There have been a number of threads in this forum and related forums dealing with measurement of the amount of organic matter in the soil. Some folks use a soil lab to get a measurement, others put a quantity of soil in a jar with water, agitate the jar, and allow the contents to settle. I thought a few bits of information might help in understanding some of the situations.

There are a number of methods soil labs use to measure soil organic matter. Some use an acid to measure the oxidizable carbon content of the soil. If you are curious, look up the Walkley-Black method. This method has been around since at least the 40's, and is particularly useful if the soil has a low amount of organic matter. However, it involves the use of dangerous chemicals, and careful laboratory workers. There is also a heavy metal waste stream to dispose of.

Another method of measuring organic matter is to heat a dry sample up to a high temperature (at least 340C), and measuring the weight loss as organic compounds are volatilized. There is a correlation between the weight loss and oxidizable carbon in the soil. This method has the advantage of less hazardous laboratory work, and no dangerous waste stream. It is particularly suited to soils with high organic matter. Look up the weight loss on ignition method for more details.

There are a number of other tests available, and a lab may choose one based on customer requests, or the nature of a specific soil.

If one were to measure soil organic matter at home, taking a jar filled with soil and water, agitating it, and it to settle, there are different results that can be seen, for the same amount of organic matter, depending on the soil chemistry, and maturity of the organic matter in the soil.

If the soil in the jar test contains mostly very mature organic matter (that is, the organic matter is no longer rapidly decaying in the soil food web), the jar test will likely not reveal the true soil content of organics. The organic compounds are no longer visible particles that float, and are instead chemicals bound to other particles in the soil. What you may see instead is a cloudiness in the water after settling which might indicate dissolved or suspended particles. You will likely still see layers corresponding to the sand and clay components of the soil, but many of the organic compounds will be bound (unseen to the naked eye) to these soil constituents.

If the jar test is performed on a soil which has had recent inputs of organics, there will, indeed, likely be materials floating or settling at the top of the contents. However, these materials do not represent all the organic content; the more mature chemical structures are not seen.

Hope this helps in interpretation of the jar soil test. I find that for my soil, the test is suitable for identifying clay and sand volumes, but completely inaccurate for soil organic matter. Much of my soil had a huge input of organic materials about 20 years ago, and the mature compounds now in the soil from that input do not show at all in a jar test.

Renais