Not only the organic matter , but i would say , the other soil parameters like pH, CaCO3 , textural variation, the fertility index , they all have to be standardized for major soil types or at soil sries level in response to different crops  and forest species having differential agro-pedological requiremets,, if sound land use planning has be realised. that is why , we always say SOIL SUITABILITY CRITERIA  in response to specific crop and soil is urgently needed in order to exploit the potential of soil resource inventory of ant land unit.

"official standards" for organic matter would not be helpful, as the organic matter contents of soils differ widely due to their history of cultivation, particle size distribution, pH et cetera. Further, the percentage of organic matter in soils underlies seasonal variability (to a certain extent). Therefore, it is not possible to have an "official standard" target level. Rather, it is necessary to observe the momentary levels of organic matter and to observe whether or not measures you are taking to increase organic matter content are successful. I am not sure about information from the UK, but TEAGASC from Ireland has some good information on this topic.

I agree with  Anoop and Thomas, that it depends on many factors, and the "content of organic matter" itself is not very relevant. Although it is considered "more organic matter - better" actualy there very good soil containing about 1% of organic matter (0,58% organic C) or less, and soils not so good due to drainage problems with 3% of organic matter. However, as the question refers to "official standards" see these papers.

The first one refers to Poland, the data are not in the table but in the text, at the bottom of the page 75 according to numeration, when printed, or 77 (numeration in the file).

For Peru, you can see this information in the last page (18), as "materia organica" - it is in Spanish, "alto" - "high", "medio" - "medium", "bajo" - "low".

Two examples from "opposite" countries... ;-)

There are really no "official " standards for soils in either forestry or agricultural soils.  One must take into account the initial state (undisturbed by humans) at a site to evaluate what the organic matter content really should be.  However, this is impossible because most soils are perturbed in some way.  Each soil type has its own capacity for storing organic matter and is impacted by the five soil forming factors.  When we impose human practices on the soil, we change the soil as the organism component of the formation factors.  A survey of similar soils under similar forestry practices in a region would be helpful in evaluating if the soil in question is better, worse or similar to the soil in question.  Beginning monitoring of a soil by establishing a current baseline of the soil by sampling now and then again in the future will help determining if the status of the soil is improving, declining or remaining the same.

I agree with all of you, thank you for the answers.

My concern is that targets are often advised. Maybe the word "standards" is quite  strong, but I have been taught for example that 5% of organic matter (LOI) is a "good" level for arable soils.

I totally acknowledge that the interpretation of the content of organic matter is highly dependent on the context (soil type, climate, etc).

A colleague and me are looking for common values, corresponding to specific land use and climate for example.

I was thinking that maybe databases exist with information about the relative frequency of the observed organic matter contents (mean, standard deviation, or even better the probability distributions / density curves).

Dear Thomas, according to my observations in Poland, for arable soils, 5% of organic matter would indicate excess of water (in other words - insufficient aeration). Some information about percentage od soil with different humus content classes you can see in the paper I attached (about Poland and Baltic Sea), table 2.8, page  74 and 75 (or 76-77 according to the file) you can see, that about 47% of Polish soils have "average" level of organic matter between 1 and 1% of it (0,58-1,16%C). It is interesting to compare Polish and Peruvian critical levels:

low 3% (only Poland)

And now, I saw, that there is an error in Peruvian table "alto" (high) should be changed by "bajo" (low) and vice versa :-(. Well errors are common, in some Polish documents also exist... (and not only Polish).

When considering target values, they should/must be different depending on land use, soil texture and other factors, as it was written in other answers. In sandy soil it is impossible to increase C content significantly even using 100t/ha of farmyard manure (it would be illegal- Nitrate Directive), because in such soil organic-clay complexes cannot be formed and other form of organic matter is easily mineralised.

Other interesting aspect - in

Nair P., K., R. 1993. An introduction to agroforestry. 489ss, dostępny pod adresem:

http://www.worldagroforestry.org/units/library/books/PDFs/32_An_introduction_to_agroforestry.

pdf?n=161

in table 14.3 it can be seen, that high organic matter content i so considered a limiting factor. It probably refers to organic soils, but, perhaps also some mineral ones?

 Normally, soil testing laboratories attached to agricultural universities or state governments estimate organic carbon in soil.The generated data is summarized  at district or state level annually or at some periodicity and published.This will help to know the soil fertility status and also help to monitor over time .One can contact  concerned state government official or university professor.By multiplying the organic carbon content with 1.724 you will get organic matter.Some institutes like Rothamsted  Experimental Station are maintaining data base of organic carbon status of long term experiments  being carried out world wide.

Thomas , best way could be , you can try out some of the standards already in practice ( Century , Roth C etc ). The other possibility is  , to develop the  organic matter standards  through your long term experiment , but this would be a very strenuous process , since so many variables will come into picture , right from soil -related issues  to environment- related issues, in addition to various land uses. Find below some PDFs , which could be of some use .