Why is the grass so good, but the soil test results so bad?
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Misunderstanding the MLSN Guidelines

It is great to see the Turf Nutrition Week webinar series on TurfNet. This program, sponsored by Grigg Brothers, is on an important topic. If turf is not supplied with the right amount of nutrients, it will not perform as it should. 

I was pleased that the MLSN guidelines were discussed in the first webinar (you can watch it here - Nutrient Fate and Use: Solutions for a Changing Landscape), although I was disappointed that the guidelines, the way they work, and the thinking behind them, were not understood. I've written a lot about how to understand and use these guidelines, for example here, here, and here, although I understand that not everyone will have read about this. Let me try, then, to give a real quick overview of MLSN.

  1. The guidelines are not themselves a target number. They are the level one doesn't want to go below. That is why they are called minimum
  2. Fertilizer recommendations are based on ensuring the soil does not drop below this minimum level.
  3. Using this approach, the amount of fertilizer recommended is the amount that the grass is expected to use, plus the amount necessary to keep the soil at or above the MLSN guideline level, if necessary.
  4. Use of the MLSN guidelines then ensures that the maximum amount of nutrients the grass will use are supplied.

It is no secret that conventional guidelines are broken. You can take my word for it, read what another soil scientist has to say about this, or just take it from the textbook on this subject, Soil Fertility and Chemical Problems by Carrow, Waddington, and Rieke:

More research is needed on soil test calibration for turfgrasses ...

In some cases, turfgrasses have been placed in a "high" P and K requirement category, while pasture grasses were in a "low" category. This decision was based on economics, not agronomics. The cost of fertilization was not considered of primary importance for turf. 

The MLSN guidelines and the resultant fertilizer recommendations are designed to supply the grass what it needs, based on agronomics, not economics. We absolutely have to supply the grass with all the nutrients required to produce the desired surface. And that is just what the MLSN guidelines do.

In the question and answer session of today's webinar, Jim Murphy and Jon Scott discussed whether the use of the MLSN guidelines leave turf managers exposed to potential problems. Since the fertilizer guidelines based on the MLSN guidelines will supply any grass, anywhere in the world, with the maximum amount of an element it is expected to use, while still keeping soil at or above the minimum safe level (the MLSN guideline), I don't think we are leaving ourselves open to those kind of problems at all.

And that leads me to pose another question. What justification is there for recommending application of more of an element than the grass can use, or that the soil can hold?


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