When I make 5-1-1 or gritty mix, I add 1 T per gallon. That is a low dose and, when growing vegetables or annual flowering plants outside in the summer, I also use a soluble fertilizer at full strength (according to label) starting about a month after the plants start steady growth. The rate at which a controlled release fertilizer releases fertilizer depends on temperatures and plant's rate of growth. So, even though osmocote plus is labeled as lasting 6 months, it probably only lasts 3-4 months in a hot summer outside.

I use 1 pound of Nitrogen per cubic yard of medium.

1 tablespoon per gallon? I had been putting in the recommended dose of regular osmocote (prior to purchasing plus) and it was about 1.5 capfuls for a 16 inch container i thought

I don't know the volume of a capful or a 16 inch container, so I dont know what that means. I am basing my recommendation on Al's 5-1-1 recipe. I've used this for the past two years, and it has worked well.

Thanks. I meant a capful of osmocote (that is how the regular osmocote tells you to measure it). A 16 inch container is somewhere around 10 gallons i would guess. They are already planted and it seems that 10 TBs would cover the whole surface.

There's a difference between adding a CRF to the surface of an existing container, and incorporating a CRF when making up a batch of 511. The general formula "if" adding to a batch of 511 is 1 tablespoon per gallon of mix. The instructions on my CRF for fertilizing containers is based on the diameter of the container and is just meant to be the amount needed to be scratched into the surface of an existing container. So you are both right.

Err on the side of caution, I'd say.
Both Ohio and I use a lighter dose of Osmocote, and then fertilize with a liquid throughout the season. If needed, you can always add more.

Josh

Mixing in with the entire medium isn't considered a best practice, because of leaching. Best practice is mixing in in the upper third of the container so that the water takes the nutes down the rest of the way and they have a greater chance of being used by the plant. Topdressing also works, but it definitely needs to be worked in because if it dries out it isn't fertilizing. How much you use depends on the concentration of the fertilizer, but the CRF I use that, off the top of my head, is 15% N when applied at 1#/cuyd it makes a fairly solid layer across the top of a #5 nursery container. I weighed out the proper amount of fertilizer for a 1#/cuyd rate for the volume of each nursery container, #1,#5,and#15, and then took some scrap pvc pipe I had lying around and made scoops for each size. That means I just fill the container 2/3 full of medium. Scoop out the appropriate amount for the container size and dump it in. I add a little more medium and mix the CRF with it in the container. Then I plant on top of that.

Now this 1#/cuyd dose rate has been established for nursery and greenhouse production in conjunction with fertigation. If you are not going to fertigate, I would imagine you would want a heavier dose.

I don't think people should pay attention to the application rates on the bag. generally speaking, if you calculate those rates out to acre quantity applications you get rates that are much higher than even corn farmers use. That's only good for the manufacturer's bottom line, not the consumer or the environment.

Mixing in with the entire medium isn't considered a best practice, because of leaching. Best practice is mixing in in the upper third of the container so that the water takes the nutes down the rest of the way and they have a greater chance of being used by the plant. Topdressing also works, but it definitely needs to be worked in because if it dries out it isn't fertilizing. How much you use depends on the concentration of the fertilizer, but the CRF I use that, off the top of my head, is 15% N when applied at 1#/cuyd it makes a fairly solid layer across the top of a #5 nursery container. I weighed out the proper amount of fertilizer for a 1#/cuyd rate for the volume of each nursery container, #1,#5,and#15, and then took some scrap pvc pipe I had lying around and made scoops for each size. That means I just fill the container 2/3 full of medium. Scoop out the appropriate amount for the container size and dump it in. I add a little more medium and mix the CRF with it in the container. Then I plant on top of that.

Now this 1#/cuyd dose rate has been established for nursery and greenhouse production in conjunction with fertigation. If you are not going to fertigate, I would imagine you would want a heavier dose.

I don't think people should pay attention to the application rates on the bag. generally speaking, if you calculate those rates out to acre quantity applications you get rates that are much higher than even corn farmers use. That's only good for the manufacturer's bottom line, not the consumer or the environment.

I'm sorry, but I find your formula pretty confusing. Can you give measurements in volume instead of weight? It sounds like you're recommending 1 pound of Osmocote per cubic yard of mix. IIRC, 1 cy is equal to 27 cubic feet or roughly 202 gallons. Measuring my Osmocote, I estimate that one pound is roughly equal to 2 cups or 32 Tablespoons. So your formula roughly equals 1 Tablespoon per 6.3 gallons. Is that right? Are you saying that Al's formulas call for about 6 times too much CRF?

In doing some CRF research, I came across a series of interviews with a CRF formulator. Using your numbers as a reference, ohiofem, I think his recommendation is very similar to Al's*...

From my observations, the most successful growers are using approximately 1 pound of elemental Nitrogen incorporated per cubic yard.

He also had some interesting thoughts on topdressing vs incorporating. Perhaps nil13's approach would be a way to balance the downsides of both (underfertilizing and leaching).

Let's remember that CRF fertilizers are released by temperature and research shows that different technologies are affected by the temperature changes around the CRF prill that can alter their expected release. Just think about the temperature fluctuations and moisture consistency in the media versus the temperature fluctuation and moisture consistency on top of the media. In general, there is likely to be less temperature fluctuation and more moisture consistency in the pot than on the surface of the pot.

The surface of the media is subject to greater temperature changes between night to day and from cloudy to sunny days. Growers can expect the release of incorporated fertilizers to be more consistent than fertilizers that are topdressed.

These quotations are from a series of five articles available at perennialsolutions.com (linked below).

*If nil13 is using 1 lb of 15% N per cu yd, then that provides 2.4 oz of N per cu yd. If Al recommends 6x that much, that provides 14.4 oz of N per cu yd.

Here is a link that might be useful: Controlled Release Fertilizers

This post was edited by shazaam on Fri, Apr 12, 13 at 15:39

well...now i am thoroughly confused. I guess i am going to have to get out the ruler and do some math as to how big these containers actually are and then see where this thread ends up on the amounts to add.

No, the recommendation is 1# Nitrogen/ cuyd. That's elemental nitrogen not of the fertilizer. So if I am using 15%N fertilizer I would have to add almost 7# of that fertilizer to a cubic yard to reach 1#N/cuyd.

6.67# x 0.15 = 1.0005#
or
1# / 0.15 = 6.666667#

If say Osmocote was 10% Nitrogen you would add 10# of that per cuyd.

1# / .10 = 10#

I can't give volume amounts for osmocote because I don't have any idea how much say a tablespoon weighs. I don't even know off hand what the Nitrogen percentage is. The NPK numbers on the bag are percentages by weight so weight is the typical unit of measurement.

You don't necessarily have to measure all you pots. You can fill them with compost and empty into 5 gal buckets, (It's 5 gallons completely full) and determine volume that way. Once you get an idea of what the volume looks like you can pretty effectively guess. Or you can measure out potting medium, add the appropriate amount of fertilizer for what you have added and then just top dress a little to cover the rest. Adding the fertilizer right into the medium and completely incorporating it is easiest if you have a bunch of oddly shaped containers. Just remember that once the CRF is damp in the medium, it starts releasing fertilizer, so you want to use you mix right away.

I should have read your post better. You tell me how much a tablespoon weighs. lol

Osmocote plus is 15-9-12. So, if you were talking about 1 pound of pure N per cubic yard, then you would need roughly 6.6 pounds of Osmocote 15-9-12 to get that. In other words, about 1 tablespoon per gallon. ;-)

Some notes:

I prefer incorporating CRFs. Release is more uniform, there is less (no?) loss if the container gets tipped on it's side, and rain/wind can't splash/blow the prills out of the container as they can when the container is top-dressed.

When using NPK %s like 17-7-12, 18-6-12, or 19-5-9, it's common for nursery/greenhouse ops to incorporate from 8-15 lbs of CRF/cu yd, which is 27 cu ft - which is about 5-9 oz per cu ft, or about 2/3 - 1-1/4 oz per gallon (by weight). There's prolly about 1/2 gallon of product (64 oz by volume) in one of the 1.25 lb containers of Osmocote, so each cup of Osmocote will weigh about 2.5 oz. Each ounce by volume weighs about 1/3 oz. Based on those calculations, you could use as much as 1-1/3 to 3-3/4 oz (volume) per gallon of soil. I'm guessing at the volume of a 1.25 lb jug of Osmocote. If someone will measure how much the jug holds by volume (up to the normal fill line for the fertilizer), let me know & I'll recalculate, but I should be close with the figures above.

I used Osmocote Pro 17-5-11, 5-6 month formula, incorporated into the soil of most of my mixed (floral) display containers last summer, and I have to say it worked well. I'm going to try it this year on a large % of the material I'm developing as future bonsai, and on the veggies. I'll let you know how that works out, too. I'll be using it again for my mixed containers. It was about $75/50 lbs.

That's all I can think of for now.

Al

Hi, Al. I'm glad you checked in. I did convert weight to volume of osmocote plus in my post, although it may not be clear. The Plus 15-9-12 version comes in 4.5 pound containers. That is almost 9 cups by volume. Which means 1 pound=2 cups=32 tablespoons.

No, the recommendation is 1# Nitrogen/ cuyd. That's elemental nitrogen not of the fertilizer. So if I am using 15%N fertilizer I would have to add almost 7# of that fertilizer to a cubic yard to reach 1#N/cuyd.

Sorry about the misunderstanding. I read your second post as 1 lb of fertilizer rather than 1 lb of N (and completely overlooked the first one in which you stated that you "use 1 pound of Nitrogen per cubic yard of medium"), which seemed odd since everything I was reading (I'm new to CRFs) suggested much higher rates. That should have been the tipoff that I was missing something -- thanks for the clarification.

This post was edited by shazaam on Fri, Apr 12, 13 at 21:01

I have a number of medium to very large pots and no mental picture of the appropriate amount of fertilizer to add to potting mix. (Have only used water soluble fertilizer until now.) I'm also working on a deck, so I'm limited to making small batches of soil at a time.

I found an online calculator to get a fast estimate of the volume of my various containers (link below).

Based on posts above, would this be correct amount of Osmocote:

Sample pot: 19" in diameter x 10" tall = 12.27 gal

Al says "1-1/3 to 3-3/4 oz (volume) per gallon of soil"

1.3 oz * 12.27 gal = 15.95 oz of Osmocote

Ohiofem says "1 pound=2 cups=32 tablespoons"

15.95 oz = almost 1 pound, so I could use 2 cups Osmocote for my 12 gallon pot. Did I do that correctly? And the 2 cups should go in the top 1/3 of the potting mix?

Here is a link that might be useful: Online volume calculator

Dsb22: I hope Al comes back to review what he wrote. He has always recommended 1 tablespoon of CRF per gallon of mix, but what he says above works out to more than 3 times that much. There are 2 T in an ounce (by volume). Your 12.27 gallons of mix would call for about 12 T or 6 ounces of fertilizer (volume again). Please don't put 2 cups of fertilizer in your mix. That would be 32 tablespoons. I think the problem is that Al was guestimating the weight of the osmocote. I actually measured it.

Balkong: SIP containers require a different potting mix and different fertilizing strategy than regular containers. No one recommends using CRFs or soluble fertilizers in them. I suggest you ask your question in a new thread.

Thanks, Ohiofem. No plants have been burned yet :-) 1 tablespoon per 1 oz by volume will be easy to go by.

DSB: I hope that was a typo. 1 tablespoon per 1 gallon, not one tablespoon per ounce is what is recommended. I highly recommend that growers find or purchase measuring spoons and cups plus larger containers of known volume and use those to determine how much potting mix their containers hold. Hardware stores sell 5 gallon buckets cheap. I cook, so i also have also have quart and gallon size containers. Take some dry ingredient like peat or perlite and fill your container then measure how much it holds.

Going back to the original question, I have a 16-inch high by 16-inch wide round container that holds about 10 gallons of mix (I measured). I incorporated 10 T of Osmocote into the soil from top to bottom. Like Al said I believe it should be well mixed in. If the pot was already planted, I would have scratched it into the top couple inches and covered it with another inch of mix.

Ohiofem, yikes, yes, typo/brain glitch--I meant 1 tbsp per gallon.

I do have a white five gallon bucket and measuring spoons that will be getting a lot of use this spring. Thank you for the reply on mixing in it as well.

Thanks everyone. Didn't think this would end up being so long a discussion. My pot aren't too far off your size ohiofem so I will add in a bunch more osmocote and mix it in. A few of the plants are very heave feeders so I am not worried about over fertilizing

I went to some greenhouse mgt books I have & noted their recommendation for incorporating CRFs with NPK %s close to 18-6-12 was from 8-15 lbs/yd. That would be about 5 - 9 oz/cu ft (by weight - for how much to add to a gallon, you would need to divide the dose/cu ft by 7.5 and convert the weight measurement to volume).

Usually, when I was discussing CRFs it was in conjunction with using other soluble fertilizers as the primary source of nutrients. The CRF was just a starter charge or something to fall back on when/if you forgot to fertilize. When thinking about CRFs, it should be apparent that because they are designed to last so long, you would need considerably more CRF by weight when you apply it than you would slow release fertilizers, and MUCH more than if you were using readily soluble fertilizers.

Al

Please help!
Sorry to bother you about this, but I've got dyscalculia (a learning disability like dyslexia, but regarding math and numbers), so for numbers I often have to rely on the kindness of strangers.
This year I'm experimenting with some containers of pure cat litter (diatomite/ kieselgur), it's a bit like vermiculite, I guess, but more water retentive. I've just bought some organic NPK slow release fertiliser, but I have NO idea how much to use. -I've read through the above posts three times to no use.

The product I have is:
5 liter = 4 kilo
NPK 9-2-5

N: 9.0%
P: 2.0%
K: 5.0%
Magnesium (MG) 0.2%
Content of dry matter: 88%

How much do I need to use (preferably in volume)?
Even though I'm on metric here, you can tell me in fluid oz & gallons, as long as you tell me if it's the American or British system (I can convert it via an app on my phone).

If any of you could help me, you would make me VERY happy!

VG

PS: I noticed a couple of places above "#" used after a number... What does that mean? Here it's used in front of numbers, and simply means "number", like #1 is "number one" or #5 is "number five".

I usually keep plants in one container for years, so this adding CRF to the soil is all nice and easy, but does little for the life of the plant. I guess this is more important if you're selling plants and always using a new mix, but us growers are not. I would not stress much over this as we are talking such a short term strategy for the life of the plant. A better long term strategy is what you should be worrying about!

I usually keep plants in one container for years, so this adding CRF to the soil is all nice and easy, but does little for the life of the plant.

I don't really follow your argument, Drew -- using a CRF seems like a good long term strategy for maintaining the health and vigor of containerized plants. If you're growing woody plants or perennials and aren't repotting annually, you'd mix the CRF in to the potting mix preplant for the first year, then you'd top dress with CRF in subsequent years.

Vgtar: I am sticking to my original answer: when making your potting mix from scratch, add one U.S. tablespoon of Osmocote to one U.S. gallon of your mix. If you use Osmocote Plus 15-9-12 on indoor houseplants it will last about 6 months. If you are using it on fast growing outdoor plants in high summer temperatures, it will last about 3-4 months. After that, add some to the top of your container according to the directions on the fertilizer package. This is actually a low, safe dose. If you want more than minimum growth in the summer, you should also use a soluble fertilizer regularly according to package directions.

I think the # symbol is used for pounds (U.S.) by some on this forum. I have also seen people use it for gallons (U.S., volume). These are not standard uses, just some of the many shortcuts forum regulars use.

Vgtar: I just retread your question and see that you are not talking about Osmocote 15-9-12 but a different CRF that is 9-2-5. That is roughly half as concentrated as Osmocote, so you could safely use 2 tablespoons per gallon. But please be sure your fertilizer is a controlled release formula. Someone else on the forum accidentally used a regular fertilizer that was meant to be watered in instead of a CRF. That can be a disaster.

Thank you so much Ohiofem!
I planted a rose in what might be too strong a mix. It's a self-watering container, so I'm hoping it will only pick up the nutrients that are just beneath it in the beginning, and that I can then water from the top later in the season. Just to be safe, I've saved a bit of the mix, without any fertiliser, in case it starts to show symptoms of burn. But with roses being quite heavy feeders, I'm hoping it wont come to that.
I think this fertiliser should be OK. It says on the side of the tin, that it is suitable for containerised plants... it just doesn't tell how much to use in those containers, only how much to use in the garden.

Again, thank you ever so much
VG