What you are really asking?

If you are asking if doubling the dosage gives you 20:20:20 the answer is no. The NPK numbers are the % of those nutrients by weight in a given amount of fertilizer. A 10:10:10 fertilizer means that 10% by weight of the fertilizer is Nitrogen and so forth. If you double the amount of fertilizer you use then it is still only 10% nitrogen by weight. But you've also still doubled the amount of fertilizer you've given the plant (as well as doubling the amount of every other nutrient).

This is why when people suggest a 5:1:3 ratio for citrus (as they sometimes do) you can use a fertilizer that is 5-1-3 or 10-2-6 or 15-3-9 and so forth. The ratio on all of those is still 5:1:3. The quantity of nutrients available to the plant is higher or lower but the ratio of the various elements to each other is the same.

Recommended dose has no direct relationship to the NPK guaranteed analysis numbers. Or, rather, it has a relationship but it's not the one that you're asking. If you want to mix fertilizers to obtain a particular ratio then you don't want to use the recommended rates. You need to do the math based on the % by weight and know the weight of a given quantity of the 2 and so on.

So in short: No. No. and Maybe, it depends on the quantities. But it is possible to mix fertilizer and change the NPK of the final mixture. Do the math. You also can't just take 1 lb of one and 1 lb of another and add them together. For instance: 1 lb of 3-1-2 contains .03 lbs of N. 1 lb of 8-2-3 contains .08 lbs of N. Add them together and your new 2 lb mix contains .11 lbs of N. Which means the % of N in your new 2 lb of fertilizer is .11 / 2 = 5.5%. So when you mix the two in equal parts you get an N value of 5.5.

If you don't want to bother doing the math then you could try using something like HydroBuddy but using finished fertilizers you might have difficulty nailing down a particular ratio (sometimes you can't get to where you need to be with just 2 finished fertilizers - if you use elemental or basic fertilizers you will get there more easily).

There are some other things to consider when combining two fertilizers. You can get interactions and precipitations and heat reactions and etc. Are you in need of an 8:2:3 ratio for some particular reason?

And to add to redshirtcat's excellent math :-), if you're using a water soluble fertilizer that requires you to use water to mix it up, then, if you use 1/2 the amount of water recommended to dissolve the fertilizer, then yes, it would be twice as strong. But if not, and this fertilizer is applied in granular form, then what redshirtcat has explained would apply. And redshirtcat has mentioned the "magic" ratio for citrus, so if you can find a pre-mixed fertilizer, such as Jack's Professional 25:5:15 High Performance, you're good to go. And you can skip the math, and possible issues with mixing different components.

Patty S.

I was asking about water soluble ferts. So hoosierquilt, does twice as strong the same as doubling NPK? And I was asking because I was curios.

Sorry if I wasn't clear.

The answer is that it depends on what you mean by "same."

If you mean does adding 2 pounds of 10-10-10 fertilizer to a container make the same number of nutrients available to the plant as adding 1 pound of 20-20-20 the answer is: yes.

But remember: since the NPK ratios are by *weight* you don't want to try to do things by volume if you are comparing labels and substituting. If, for instance, you have a 10-10-10 label and you read that you should use "1 Tablespoon per gallon of water" and you want to use a 20-20-20 fertilizer instead of the 10-10-10 then you need to do a little weighing. You need to weigh a Tablespoon of the 10-10-10 - remember the weight and then use half that amount (by weight) of the 20-20-20.

If you do things by volume you can get pretty far off. Make sure you are working by weight if you want to be accurate.

Also: remember they are not the "same" when you need to start combining fertilizers to make your own ratios.

Well, redshirtcat, then 2 Tbsp. would be twice the weight, thus twice the concentration. So, if the directions call for 1 Tbsp per gallon to give you 10-10-10, then 2 Tbsp. in 1 gallon would then give you 20-20-20. The same dry volume of the concentrated fertilizer granules would also be the same and the same dry weight in this case. Now, AFTER dilution, that's a different thing.

Patty S.

This is why I was trying to understand what, exactly, he is trying to do.

If he has 10-10-10 and he wants to fertilize as if he had 20-20-20 then yes, he can take the weight of 20-20-20 he would have used and then use twice that weight of 10-10-10. This is what I said above when I said that using twice as much 10-10-10 as you would have will make the same number of nutrients available as if you had been using 20-20-20.

But he asked about recommended rates (usually given in volumes). So if it is the case that he has a label on one bag of fertilizer that is 10-10-10 and that label suggests a fertilizer rate of 1 TBSP per gallon of water. That particular 10-10-10 fertilizer has a certain density - a certain weight per tablespoon.

Now let's say he wants to fertilize at exactly the rate that he had been. At the rate of 1 TBSP per gallon of 10-10-10 fertilizer but he only has a bag of 20-20-20.

The 20-20-20 fertilizer will not necessarily have the same density as the 10-10-10. In fact it almost certainly will *not* have the same density. That is: you cannot just take 1/2 TBSP of 20-20-20 and substitute it for 1 TBSP of 10-10-10.

That is what I'm trying to communicate. The labels tell you the % by weight of a nutrient so you cannot substitute by volume.

I'm not sure which he is trying to do - so I tried to explain both. I guess I'm not doing a very good job.

Correct, agreed :-) Hah, you did a fine job! The problem with the question I think is we're not sure if we're talking about dry (by weight) fertilizer, or fertilizer designed to be dissolved in water (volume). So, yes, you did a great job, just wanted to point that part out. So, Jack (that was my dad's name, kinda partial to that name), are you talking about water soluble fertilizer you dissolve in water, then apply? There are also dry, water soluble ferts out there you apply dry (by weight).

Patty S.

Ok thank you, that makes sense.:)Jack

The 1st answer was correct but he did not understand what was really being asked. I would have said yes.