.   

Dr. Rajakumar and Dr. Rajendiran,good responses. Dr. Rajendiran,your last sentence is interesting.

I  aggree  with Dr Rao. Humic and Fulvic acids are not homogenous entity; in case of soils rich in organic matter (>5%), their corresponding increased presence, it makes easy to study their efficient role to soil bio-chemical properties; the problem arise during their study in cultivated fields with low contents in organic matter (

Himic acids are homogenous in somw propeeries and in general structure. Foro example in the type of functilnal groups. My thesis was around the caracterización of humic gractions in River sediments