By far the most simple among the chemical properties to analyse. In my view, it should be sufficient. In fact, SOC is closely correlated with most of the soil biological properties and the physical properties as well. Therefore, SOC would be a fair indicator of the soil quality.

One can go about in detail at a later point of time, if two sets of samples are having similar SOC content. To understand what is different between these sets. 

SOM is recognised as a key soil characteristic that affects multiple soil functions and ecosystem services, so I would say yes it could, but only in conjunction with a sufficient level of detail i.e. soil type, land use, texture, etc.  See e.g. doi:10.1016/j.foodpol.2010.11.025 and doi: 10.1111/j.1365-2486.2012.02665.x for some discussion in a broader context

definitively NOT!

This is such  a popular and so widely  discussed  question , that still we feel,  it is  only half discussed . There are difference of opinions about the fact that soil organic matter is  the only impact soil property as an indicator of soil quality . I would say organic matter for sure , since it has most ominous and visible effects on majority of other soil properties like aggregate formation including aggregate size , water transmission properties, nutrient movement including the nutrient pool diversity  , energy source for microbes not only to sustain but proliferate as microbial biomass nutrients besides microbial diversity , rate of soil respiration ( Debated earlier  so comprehensively on ResearchGate ) etc etc  , they could collectively be the part of soil quality indicators . However many others vouch for soil pH, if at all,  if we have  to single  out  the soil property with regard to soil quality parameter. But again when we have to single out soil quality parameter like soil organic matter , we further need to see the functionality of the labile pool of organic matter.

I reckon organic matter, or more specifically soil carbon, overcome all the three categories of soil quality indicators (physical, chemical and biological). It can affect other quality indicators such as aggregate stability,soil structure, water storage and permeability (physical), soil micro-organisms by providing them with energy and nutrients as well as nutrient recycling (biological). moreover, It can also affect the chemical indicators such as  CEC (Cation Exchange Capacity), pH and sodcity and salinity of the soil. So I would say yes , organic matter could be itself an indicator of soil quality  to some extent . 

No. It is very diffcult to interpret without knowing the texture of the soil. I would also argue that change in SOM over time is just as important as the current concentration.

I would say no to using soil organic matter as the sole or only indicator of soil quality.  However, it is an extremely important component of soil quality.  It is extremely important for maintaining  a highly productive soil but adequate soil physical, nutrient and moisture conditions are required for good productivity (sometimes equated with soil quality).

The SOM is the most important and influence the other soil properties or alter the effects of other properties. But the sole dependency on SOM as the soil quality indicator could be misleading while interpreting the data.

Thanks all for your interesting responses.

Perhaps we could say the SOM is not an appropriate indicator for soil quality when different systems with different soil textures are involved. For example, in a system with the cultivation of various crops or a systems with single crop with different crop residue management (residues burning or soil incorporation), the SOC could represent soil quality? in the other word, when an amount of extra carbon is temprorarily added or removed from the soil in the system studied (straw soil incorporation) compared to a reference system (straw burning).

Results of long-term fertilizer experiments  in Indian sub-continent with rice-based double or triple cropping systems indicate soil’s capacity to store greater C, and maintain higher C in passive pools and that active fraction of soil C can be used as an indicator of soil health and not the total C content. The attached pdf may be referred.

Solely is an incomplete indicator for soil quality, the different pools of C in soil presents dynamics, transformations and movements through biodiversity (Mega-Macro-Meso-Microfauna and microbiota) which responses to environmental issues. For SOM complete assessment its recommended quantified:  SOM content, Soil biomass C, Soil CO2 respiration and calculate Microbial quotient (Soil biomass C/SOM content), Metabolic quotient (Soil CO2 respiration/Soil biomass C) and C mineralization rate (Soil CO2 respiration/SOM content).

If one  pays attention to soil quality with focus on soil functions,soil humus has greater role  to play .Active pool  or labile pool or microbial biomass pool fluctuates in tune with inputs of organic matter every season or year.Soil humus may not change so frequently.Humus has great role in soil aggregation, stability, nutrient retention, release and erosion control.Water retention and mobility is also governed by aggregation.Humus provides anchorage or shelter to microorganisms and its slow decomposition supports certain minimum microbial population in the absence  of fresh input of organic matter.Holds the cationic and anionic form(phosphate through Ca)of nutrients in plant available form.Humus also buffers soil pH and resists changes in soil pH with addition acid forming  fertilizers. So organic matter determination or humus estimation plays greater role in soil quality assessment.We take narrow view of soil quality or health with focus on crop production.Healthy soil has to maintain or enhance water and air quality and support human health and habitation.In that case we have take care of soil erosion,soil pollution in turn water pollution,transport of pathogens,heavy metal accumulation and transport.So we have to broaden the scope of minimum data set for soil quality /health assessment.

Another look at this question refers us to Life Cycle Assessment (LCA) method. The LCA is the instrument of choice for quantifying environmental impacts of products and the systems that produce them throughout their full life cycle. Is soil organic carbon taken into account within current LCA methodologies? So how the LCA and soil carbon could be combined?

For instance, we have two systems: System A in which straw is burned in the field and all carbon in the straw releases into the atmosphere, while in System B, the straw is incorporated into the soil. So could soil quality or soil fertility impact of systems A and B be quantified using the LCA methodology via soil organic carbon changes? 

Life cycle analysis can be applied to crop residues applied to soil or manure applied to soil.Crop residues can be applied to soil as in conservation agriculture,they can be burned on the field after harvest  of crop and collecting the economic or grain portion.The residues are also fed to animal as feed and the manure is returned to soil.In case of burning the carbon,nitrogen ,sulphur etc are burnt and released as oxides and silica, cationic nutrients, K,Ca,Mg and anions,phosphate,chloride,  etc  are returned to soil.In case of manure application recycling may not exceed 50% of that in original straw.The fresh organic matter added in the thee cases is equal to roots and straw(in case of residue recycling),only roots in case of burning and roots plus manure in case of manure application.The residue (including root/straw/manure)first form part of labile pool of carbon and then microbial biomass pool.Whatever portion remains after decomposition and mineralization processes transforms in to humus.It is relatively resist to microbial attack and remains soil for decades to centuries performing several functions mentioned by me earlier.All the pools like manures,residues,labile pool/particulate carbon,microbial  biomass carbon and humus can be quantified in the three systems.