Organic material accumulation rate

We find it useful to calculate the accumulation rate of organic material for a few reasons.

First, knowing the accumulation rate allows us to predict exactly how much sand is required to adjust the surface layer of the rootzone to a particular total organic material percentage.

Second, by checking the changes in accumulation rate over time, and between zones, you can learn how differences in maintenance practices (like nitrogen rate, compost additions, thatch decomposing agents, core removal, biostimulants, or sand addition itself) or changes in growing environment, affect the accumulation rate.

Third, knowing the accumulation rate makes it possible to compare with normal accumulation rates for a particular species or region. This can be useful when planning adjustments to maintenance practices.

What do we mean by accumulation rate? We mean how much total organic material is produced per unit of soil per unit of time. A standard unit would be grams of organic material per kg of soil per year.

Calculating this is not as simple as comparing OM2 test results from one year to the next. If the OM2 was 5% (5% by mass is 50 g/kg) last year, and the OM2 test this year was 5.5% (55 g/kg), we can’t simply say the accumulation rate was 5 g/kg/year. That’s because almost every location with professionally managed turfgrass adds some amount of topdressing. That topdressing material, usually sand, changes the mass of the soil, and changes the depth of the soil layer being assessed, and this needs to be accounted for.

If we applied no topdressing sand, and if the OM2 went from 5% to 5.5%, then the total organic material accumulation rate would be 5 g/kg/year. That’s rare, though, to apply no topdressing sand. Let’s say 2 mm of sand (31.2 tons/ha, or 638 lbs/1000 ft²) were applied through the year. If 2 mm of sand were applied and the OM2 went from 5% to 5.5%, that’s a much higher organic material accumulation rate of 11.3 g/kg/year. Why is this? It’s because the sand addition should cause the organic material to decrease, but after accounting for that expected decrease, and the changes in soil mass and layer depth with the added material, the amount of organic material produced to reach the 5.5% OM level needs to be 11.3 g/kg/year.

Let’s look at the 0–6 cm surface layer, and consider the minimum standard sand recommendation of 6 mm (94 tons/ha, 1,915 lbs/1,000 ft²). If the total organic material is 4% in the 0–6 cm layer, and after one year and 6 mm of sand addition, the total organic material remains at 4%, the organic material accumulation rate is 4.86 g/kg/year.

Why is the accumulation rate not 0 when the starting and ending organic material stays at the same 4%? It’s because the sand addition should make the organic material percentage in the soil go down. But the organic material in this case didn’t go down, because there is an accumulation rate.

This turns out to be useful for the three reasons noted at the start of this update. I’ve called this the killer feature of OM246 testing, and I encourage everyone who does OM246 testing to calculate the accumulation rate.

You can make these calculations with the OM246 calculator.