A little more data to support an anecdote
27 May 2016
Yesterday I wrote about soil organic matter decreasing over a 3 year period, even though the greens had only been cored twice in that time, and sand topdressing amounts had been reduced each year.
When I think about reducing organic matter, I usually think of removal or dilution. Removal would be through coring or scarification; dilution would be by mixing sand with the organic matter.
But in this case, I think the organic matter in the soil is going down because the organic matter production is less than the organic matter decomposition. The reason I think this is simple. There hasn't been much removal or dilution of organic matter in the past 3 years, but the organic matter has still gone down.
In the comments to yesterday's post, there was some discussion of layering if sand was not applied often enough. I agree that undesirable layering might occur, but only if the grass was producing organic matter faster than it was decomposing.
To put this into context, I added up the volume of clippings from the greens in 2015, to give some idea of the growth rate at which the maintenance work described yesterday has led to a decrease in soil organic matter.
Add that up for the year and it is 270 L/100 m2. Measurements of the fresh weight of clippings on these greens give 0.3165 kg for each liter of clippings, so that is 85 kg of fresh clippings per 100 m2. I expect these clippings are about 70% water and 30% dry matter, so I've estimated the dry weight of the clippings at 26 kg/100 m2.
That gives three estimates of how much the grass is growing at this location. Those numbers might be useful if you'd like to compare the growth of grass where you are.
As an aside, these types of calculations are how I estimate nutrient harvest. If you've been to one of my seminars about how to use the MLSN guidelines, I will have described that the use of the guidelines involves taking the amount the grass will use (I'll call that a), adding that to the amount I want to make sure remains in the soil, which is the MLSN guideline (I'll call that b). These values a and b, together, are the amount of an element we want to be sure is present. a + b represent the amount we want to have. The amount we actually have is measured by the soil test, and I call that c. It follows that the amount of an element required as fertilizer is the amount we want to have, minus the amount we do have, represented in an equation as a + b - c.
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