Richard

My impression is that too many people are focused on what formula to use without understanding what quantity to apply.

The N-P-K rating of a fertilizer is percentages, not quantity. Fruiting bananas in the ground or a very large container will utilize up to 1 net pound of nitrogen and 1.5 net pounds of potash per year. Note that the potash can only be utilized to the extent that nitrogen is present.

For fruiting bananas in non-tropical regions, the proportion of 1 part nitrogen (N) to 1.5 parts potash (K) is an excellent ratio. The following fertilizers can all provide it: Alfalfa Meal (2-0-3), Geritz Garden Mix (10-5-15), Scotts Miracle-Gro For Fruits (16-8-24), Grow More 20-5-30.

Getting a good formula is not enough. You also need to compute the amount to use.

Alfalfa Meal 2-0-3: 1-lb Nitrogen divided by 2% ==> 50 lbs per year per plant.
Geritz 10-5-15: 1-lb Nitrogen divided by 10% ==> 10 lbs per year per plant.
Scotts 16-8-24: 1-lb Nitrogen divided by 16% ==> 6.25 lbs per year per plant.

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Bob

FINALLY I think I get it now.

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turtile

To make it more confusing, P and K aren't really percentages of the element. P is measured as the percentage of P2O5 (around 44% P), and K is K2O (around 83% K). You would have to add more K to the ratios above to really net 1.5lbs of K.

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conejov

Interpreter please? Some one have a Dummies to fertilizer ratios guide?...Im sorry but Im alittle slow..

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turtile

If you buy a 100 pound bag of 10-10-10

10 N = 10 lbs Nitrogen
10 P (P2O5) = 4.4 lbs Phosphorus
10 K (K2O) = 8.3 lbs Potassium

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Richard

John,

The ratios I'm referring to are for available nitrogen (N), phosphate (P), and potash (K).

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Patty in Wisc

That's OK Alex, I got strait A's in math & science, but I don't understand what these equations are either. They just don't make sense to me.
I use all water soluable stuff, so 50 lbs of anything sounds like a lot to me!
I just put my compost on & fert w/1 Tblsp or 1 tsp. per gal of water depending on which fert I use, & it works fine. Sorry if I'm ignorant Gang!

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Richard

These equations are "proportions" which in the U.S. are now taught in 7th grade math. As an example:

Suppose your fertilizer is 15% available nitrogen and you want to know how much will provide 1 net pound of nitrogen. Your equation to solve is:

X * 15% = 1 lb

so

X = 1 lb / 15%

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turtile

All of the ratios you provided use P2O5 and K2O. Unless you mean that the ratios net 1.5lbs of K2O equivalent and not just K.

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chong

I think that by introducing the issue of the chemical compounds, from where each of the three N-P-K elements are derived, it is just making things more complicated than we need to be. We can probably all agree that those elements, N-P-K, are not in their elemental form, but rather as part of a compound, e.g., NO3, P2O5, K2O, etc. The labeling that we read is mandated by law to reflect the percentage availability of the elements and not the compound that they are part of. If percentage of the compounds were to be listed on the labels, i.e., NO3-P2O5-K2O, they would deviate from the intent of the labeling, which is to confirm the availability of each of the elements N-P-K.

As an example, an N-P-K formula of 10-10-10 might have an equivalent to 16-25-10 for NO3-P2O5-K2O. But if you are calculating the amount of each element you would like to feed your plants, it would be impractical to list the latter. See the link below:

Understanding Fertilizer Numbers: N-P-K

turtile

It doesn't matter which compounds are present in the fertilizer. P and K must be represented as P2O5 and K2O by law. Before modern technology, fertilizer was tested in the oxide form.

If you buy muriate of potash (KCl), the label will read 0-0-60. While the actual K content is more around 50%.

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adrift

Golly gee, it's easy to see y'all are plant people cause you re-use and mix up the names of everything.

Correctly restating your quote: The ratios I'm referring to are for available nitrogen (N), phosphate (P2O3), and potash [fertilizer](K2O)

P is phosphorus, an element; not phosphate.
K is potassium, an element; not potash.

To keep everyone on their toes...
"Real" potash is a mixture of compounds, mostly K2CO3 = potassium carbonate but also many (MANY) naturally occurring salts of this compound. Probably because this stuff was first used hundreds of years before people knew enough chemistry to figure out what the compounds in their ashes really were.

BUT, there is a chemical called "potash fertilizer" which is potassium oxide K2O, which isn't even used as fertilizer because it is so caustic. But it IS the standard measure of the amount of potash IN fertilizer.

So, in reality the potash fertilizer IN fertilizer is not "potash fertilizer" but some mixture of potassium carbonate, potassium nitrate (salt peter), potassium chloride (muriate of potash; "salt substitute"), potassium sulfate, potassium permanganate, or potassium hydroxide (potassium lye).

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Patty in Wisc

Thanks Chong & KJ.

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adrift

Perhaps, but it is what it is.

True.

False.

While, for simplicity, many people say it is "N-P-K" and imply that it is the pure elements, (and say so on web sites), by government definition it is nitrogen, phosPHATE, and potASH.

Don't go to gardens-R-us.com, look at a university or government site.

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adrift

Bingo!

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adrift

Here are your homework problems for tonight. Don't skip #3 or #5 they are informative about labeling.

Practice Problems for Fertilizer Analysis

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CValentine

WOW!!!
I actually enjoyed working through that!
I have learned something new today, Thank you!! ~Cheryl

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island cassie

Haha! hopefully I won't be tested on that! My excuse is that I can't go with a shopping list and buy anything I need - whatever I see - I buy in case I never see it again - then figure out how to use it. A different slant perhaps?

chong

That opening sentence was the whole purpose for the post. It was an appeal to simplify the discussions in layman’s terms rather than to spark additional motions or arguments that only most people in academia will understand. I do not believe that Richard’s initial post was anywhere near as complicated as this is now turning out to be. If we were to go back to that post, what I think Richard wanted to relay was that if you want to feed a plant a certain amount of nutrient per year, i.e., N, P, or K, all you need to divide the pertinent nutrient by the percentage of the nutrient to arrive at the amount of fertilizer you will need to feed that plant.


Hold that thought . . .


Indeed you are right. And I will concede that if accuracy is critical, a university or government site would the place to go. But I haven’t found any of those sources that clearly explain these facts to the same detail as you, when applying those formulas for plants. In fact, most of the ones I’ve seen only go so far as the way Richard figured the amounts in his original post.

As I explained in my opening statement, the bottom line is: to what degree do we carry this accuracy? Do we use a scale and weigh the fertilizer every time we fertilize? Or, do we just rely on the label of the product, and then just eyeball the amounts when dividing? And if we have several plants of varying requirements, would we need to meter the amount of fertilizer at each plant and record the number of times that they’ve been watered? Perhaps your point is somewhere in between. If so, why worry about the 10% in N actually amounts to only 8%. I don’t think the plants will know the difference.

I believe that when garden writers give advice, the advice they give will help in almost all cases. If they refer to the percentages in the label as a gage to apply the fertilizer, I believe that in most cases that’s what they use themselves.

I don’t know if anybody else actually figures exactly how much to feed their plant like you. Personally, I just use the formula that Richard listed in his initial post on this thread. There are some active farmers in this forum, and I doubt if any of them do not calculate their needs that same way as Richard.

Richard

You know, that was my thought when I first started reading agricultural extension papers from Cal Poly San Luis Obispo, UC Davis, Yale, and U of Florida. In mathematics, we call it "abuse of notation" when something is abbreviated to the point of ambiguity. The fact is though, that when you read a plant science report recommending N-P-K ratios of 3:1:2 for citrus or 16:1:24 for bananas, they are referring to the N-P-K of fertilizer labels and not the literal meaning of the symbols in chemistry.

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