If memory serves, it has something to do with the bio-avalability of the nutrients. Some of that is not able to be assimilated by a given plant. This will probably start a war, but... Citrus grow in sandy soils that are lacking in organic material to hold nutrients (Cation-exchange capacity or CEC) and are subject to daily rains that wash nutrients away. Potted plants don't need 5-1-2 or 10-1-2 or whatever, but someone will throw an article at me (from a book that is covering citrus grown in the ground!) that says you need more N!!! My $.02 worth, at any rate.

I would only add that the 3-1-2 ratio is the same ratio in which the vast majority of plants require and consume nutrients....3 times more N than P and a third more N than K.

As mblan noted, trees planted in the ground do not necessarily require a similar formulation as they have free access to whatever is available in the soil. And a great many inground planted trees require little to no fertilization anyway!

But when you are dealing with container plantings and a nutrient-free potting medium, as most are and certainly the gritty or 5-1-1 mix, then the 3-1-2 fertilizer formulation hits the mark. And it really doesn't matter the genus or species - virtually ALL plants benefit from that ratio.

After studying fertilizers and nutrient absorption, it has become clear to me that fertilizers tailored to specific plant types are more of a marketing ploy to sell more products than an actual plant "need".

Acid loving plants will be harmed if given an alkaline fertilizer product. Forest plots can be assessed for timber production potential by studying which particular native plant species are present because among other things different kinds require differing levels (high, medium and low) of nitrogen for their growth. When used in this way they are referred to as indicator plants.

But what happens in the ground has no reflection of how a containerized plant will behave. The same rules DO NOT apply!

Gardengal48

“Plants use 6 times more N than P, so fertilizers that use more P than N are wasteful and more likely to inhibit blooms”

That was quoted from Tapla’s thread about fertilizers.
I know I’m on the right track. Just trying to understand how he got to that 3:1:2 ratio.

The Citrus fertiliser I mentioned is actually the same price as the Nu vital and most of the other products in the range. ‘Seasol’ are generally pretty trustworthy. They’re a decent brand. I’m just wondering if it would be worth buying the citrus version because as far as I can tell... the work has already been done. N has already been halved and K has already been increased as per their label.

"Acid loving plants will be harmed if given an alkaline fertilizer product." You might be able to get away with "..... could be harmed .....", but not "will be harmed". There are far too many variables to make such a broad statement.

I'm missing something, but maybe you can answer your own question if you haven't considered that the labeling on fertilizers report the amount P (by weight) as phosphorous pentoxide (P2O5), and the amount of K (potassium) as potassium oxide (K2O). To determine how much phosphorous and/or potassium is actually in the product, you'll need to multiply the reported weight of P by .43, and the reported weight of K by .83.

9-3-6 is a common liquid synthetic made by Dyna-Gro. It's actual NPK %, after factoring for the actual elemental weight of the nutrients is 9-1.9-5.

BTW, I don't think I said ALL plants use the same ratio of nutrients because I tend to avoid broad statements. I usually say that a very high percentage of plants use nutrients at the ratio provided by 3:1:2 RATIO fertilizers. Plants vary far more in the o/a amount of nutrients they use than they vary in the ratio at which nutrients are absorbed.

Al

Sorry AL, but this really didn't answer any of the the three questions I posted.

Perhapse I didn't simply my question(s) enough.

"I use a liquid fertilizer with a full compliment of nutrients and micro-nutrients in a 3:1:2 ratio"

"Tissue analysis of plants will nearly always show NPK to be in the ratio of approximately 10:1.5:7"

"All we need to do is supply nutrients in approximately the same ratio as plants use them"

1. My question is why aren't you using a product closer to 6:1:4 if that's the case.

2. If I can find a fertiliser which provides nutrients at the average ratio that the plant uses (6:1:4), shouldn't I be using that?

3. Can you suggest why are so many people suggesting that I use a 5:1:2 ratio for citrus?

I think once, I've understood these points, I'm going to be fine with working out the other stuff on my own.

"My question is why aren't you using a product closer to 6:1:4 if that's the case? If I can find a fertilizer which provides nutrients at the average ratio that the plant uses (6:1:4), shouldn't I be using that?" 9-1.9-5 (the actual NPK ratio of FP 9-3-6 after factoring for how P and K are reported on packaging) adjusted to reflect 6 parts of N would be 6: 1.2: 3.3, which is pretty darn close to what you're asking about (Why am I not using 6:1:4?). I suppose, if I ever find the perfect ratio in a fertilizer that includes all the essentials in an appropriate ratio, and derives none of its N from urea, I'd probably change what I use; or, if you want to fiddle with additional chemicals to fine tune the ratio to whatever you prefer, you could do that. "Can you suggest why are so many people suggesting that I use a 5:1:2 ratio for citrus?" No. I can only be responsible for what I say. If I had to guess, I'd say it probably comes from the fact that most literature re citrus is aimed at agronomy, and citrus generally prefer sandy soil, which doesn't have a very good CEC, and from which N is easily/quickly leached; this, because sand has no internal porosity and has far less surface area than organic and other types of naturally porous mineral products - so fewer attachment sites.

Al

Thanks for that.

I'm assuming you meant Dyna Grow 9-3-6 actual ratio is 9 : 1.29 : 5 though, because 3x0.43 is 1.29 (not 1.9)

Not sure how I made the calculation error, but you're correct - good catch. Did that cover your questions, D?

Al

It does. Everything is starting to make perfect sense. I really appreciate it Al!

So, when we look for the ratio of a fertilizer, is the real ratio more important (after multiplying by .43 and .83 respectively) than the ratio before doing the math?

I guess what I’m really asking is; should i be looking for a fertilizer product that is similar to the plants nutrient uptake AFTER the multiplication process mentioned?

And finally, just to understand controlling ratios a bit better:

How can I increase P in NPK a bit? If I half the dose of NPK and it leaves me with a P deficit, how can I correct it?

I have no more questions. Thank you for taking the time out of your day. I've learnt a TON in the last two weeks and my plants are already chirping up! :)

bump

Of course, the ratios don't actually change because we applied factoring, so it makes sense to default to how using the reported NPK content by weight % to establish the fertilizer's ratio. If you use a 2:1:2 or 3:1:2 ratio, it's not likely you'll go far wrong, though there are exceptions. E.G., Hibiscus doesn't like too much P and actually prefers more K than N; so, something like a 5:1:7 ratio fits that plant well. If you're going to try to work hard to ensure your plants have the very best opportunity to realize as much of their genetic potential as possible, you'll need to A) learn how to doctor commercial fertilizers or mix your own B) Research what the plant needs and when it needs it. That is to say some plants need more or less of certain nutrients at different parts of the growth cycle.

If you use a half-strength 9-3-6 (or any other 3:1:2) fertilizer solution, P won't be any more deficient the other 16 essentials unless the plant you're fertilizing is a known P hog. then there's the question of how you'll recognize it at this point in your trek toward green thumb status. I mention that because it's not at all easy to accurately diagnose some deficiencies or toxicities because they present differently depending on the plant and depending on other cultural influences. Often, a deficiency of P will resolve itself when the soil warms or you use an additive to change the soil pH.

However, if you need to, the best way to supplement P for containerized plants would be by purchasing P2O5 as a reagent. The easiest way would only require buying a container of bloom booster fertilizer (i.e. 10-52-10), and mixing an appropriate amount into your regular fertilizer solution to increase the P content. I suppose you could buy hydrogen phosphate (HPO4 2−) or dihydrogen phosphate (H2PO4−), but you'd need to research how to use them most effectively.

Al