This email inquiry arrived recently:
“Hi Micah, looking at the MLSN guidelines and our recent test results, all our levels are above what’s required. Does this mean, we only need to apply N until levels come down?”
Here’s how I answered, and I think that the soil report extract at the end is the better explanation.
It sort of means that, but you want to predict how fast the levels will come down. You don’t want to just wait until the next time of soil testing. See for example MLSN newsletter #23, and see specifically the section beginning with this text:
With MLSN, the interpretation is different. There’s no medium range for an element. There’s no high range either. There is simply the MLSN value, and the interpretation goes something like this. If the soil tests above the MLSN minimum, then one can be confident that there was enough of that element present to meet the grass requirements at the time the sample was collected. If the soil test result for an element is below the MLSN minimum, there still might be enough of the element to meet the grass requirements, but I’m not so confident about that anymore. I’m not so confident because a soil testing below the MLSN value has a test result for that element in the lowest 10% of turfgrass soils that still produce good turf. That is, we can certainly have good turf at that level of an element in the soil, but there aren’t a lot of soils producing good turf with the element that low, so proceed with caution—you’ll probably want to add that element as fertilizer. And keeping in mind that the grass is alive, and that the soil nutrient levels today are already lower than they were a couple weeks ago when you collected that soil sample, one needs to consider how much of an element is required to keep the soil from dropping below the MLSN value.
This too is relevant. This is from a section of ATC soil test reports:
If your soil test is above the MLSN guideline in Table 1, then you don’t need to add that element—at least not today. You can be confident that high quality turfgrass can be produced in soils with that quantity of the element. If your soil is below the guideline, you can have high quality turfgrass too. But you should be aware that there are few soils producing high quality turfgrass with nutrient levels that low. You’ll probably want to add enough of the element to raise it above the MLSN guideline. That’s only the situation for today. Is your grass alive? We hope so! If it is, then it is using nutrients. The nutrient content in the soil is going to be lower tomorrow than it was today—unless your grass is dead or dormant—because the grass uses nutrients. To really use the MLSN guidelines, we need to look not at today, but into the future. And that’s what I have done to generate the recommendations in this report.