"Do the MLSN guidelines use 17 essential nutrients?" a friend asked me last month. "I've found a couple good articles but couldn't really find a number on the MLSN website."
"That's an interesting question," I replied, "and I may answer it on my blog. I can explain the 17 and you'll be an expert."
Here's how it works, and how soil testing and the MLSN guidelines fit in. I'll describe this in four sections.
First, what is an essential element?
To be classified as essential, the element must meet three criteria, as described in a classic 1939 article by Arnon and Stout. An element is essential if:
- a deficiency of that element makes it impossible for the plant to complete its life cycle.
- the deficiency can only be corrected by supplying that element; the function of that element cannot be substituted by another.
- it is directly involved in plant nutrition (plant metabolism), and is not merely correcting a soil chemical or microbiological condition.
Second, what are the essential elements?
Or more specifically, where does the 17 (or sometimes 14) number come from?
Well, we can start with carbon, hydrogen, and oxygen. These elements are what Carrow et al. call the basic nutrients. These elements are never required as fertilizer, because they are never deficient. The grass gets them from CO2 and H2O.
Then there are what are usually called the macronutrients. These are nitrogen, potassium, and phosphorus. These elements are often required as fertilizer.
Next come the secondary nutrients. These are still in the macronutrient range (more than 0.1% [1000 ppm] by dry weight), but are rarely required as fertilizer. The secondary nutrients are calcium, magnesium, and sulfur.
And then there are the micronutrients. These are used in small amounts by the grass, from less than 1 to 500 ppm in the leaves. The micronutrients are iron, manganese, copper, zinc, boron, molybdenum, chlorine, and nickel.
If you add all those up, the basic, macro, secondary, and micronutrients, you get 17 in total. That's where the 17 number comes from. Because the basic elements carbon, hydrogen, and oxygen are ubiquitous and are not applied as fertilizer, those three are often omitted from discussion and the list of essential elements is given as 14 in total. And occasionally there will be one of the micronutrients omitted; for example, the excellent
Turfgrass Fertilization: a basic guide for professional turfgrass managers from Penn State omits nickel and gives the total as 16 elements. Which is fine, as I'll explain in the fourth section, below.
Third, do the MLSN guidelines use all 17?
Yes. And no. The MLSN guidelines provide a framework for ensuring that any grass, at any location, will be supplied with all the nutrients required by the grass. And yet the MLSN guidelines only list a minimum value for five elements: potassium, phosphorus, calcium, magnesium, and sulfur.
The MLSN guidelines are used to interpret soil test results. We don't soil test for the basic elements. Those are never deficient. And one doesn't make fertilizer decisions about nitrogen for turfgrass based on soil tests either, so we don't include nitrogen in MLSN (see N & MLSN, what's the connection). All the other macronutrients and secondary nutrients have a minimum guideline using MLSN. And we deliberately don't worry about micronutrients too.
Fourth, what about micronutrients?
I don't worry about them very much. I've explained this in detail in these two posts:
Quoting from my comment in one of those posts:
the quantity of micronutrients the grass uses is so tiny as to be almost negligible. And we constantly keep the growth of the turf -- and consequently its demand for nutrients -- restricted by applying less nitrogen than the grass can use. Thus the probability of a micronutrient being deficient is very low ... Let's say grass uses 10 g N/m2 per year. It uses progressively less K, then P, then Ca, Mg, & S. By the time we get to the most used micronutrient (Fe), we are looking at only 0.025 g/m2 per year. 25 mg! And the other micronutrients are a fraction of that. In practical terms, there is almost no way a micronutrient can be deficient in turfgrass, and it is so easy and cheap to just spray out a complete micronutrient package at a tiny dose. There is really no need for soil testing for micronutrients in turf.
Which is why I think it is fine that the guide from Penn State omits nickel. When it comes to the micronutrients, the grass uses such a tiny amount that one doesn't have to worry much about them. And if you are worried about a micronutrient deficiency, or want to be especially sure that the grass has enough micronutrients, then it is easy and inexpensive to apply all the micronutrients that the grass can use. There is no excuse for a micronutrient deficiency.