OM246

Soil organic matter, and the total organic material measured by OM246 tests, are reported on a mass basis. That is, mass of organic matter per mass of soil. For convenience, this is usually reported as a percentage (%).

One of the things I’ve started looking at when doing OM246 reports is the combined OM in the top 6 cm (2.4 inches) of the rootzone. This shows the big picture of total organic material change at the surface of the rootzone.

I’ve written a lot about OM246 testing for total organic material by depth in the soil, and why I think this test is so useful. Something I hadn’t written, until this week, is my own set of sampling instructions.

Organic material measurement by mass loss on ignition is reported on a mass basis. For OM246 testing, I prefer doing a burn at 440 °C.1 The equation for loss on ignition (LOI) as a percentage takes the mass of sample dried at 105 °C ($m_{105}$), subtracts the mass of the sample after burning at 440 °C ($m_{440}$), divides by the original mass, and multiplies that by 100.

I’ve made some updates to the standard OM246 report this year. The portion of the report with charts is showing four distinct things, and I showed examples and explained them in this video.

I like to test the total organic material at the surface of a turfgrass rootzone in autumn. Doing the testing in autumn captures the majority of the growth, accumulation, decomposition, removal, and dilution that has happened during the previous year.

One of the many valuable things I utilize from OM246 testing is what I call the organic matter accumulation rate. I’m actually referring to the net accumulation rate—how the total organic material in the rootzone would change if no topdressing had been applied.

A substantial amount of organic material accumulated at the surface of a Tifdwarf bermudagrass putting green at Rota in the Northern Mariana Islands. I don’t recall if I read this exact statement, or if it occurred to me when I read something else about invigilation.

You can use OM246 test results to learn how total organic matter compares to general recommendations; better yet, you can evaluate how maintenance practices, growth, weather, and decomposition are changing total organic matter at a single site; and then there is what I call the killer feature of this testing: you can make a precise and site-specific calculation of organic matter accumulation rate and sand topdressing requirement.

When the total organic material is known at time A and at time B, the accumulation rate can be calculated. By accounting for the sand that was applied between time A and time B, another accumulation rate can be calculated: the organic matter accumulation rate independent of sand application.