Measuring organic matter

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.” Stardard 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.

This unmown manilagrass is an extreme example. There is a lot of organic matter in that profile. Let’s say we cut off the above ground plant material, and just do a standard organic matter test on the thatch. A lot of that material will be removed from the sample. Almost all of it would be removed, actually, and not tested, because it won’t pass through the screen.

grass with a lot of undecayed organic matter The thatch layer of this unmown manilagrass would be almost enitrely removed before testing for organic matter.

That was an extreme case. How about from a putting green?

a soil profile Soil profile from a bentgrass putting green.

The screening will remove most of the undecayed plant material.

organic matter screened from a soil sample Undecayed plant material resting on the screen while the soil that will be tested for organic matter has passed through the screen.

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?

Micah Woods
Micah Woods

Scientist, author, consultant, and founder of the Asian Turfgrass Center