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The soil test numbers are almost double MLSN standards and I'm still getting recommendations to apply more

That arrived in my inbox recently, plus a few questions about calculating K requirements using the MLSN guidelines, and whether if there are minimum levels, are there optimum levels too? Here's how I answered.

It sounds like you are on the right track. Here's a few general remarks based on what you described/asked:

If the lowest K on that course's greens is 57 ppm, you are great. You are mostly Poa on those greens, right? The grass will use about twice as much N as it does K. Therefore, my suggestion is to apply half as much K as you do N, and that should keep the soil at about that level well above MLSN. Check a year later and see if the trend is going up or down. If going up, you can cut the fertilizer, and if going down you might increase it. What you are basically doing is applying 100% of what the grass is using plus you are keeping a nice reserve in the soil. That is a safe way to do it and it is not gratuitous overapplication.

For the other courses, it makes sense to let the numbers get a little closer to the MLSN minimum. I would make the calculation based on the soil test and the expected N application rate. The 22 ppm change in soil concentration is correct for a pound, if you are thinking of a 6 inch deep rootzone. For putting greens -- actually for most mowed turf in general -- I think of the rootzone as being 4 inches. In that case you can expect a pound of application per 1000 ft2 to increase the soil by 33 ppm. You can expect the harvest of a pound per 1000 by the grass to reduce the soil by 33 ppm.

If you apply 4 pounds N, expect the grass to harvest about 2 pounds K (50% K use compared to N). MLSN at 37 ppm is about 1 pound of K. That's always going to be there in reserve; we don't want the soil to drop below that. If you have a soil test at 45 ppm, then the way to calculate a fertilizer requirement is like this.

Amount needed is amount to keep in reserve (the MLSN minimum) plus the amount the grass will use minus the amount actually present. I'm going to say you calculate this for a 4 inch rootzone depth and you will apply 4 pounds of N.

That is 2 lbs of K use + about 1 lb K needed as the MLSN minimum - (45/37 = 1.2 lb in the soil now). That is 3 - 1.2 = 1.8 pounds of K required to keep the grass from dropping below the MLSN guideline. I suggest dividing that 1.8 pounds (your number is going to be different, I am just showing how I make the calculation) into as many apps as possible and then applying it through the season. Or, if you are applying 4 pounds N, then you have a 4 to 1.8 ratio of N to K, and just apply close to that ratio at every application.

It can be simpler than that, but that is the detailed work through of the calculations. The really simple way is if the soil is less than 50 ppm, apply N and K in a 1:1 ratio. That is bound to make the soil K go up, so you will be sure to be staying above MLSN. If the soil is in the 50 to 75 ppm range, apply 2:1, and that should keep the soil at a similar level. When the soil is above 75 ppm, so long as you aren't applying a ton of N, you probably can get by with little or no K.

I prefer to make the exact calculation. And then check the grass response to the fertilizer and adjust inputs accordingly.

Oh, is there an optimum? I don't think there is. I think there are problems if you get too low with any element, but once you move from "too low" to "enough", then all the benefits from that element have happened. The MLSN approach is designed to prevent you from getting "too low" and to make sure you are always in the "enough" range. Once there is enough, the problems are expected to be from traffic, or dry spots, or shade, or whatever. The idea with MLSN is to provide a framework to put every element in the "enough" range and then you can focus your limited time and energy on the other things that might be affecting turfgrass performance.


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