Reading and Interpreting Fertilizer Labels

[BobL]

Chemically, salt is a general term used to describe an ionic compound formed from the reaction of an acid and a base.

NaCl, Na2SO4, KCl, KAI(SO4)2 are all salts (the latter is a double salt) as are hundreds of other compounds, with a broad range of color, melting point, taste, solubility, Oder, etc. characteristics.

Table salt -NaCl- has acquired the generic name “salt” in common usage, but it’s quite a bit different from the molten BaCl2 salt we used to use to temper/carburize steel.

[BobL]

A majority of fertilizer ingredients are salts. The salt index relates their osmotic potential to that of sodium nitrate. 100 is just a defined index, much like you can buy gas with an octane rating from 85 to 110+. 100 isn’t a magic number, it’s just the knock resistance if pure octane, to which others are compared/indexed.

Greater osmotic potential makes it more difficult for plants to extract water from the soil. This can be especially damaging for new seeds, preventing them from germinating.

At least this is what I remember on the topic from research years ago...

[andy10917]

OK, so let's look at what happens in the plant (grass in this case) rather than focusing on the number used in the Salt Index...

Osmosis (or "osmotic pressure") is the process of a solvent (water in our discussion) moving from an area of low concentrations of a solute (fertilizer salts/nutrients in our discussion). Normally, the salts/nutrients are more concentrated in the plant and therefore the water moves into the roots from the soil. If we use too much fertilizer at once, though, the salts are more concentrated in the soil - and the water movement slows and eventually stops. We see this as slow growth and wilting. It can also mean that nutrients that are needed in tiny amounts but toxic at higher concentrations can reach toxic levels in the grass and directly cause damage.

The Salt Index measures the potential of a specific fertilizing nutrient to cause these problems. It's far from perfect, as it doesn't consider the amount of the nutrient applied, but it does provide some relative data about the potential of a fertilizer to be a problem. For example, if we want to apply Potassium, we have an Index of 42.6 if we use Sulfate of Potash ("SOP") but the Index value is 116.2 if we use Muriate of Potash ("MOP") instead. The potential for damage is higher for the MOP. So, why do people use MOP instead of SOP? It's much cheaper.

This is why our fertilizer plans don't try to apply the full amount of a nutrient to address a deficiency at once in a single application. instead, we try to spread it out over time and multiple applications that will not cause wilting or burning. We stay below the levels that would cause problems. It's important to note, however, that the assumptions include that adequate water is applied when fertilizers are applied.

If you do further research, be careful not to read too much material related to agricultural crops - these often contain solutions that can't be used on lawns, such as placing fertilizers in furrows between rows -- there are no furrows or areas between rows in a lawn, and you can't mix it into the soil by plowing, without destroying the lawn.

There are other effects of fertilizer salts on the microherd that are problems, but let's concentrate on one item at a time and understand it before moving on...

Questions/comments?

[andy10917]

Anyone else with questions/comments before we move on?

[andy10917]

Last chance - then I move on.

[andy10917]

I think I'm going to end this thread - there just aren't enough folks using it as a directed-study thing.

[micvog]

Don't do that!

What's the next thing to understand?

[turf_toes]

I suspect this thread has more lurkers than active posters. I decibels have been following along at home

[likeasponge]

Following.

[andy10917]

It doesn't need followers. It is self-study with me providing guidance and answering questions. I know the answers to this stuff, so I'm not going to do the amount of reading to get comfortable for folks.

I'll keep it going, but I need some indication that folks are doing more than reading the few paragraphs I write...

[bpgreen]

I've got experience tesching and mentoring and I understand the approach I'd trying to guide people to the right answers (or sometimes to the right questions). I also consider myself fairly literate in reading and interpreting labels.

But I'll admit that I've been befuddled at the direction you're trying to guide us in much of this exercise, so after my first few attempts to engage, I've kind of taken a back seat.

I'm not saying it's your fault.

Wait. Yes, I am.

You need to do a better job explaining what you want from us.

Many of us are willing to learn and at least some of us are willing to engage. But after the first few posts, I haven't been able to figure out what you want that we're not giving.

I don't like feeling like an idiot, and I suspect that is true of most reasonably intelligent people. Maybe if you tried to be less condescending and more helpful, you'd get more/better responses.

But maybe that's just me.

[andy10917]

My intention was to provide some basic topics to research, and then have folks research them until they got confused or had questions, and hopefully more-experienced members (or me) would break the logjam for the less-experienced. Guided research with discussion where the going gets tough. I got a little frustrated when there was just me posting, and I didn't think that a couple of posts by me were going to achieve the intended result.

Maybe this topic is better in-season, when there is more traffic. No intention to be condescending, just trying to make some deeper conversation going during the quiet time of the year.

[Spacklerstyle]

Hi,

I am new to this forum, but have been reading posts for a while now. This is an intriguing topic/excercise, so I thought I’d jump in. Howdy all.

Research tells me the definition of a salt is any chemical compound with a positive and negatively charged ion, which means all fertilizers are salts, right? Once they are dissolved in the soil, they increase the salt concentration of the soil solution, which in turn increases the solution's osmotic potential. The greater the osmotic potential, the more difficult it is for grass to extract the soil water they need for growth.

So, I guess the question and answer is, which fertilizers are “better” (i.e., least potential for burn, and greater “effieciency” given the same elemental weight), which I would guess would be the ones with ingredients at the lower end of the table? (Hence the reason SOP is “better” than SOM)?

[andy10917]

I look at it more like a set of options to be balanced and compared, or like a pallet of colors that a painter mixes and uses. If you have a lot of deficiencies in nutrient levels and a large lawn (acres), it may be prohibitively expensive to use the "best" fertilizers (top ingredients) for a couple/few years until the levels rise to reasonable levels. That may not apply if your lawn is 3000 sq ft - the increased costs will be tolerable. In the large-lawn case, using cheaper ingredients carefully will keep costs down and avoid the possibility of the lawn owner quitting the remediation process, and then switch to higher-quality fertilizers when the "tweaking/maintenance phase" is reached. Optimal? No. Pragmatic? Probably.

Research tells me the definition of a salt is any chemical compound with a positive and negatively charged ion, which means all fertilizers are salts, right?

If by "chemical" you mean "synthetically produced chemical", then yeah that's probably pretty true. But numerous fertilizers are organic molecules of chemicals (soy bean meal, feather meal, Milorganite, Bay State, etc) and while they are technically "chemicals", they have very, very low salt indexes - Milorganite has a salt index of 2, IIRC. They use the Nitrogen Cycle and the microherd to convert the organic molecules to available nutrients. But they are slower to act and tough to use alone in an extended remediation program.

So, there is a place for lots of fertilizers, but it takes a bit of thinking about where they fit in your lawn's planning.

[Spacklerstyle]

Interesting. Makes total sense.

So, here’s a follow up question to link the previous topics... In the label example used earlier, if we were limiting Potassium to 1lb/K, we’d only have 1/2lb N /K. If the program called for 1 lb N/K, I am guessing we’d want to supplement an additional .5lb/K with an N source that is much lower on the salt index (since Urea and Ammonium Sulfate are on the higher end). Something like IBDU or Milorganite (assuming the P in Milorganite was not discouraged as a result of a soil test)?

So, the dumb question I have now is... are most slow release N sources going to also be way down on the salt index and/or vice versa? The suggested science behind this makes one think that would be true.

[andy10917]

In the label example used earlier, if we were limiting Potassium to 1lb/K, we’d only have 1/2lb N /K. If the program called for 1 lb N/K, I am guessing we’d want to supplement an additional .5lb/K with an N source that is much lower on the salt index (since Urea and Ammonium Sulfate are on the higher end). Something like IBDU or Milorganite (assuming the P in Milorganite was not discouraged as a result of a soil test)?

Yup! Assuming that you want/need to apply a lb/K of Nitrogen per month, you could supplement the fertilizer app with Milorganite, etc to get to the full lb/K monthly of Nitrogen. I often recommend the "Vitamin M" off-cycle, meaning that if you're applying on the first of the month, apply the Vitamin M on the 15th, halfway between the main fertilizer apps. Will you technically go over the Phosphorus maximum? Yes, but it's in a different form (organic molecule) and I've never seen an issue when I do that (and I do it a lot). This approach smooths out the boom/bust cycle of Nitrogen supply too.

are most slow release N sources going to also be way down on the salt index and/or vice versa? The suggested science behind this makes one think that would be true.

Good question and not-so-straightforward to answer. The original Salt Index was designed in the 1940's, and is the potential of the ingredients in the fertilizer to "burn" the plants it is applied to. It is indexed (value 100) to the potential of Sodium Nitrate, a popular fertilizer during that period. Numbers higher than 100 are more likely to burn and those under 100 are less-likely to burn plants. There were (and are!) problems with the index - it doesn't account for how much of the fertilizer components are in the particular product. There have been efforts over the years to correct for that. It also doesn't account for time-release products (Sulfur and/or Polymer coatings, etc), which is where my guess is that addresses your question. Bottom line: the Salt Index has some comparative use for ingredients, but is too old to account for almost 80 years of advances in fertilizer technology. It does work somewhat for cheaper low-tech fertilizers and comparison shopping.

[Green]

Andy,

I have at least one topic I'd still like to cover at some point. It has to do with figuring out how much of the bag content in a "hybrid" product (e.g. biosolids, urea, and AMS) is derived from biosolids...when all the label lists to help with that is the total slow release content. Can we please try to figure this one out at some point in this topic? I've struggled, sitting there with a calculator for an hour, trying to do guess-and-check to make the math come out right, and only ever got sort of close. Whenever you're ready for this one, I can post the question, an example or two, and the details of what I'm trying to figure out.

Thanks!

[andy10917]

Green, go ahead. This thread goes where there is interest...

[andy10917]

Green, do you have specific questions about "hybrid" products, or should I start a general discussion of them?

[andy10917]

It is pretty hard to figure out the proportions of nutrients coming from organic/natural ingredients from the label - when the laws were created for fertilizer labeling, the focus for the most part was inorganic fertilizers - so "hybrid" products can be difficult to discern.

But there are some general things you can look into, and there was recently a product that I could see had labeling issues. Here's the thought process that made me realize there was an issue:

The label listed Ammonium Sulfate as the primary source of Nitrogen, and Ammonium Sulfate is 21-0-0 NPK (and 24% Sulfur). Most organic/natural nutrients are much less-dense (lower in NPK concentrations), from Alfalfa down at 3% to Soybean Meal at maybe 7%. Blood Meal is an outlier at 13% Nitrogen. So, if Ammonium Sulfate is the primary Nitrogen source at 21%, any other ingredients would be diluting the percentages and certainly not raising them.

So, how was the hybrid fertilizer product listed at a HIGHER Nitrogen number than 21%? There had to be another even higher/denser Nitrogen fertilizer that was raising the Nitrogen content percentage - but it wasn't in the label info.

It turns out that there was an (probably accidental) omission on the label, and the product contains Urea (46-0-0). That would do it - Urea at 46% would raise the mixture ABOVE the 21% Nitrogen percentage overall. I believe that the vendor corrected the label.

So that's a way of thinking about the label info in a "hybrid" product - they generally will not be as high in NPK percentages, because the lower density of nutrients in organic/natural sources "dilutes" the higher nutrient density of synthetic/manufactured products.