10-10-10 vs 19-6-12 time release fertilizer

paulsiu(5a)April 6, 2012

I was looking at a way of fertilizing a container plant with the least amount of effort. Time release fertilizer seemed the way to go. At the home depot, I notice Miracle Gro sells a shake n feed with 10-10-10. Osmocote seems to sell one with 19-6-12.

I was reading Al's post about how the ratio should actually be 10-1.5-7. The Osmocote seems closer to that ratio, so does the ratio even matter?

The plan is to grow perennials and annuals. No veggies.


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tapla (mid-MI z5b-6a)

That ratio illustrates that, on average, plants use 1.5 parts of P and 7 parts of K for every 10 parts of N. The actual amounts of P and K that fertilizers supply are not what it says on the box. 10-10-10, for instance, is actually 10-4.3-8.3 after the factoring is done to account for the fact that P is reported as P2O5 (phosphorous pentoxide) and K is reported as K2O (potassium oxide).

That's the background ...... what's important is, after the factoring is done for 3:1:2 ratio fertilizers, you'll find that they supply actual amounts of NPK in a ratio almost exactly that of the average ratio of nutrients plants use. 18-6-12 would be exactly a 3:1:2 ratio, so your CRF is pretty spot on in that regard. The only drawback to using it is, you relinquish control of when nutrients are supplied to your plants. When it gets really hot, there is advantage in withholding fertilizer, and you lack that control with CRFs.


    Bookmark   April 6, 2012 at 7:45AM
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I would only add to what Al posted by saying don't worry too much about achieving any precise ratio. Why? There are a few reasons. First, plants differ widely in their nutrient requirements. Second, as Al alluded to above, nutrient requirements differ through a plant's growth cycle. Third, there is not a 1:1 correlation between the nutrients added to the soil mix and what is taken up and used by the plant. Different elements react differently to different soils or soil mixes. Some elements, e.g. phosphorus, is virtually immobile on mineral soils but will leach from organic soils. The important thing is to ensure that there are appropriate nutrient levels (i.e. no deficiencies or excesses) at any given time to satisfy the plant's requirements. Plants will use what they need when they need it.

    Bookmark   April 6, 2012 at 9:42AM
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Thanks for the info, I did not realized 10-10-10 was actually not 10-10-10. Are manufacturers consistent in the way they report NPK, so 19-6-12 is actually more like 19-2-10 for example?

Good point about losing some control over release of material. I was under the impression that stuff like Osmocote actually increase release when the temperature goes up, which may not be a good thing in the heat of summer.

On the other hand, I am not looking for perfection here, if it works well enough. I'll be ok with it.


    Bookmark   April 6, 2012 at 3:20PM
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Are manufacturers consistent in the way they report NPK

Yes, this is governed by law, so all fertilizer sold in the U.S. will follow the same conventions. Manufacturers are also limited to reporting only plant available nutrients on the label. So, if a fertilizer contains 30% total phosphate, but only 3% is plant available, they can only state 3% on the label. I recently discovered this last requirement is different overseas where manufacturers can report total percentages.

    Bookmark   April 6, 2012 at 4:09PM
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