Soil nutrients and extra beer

This is about the quantity of nutrients in reserve, the quantity of nutrients in the soil that the grass doesn’t use, when one keeps the soil nutrients at or above the MLSN values. I find it useful to think of this in reference to the well-known beer analogy.¹

In that beer example, I want to keep some amount of beer in reserve, to make sure I don’t run out of beer at a small party. I could describe the beer analogy as a way to figure out how many beers I need to buy when I’m inviting a few friends over to watch a game and I want to make sure I have an extra six-pack or two in reserve. When I was preparing for a presentation² about turfgrass nutrition, I realized that someone could argue against this by saying, “I’m not comfortable with a six-pack in reserve; I want 50 beers in reserve.”

I wrote this note:

To point out how safe MLSN is in this regard, make a calc sometime of K, P, Ca, Mg, and S minimums and show them in terms of expected time to use all, rather than as ppm.

And here’s what those expected times are. For this example:

• I’ll use creeping bentgrass leaf nutrient normals
• I’ll use the annual clipping harvest we’d get in the Chicago area with a Turf GvX of 40 and a volume to dry mass conversion of 0.063

With those straightforward calculations, for a 10 cm (4 inch) rootzone depth with a bulk density of 1.5 g/cm³, the MLSN values for each element work out to a minimum of 2.7 years of plant use for sulfur, and a maximum of 120.5 years of plant use for calcium.

Remember, the way MLSN works is we apply nutrients to keep the soil from dropping below these values. These nutrients are not used by the grass. They are just there, in the soil, as a reserve that is untouched. And if the grass were to dip into this reserve, these values represent years of plant use.