Putting green organic matter by depth in the soil

I watched a Greenkeeper App meeting about organic matter in spring of 2020. The video doesn’t seem to be available anymore. It has Doug Soldat, Bill Kreuser, and Roch Gaussoin talking about soil organic matter, rootzones, sand topdressing, and turf performance.

They brought up some really good points, and raised a few questions. I made notes as I watched because there were a few of these questions that have really practical and effective solutions.

In particular, I’m providing an alternative answer to the question Doug asked at about the 22 minute mark, “What protocol would you recommend for sampling greens for organic matter?”

1– It’s useful to think of two types of organic matter in turfgrass soils. One meets the definition of soil organic matter as defined by the Soil Science Society of America: “The organic fraction of the soil exclusive of undecayed plant and animal residues.” Gaussoin et al. wrote about this at length in Characterization, Development, and Management of Organic Matter in Turfgrass Systems. In fact, that’s where I came across the link to the SSSA soils glossary. If you get a soil test done in the USA, and get a test for organic matter, it is almost certainly going to meet that definition. And the way it meets that definition, “exclusive of undecayed plant and animal residues,” is by removing those materials from the sample prior to testing. All the grass plants, all the living and dead roots, any thatch or mat, gets removed from the sample.

2– Oh, you wanted to adjust your sand topdressing or coring or verticutting to deal with thatch and mat? You’ll need to do a different test. I find it really useful to manage the sand topdressing, and the living plant material, and the undecayed plant material near the soil surface, by measuring it. And I call that the total organic matter.

3– I think of it like this. The organic matter number that comes from almost all tests in the USA is the humified material, excluding thatch and mat and plants. This is organic matter as per the definition. This is a really useful number for evaluating potential N mineralization, contribution to soil CEC, soil C content, and soil health.

4– Then there is another number that is a lot higher, especially near the soil surface where there is undecayed plant material. This is the total organic matter, which includes the humus plus the grass and roots and rhizomes and thatch and mat, all the living, dead, dying, and humified organic matter. This number is really useful to assess the effect of sand topdressing, to relate to playability and surface performance, and to plan future topdressing and coring and scarifying.

5– The total organic matter testing has been around for a while. Two excellent articles about this are Glasgow et al. (2005) with An investigation of organic matter levels in New Zealand golf greens, and Windows & Bechelet (2012) with What’s working which summarizes year 2011 measurements by STRI. The New Zealand data are with grass (verdure) cut off the sample. STRI data I presume cut the grass off too. ATC and BRT share data for the purpose of analysis and we leave the verdure on samples from putting greens. This table summarizes the mean values from the Glasgow et al. report from New Zealand, the STRI data from the UK, and the ATC & BRT data. ATC & BRT data are from golf courses in nine countries and cover measurements from all common cool-season (C₃) and warm-season (C₄) species used on putting greens.

6– You’ll notice that these are at a specified depths. It makes sense to measure at standard depths—I call this #OM246—because one can check what is happening right at the surface, and also see what is happening a little deeper in the profile. I like doing this in 2 cm increments because that’s comparable to the large datasets already existing. Going inch by inch would be comparable. Two cm is 0.8 inches so using the #OM246 depths will be a little more precise than going in increments of inches.

7– There was some discussion about verdure on or off. I prefer leaving it on for putting green samples. The total organic matter will be about 3 g kg^-1 higher when verdure is on. In paired samples from multiple courses with different grass types, the coefficient of variation on average is higher between greens for verdure-off samples than for verdure-on. That means the samples with the verdure removed are a little more variable. And if you’ve ever tried to cut the verdure off a putting green sample, you’ll understand why.

8– Roch recommended consistency in sampling. He also suggested that if you are starting testing anew, to sample just the organic matter layer of the green, whatever depth that is. I prefer the testing by depth, because the organic matter is higher right at the surface, and that surface is where sand topdressing ends up, is what the golf ball reacts with, and is where the water enters the soil. From Glasgow et al., “Once organic matter levels increased to more than approximately 10%, infiltration rate had declined to the point where frequent ponding of water on the surface of the plots was observed. This result strongly suggests it is the surface 25 mm zone that is most likely to limit the rate of water infiltration into sand-based turf rootzones.”

9– I’ve used units of % by weight here for convenience. These total organic matter numbers are weight loss on ignition. The soil sample is dried so it contains no water, then the sample is burned (ideally at 440 °C), and then the weight loss is measured. These numbers can also be expressed as g kg^-1. That’s grams of organic matter per kg of soil. In that case, 1% is 10, 10% is 100, and 9.1% is 91. There was some discussion about the units of fractions of a percent being a bit difficult to work with, and I agree. I like to use grams per kg sometimes as an alternative.

10– Doug mentioned the quantity of N in organic matter, and that’s a big number. Keep in mind that talking about N is now the organic matter by definition, the already humified material, and is not including the thatch and the other undecomposed plant parts in the sample. You can expect that to be 5% N, with 1 to 4% of that N mineralized each year. Thus, for a soil with organic matter of 20 g kg^-1 in the top 10 cm, if the soil has a bulk density of 1.5 g cm^-3, then there will be 150 g of N m^-2. That’s 30 lbs of N per 1000 ft². From 1 to 4% of that is expected to be mineralized each year—I usually use 2.5% as an estimate. That works out to 4 g N m^-2 yr^-1, or 0.8 lbs N per 1000 ft².

11– Doug also mentioned cost of testing. And Roch recommended testing every green. I recommend testing at least 6 greens per 18 holes for nutrients, pH, and organic matter by the way organic matter is defined. For total organic matter by depth, I’d test a minimum of 3 greens at 3 depths, per 18 holes. ATC is charging USD $300 for that amount of total organic matter testing, which seems reasonable for the information gained. I’ve made some limited measurements of within green variability (in organic matter, and in total organic matter by depth, and in nutrients) and compared that to between green variability. Because the within green and between green variability in my measurements has been similar in magnitude, I’d rather do enough testing to get an accurate average for the course, and then use something other than test results to determine management green by green.

12– Frank Rossi and I talked about all these things and more on a call for his Frankly Speaking podcast a year ago, but unfortunately the audio wasn’t good on the call going halfway around the world, so it never got released. We talked about organic matter accumulation and decomposition by climate, verdure on/off and the pronunciation of verdure, sand topdressing amounts, and lots about total organic matter testing by depth. Since I don’t have that podcast to share, if you’d like to hear me talk a bit more about this, I discussed the practical use of this testing in a video I recorded called A continuous improvement system for turfgrass.