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Growth Potential, Inflection Points, and an Optimum Date for Overseeding

Are the MLSN guidelines too low?

Since the development of the MLSN guidelines (a joint project of ATC and PACE Turf) in 2012, I’ve had the opportunity to write about and to discuss these guidelines for soil nutrients with many people. One of the questions that I’m asked regularly involves the use of a temperature-based growth potential to estimate nitrogen requirements. 

PACE Turf, who developed the turfgrass growth potential (GP), have prepared a useful document that can help turfgrass managers get started with GP calculations for their site. Yesterday I shared this .xls spreadsheet, with the necessary equations embedded, on Twitter.

In response to that post, I was reminded of an important concern that I am sure many people have related to the MLSN guidelines.

That is, are these new MLSN guidelines, being substantially lower than conventional guidelines, causing nutrient recommendations to be so low that turf quality may suffer? At first glance, it may seem that this could be the case. 

Gcm_jan_2014But if we look at how the MLSN guidelines were developed, and at how they can be implemented, the chance that turf could suffer from the use of these guidelines seems unlikely. I think it highly improbable that “someday someone will lose grass trying to hit your numbers,” as was suggested in the Twitter discussion. These five points explain why: 

  1. The MLSN guidelines were developed from a database of more than 17,000 soil samples taken from good-performing turf. In each of those 17,000 samples, no matter the level of a nutrient in the soil, the grass was healthy. From those samples, we evaluated the distribution of various soil nutrients and chose a 10% threshold for the guideline. That is, we chose a level at which 10% (more than 1,700 sites) of the samples had good-performing turf at levels below the MLSN guideline for a specific element. This serves as a buffer against inadvertently setting the guideline too low. 
  2. Grass won’t take up more nitrogen than is applied as fertilizer, and nitrogen (N) controls the demand and uptake of other nutrients. This allows us to estimate how much of each element will be used by the grass, and consequently depleted from the soil, based on how much N is applied.
  3. We recommend keeping nutrient levels at or above the MLSN guidelines. We can make some calculations, based on the MLSN guideline in the soil, and how much of an element is being used by the grass, to ensure the nutrient levels always remain above the MLSN guideline.
  4. The word “minimum” is in the guidelines. And this can lead one to think we are striving to apply the minimum amount of nutrients. In a sense, we are. But the process we use involves making an estimate of the maximum amount of an element (K, P, Ca, Mg, S) that the grass can use, and ensuring that that amount is supplied to the grass in more than adequate amounts, either from the soil, or by application of that element as fertilizer.
  5. Because the MLSN guidelines are based on a conservative subset of samples from good-performing turfgrass sites, and because utilization of the guidelines involves making an estimate of the maximum amount of each element the grass can use, rather than “lose grass” using these guidelines, I am confident that turfgrass managers will see equal or improved turf quality with the use of these new guidelines.

For additional information on the MLSN guidelines and this method of estimating turfgrass nutrient use and nutrient requirements, see:

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