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Measuring organic matter

From Dr. Micah Woods’ blog posted on June 10 from the Asian Turfgrass Center:

In a previous post, I mentioned that I’ve been thinking
a lot about sand.

I expect that sand is required for managing playability
of sporting surfaces. I’ll writing in terms of golf course putting greens in
this series of posts, but the principle applies to any turfgrass sports
surface. And I expect that sand is specifically used as a way to modify the
organic matter content of the soil, especially near the surface.

I would like to be able to give a good answer to the
question, “How much sand is required,” and I want to consider for a moment how
organic matter is measured.

The Glossary of Soil Science Terms defines soil organic
matter as “the organic fraction of the soil exclusive of undecayed plant and
animal residues.” Standard soil nutrient analyses, and the organic matter
reported on those reports, are providing a measurement meeting that definition.
Undecayed plant and animal residues are removed by passing the soil through a 2
mm screen before tests are conducted.

It makes sense to me, for turfgrass soils, to do a
total organic matter test on undisturbed samples cut to depth. That is, the lab
can measure the organic matter on the samples as they receive them from the
customer, without passing the sample through a sieve.

Turfgrass managers can then know a few different
organic matter numbers. One number is the one that meets the definition of soil
organic matter: the organic fraction of the soil exclusive of undecayed plant
and animal residues. Other numbers are different—they are the total organic
matter content of the soil from unscreened samples.

I think it makes sense to standardize the depths of
these samples too, to 0 to 2 cm (that’s just under an inch), and then if you
want information from deeper in the profile, then do also from 2 to 4 cm, and
from 4 to 6 cm, and so on. This is the method of the NZSTI and Glasgow et al. wrote
about this in an excellent 2005 article. I believe Doug Linde has been doing
some testing along these lines too. And the STRI and others in the UK do
testing like this too.

There are a few other questions that come up too. Like
should the verdure be left on the sample, or cut off? And because all these
will be done by mass loss on ignition, what temperature should the burn be done
at?