overfertilizing in containers, causes?

[venturabananas]

I've been growing tomatoes in containers for several years, generally 15-20 gallon pots. For the last two years, I've overfertilized, despite following the guidelines on the bag. The plants at the start of the season are growing downward curving leaves (sort of like a fern fiddlehead) and the flowers are dropping (in varieties I know should be setting, based on past experience with them). Last year, they grew out of it. This year, I'm too early in to say how things will change.

Last year I used Osmocote Plus, this year I'm using Tomato Tone. For both, based on the instructions on the bags, I added about 2.5 cups of either, per 20 gallon pot. That doesn't seem excessive to me, but the plants are acting like it is.

I'm wondering why this is happening.

I have a few of guesses:

1) I'm not mixing the fertilizer throughout all the potting mix in the containers. It's basically all in the top half. So the concentration is higher than recommended.

2) I usually reuse about 2/3rds of the mix from the previous year. Perhaps it still has pretty high nutrient concentrations. Though I wouldn't have thought so.

3) The potting mix I'm using (EB Stone Edna's Best) does have some organic ferts in it (e.g., bat guano). Maybe there's more of this in the mix than I suspected.

Anyone have any thoughts or similar experiences?

[zipcode]

So 1 plant in one pot. Conversion tells me that's 450 grams (1 lb) of fertilizer, which is a fairly high amount (I guess it depends how much you prune). (actually 1 lb of osmocote 15-9-12 per plant sounds really high)

I think the problem here is not the amount but the fact that you add it all at once. Bags of mix usually have on them how much N, P or K they have in ppm. And it's not little, at least here in Europe (something like 1000 ppm K is common).

It also depends on watering technique. If you top water, a lot of that nitrogen in the organic will be out in like 2 weeks, and may (will) leach. You can get away with high amount of organic much better in closed system like self watering, also because the water doesn't go straight through the fertilizer every time you water releasing all the nutrients.

So, start with much less, add often, especially after fruit set. It is somewhat difficult adding organic in the middle of the season, so you can switch to inorganic.

From my observations, excess nitrogen doesn't however generally cause flower drop (at least not generalized), it's usually a very delayed fruit growth, flower sits there not dropping and not producing (it sometimes starts after a long time, and the fruit is generally small).

[venturabananas]

That's good advice. I hadn't thought of it so much as amount of fertilizer per plant, but as amount per volume of potting mix, but thinking of it as amount per plant makes much more sense.

Normally the bags of potting mix do give N, P, and K in percentages, but perhaps the organic products aren't required to. It isn't mentioned on the bags of this mix, anyway.

I use SIPs that I top water, but then the water is retained in the bottom, so the nitrogen isn't getting washed out. I may need to flush them.

Perhaps it is a combination of accidentally letting one pot in particular dry out, combined with high nitrogen that is causing flower drop in one plant.

Thanks for your advice and observations.

[Gerardo]

I killed a bunch of peppers and a few tomatoes following the Osmocote dosing instructions. I suspect the heat contributed too. Hot batches of mix with Osmocote remain that way for quite some time.

I mixed a large batch and ended up having to remove the stunted plants, flush the roots, dilute the "hot" mix with its contribution being around 1/3 of the volume, and that seemed to do the trick.

The concept of a "fertilizer strip" on top of the mix has been problematic for me, so now I mix it thoroughly.

For this year I used Osmocote sparingly and the plants are digging it.

[venturabananas]

Thanks for the helpful information. After the first plant this year started to show evidence of over fertilizing I started adding some of the fert in a strip rather than mixing it all in for later plantings. That doesn't seen to have helped. Given your experience, I guess I'll go back to incorporating the fert and just use less of it (and add more later).

[nbardo]

What instructions are you following to get your application rate? Osmocote for tomatoes recommends 1.5 capfuls per 4 square feet mixed into the top 3 inches of soil/mix. Thats about 1 tablespoon per square foot. A 20 gallon pot might be around 2-3 square feet, so 2-3 tbsp per pot. If you try a much lower rate like this and find you have a nutrient deficiency, you can always supplement later, but you cant take it away once its there


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[venturabananas]

Thanks.

Osmocote Plus has application rates for incorporation into mixes in containers. It is based on container volume, not surface area. Those rates you mention sound like they would apply to in-ground, rather than incorporation in mixes for pots.

But, yep, starting with too little and adding is better than too much! Lesson learned. Again.

(That said, I'm using Tomato Tone this year and I followed its directions, which actually called for more than I used. And for reference, EarthBox recommends 3 cups -- more than I used -- in an even smaller container.)