Q about Fertilizing Tomatoes with soluble fertilizer?

8 years ago

I have 2 tomatoes growing in pots this year.

They don't get too much care from me usually, don't really fertilize a lot since i'm not really concerned about harvest, not like i'm trying to feed a family or anything, just something I grow for fun which lives on the deck, and gives something to snack on. And usually I get a somewhat decent harvest, definitely not a lot but enough for me.

Usually I grow them in a mix of potting soil, compost, and perlite. and I only fertilize with some of those slow-release fertilizer granules that I have left over from the previous year that were used on flowers and other plants in the garden.

The other day I saw some soluble fertilizer that's gets dissolved in water on sale (garden center was closing) and decided to give it a try since it was cheap and something new for me and see if it works better. I've pretty much only used slow-release stuff and granules in the past for other plants in the garden, but never this kind of fertilizer.

And I'm not exactly sure what the instructions are telling me, this is what it says: "Dissolve 1 tsp in 1 L of water. Apply every 2 weeks. 1 L feeds 30 to 40 plants."

So my question is how much of this 1 Liter liquid solution am I suppose to water in each plant? 1 L for 30 to 40 plants...is that like for small flowers with small roots or plants like tomatoes with bigger deeper roots? According to these instructions that's like less than 1/4 cup per plant...is that really enough for a tomato in a 5gal pot when it needs to get to the roots to be used? Or would I pour in the whole liter in since it's not a big amount compared to the 5 gal pot the plant is in? And after fertilizing it with this solution am I suppose to water it in right after or should I let it absorb a bit before watering?

Also, this is an all-purpose fertilizer thats 21-19-19, isn't that pretty concentrated? Would it be better to dissolve it in 2 L instead so it's closer to 10-10-10 like a lot of other all purpose fertilizers? But then the question is exactly how much of it am I suppose to be pouring into each plant, I don't wanna burn the roots killing it.

Featured Answer

gardengal48 (PNW Z8/9)

8 years ago

With container plants you do need to supply fertilizer - there is really nothing in the soil mix that will supply the required nutrients. And liquid or fully water soluble fertilizers work best. The typical recommendation is "weekly, weakly" or at 1/3 or 1/2 the listed dilution rate applied every 7 days. And you apply the fert the same as you do water - until the solution runs freely from the drain holes in the bottom of the container.

In my world, 1L would not even be enough of a fertilizer solution for one container tomato!! But then I grow mine in much larger containers - usually 15-20G size.

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Comments (6)

Nil13 usda:10a sunset:21 LA,CA (Mount Wash.)
8 years ago
Does it tell what concentration of nitrogen in ppm that mixture yields?
A W thanked Nil13 usda:10a sunset:21 LA,CA (Mount Wash.)
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A W
Original Author
8 years ago
last modified: 8 years ago
Thanks for the replies.

Concentration in ppm is not labeled...sometimes reading and understanding these labels is a pain.

Maybe the 30-40 plants/1-L would be the recommendation if it's something that was growing in cell packs early in the season before actually being planted where amount of soil and plant size is very small.

So i'll test it out by diluting the 21-19-19 in 2 L instead of 1 just to be on the safe side.
daninthedirt (USDA 9a, HZ9, CentTX, Sunset z30, Cfa)
8 years ago
Note that the fertilizer will tend to concentrate in the mix. That's one of the underappreciated hazards of container gardening. Yep, when the water dries up, the fertilizer is left behind. You keep adding more fertilizer, without any flushing action, and the fertilizer concentration will increase precipitously. Every month or so, I just flush my pots with water. Lots of water in, lots of water draining out. That removes whatever fertilizer concentrates might have built up. In my heat, evaporation is a challenge.
Nil13 usda:10a sunset:21 LA,CA (Mount Wash.)
8 years ago
last modified: 8 years ago
Well if the concentration isn't listed you will need to calculate it. To do that you need to know what the weight of a teaspoon is grams. I'm going to guestimate the weight of a teaspoon as 6g (table salt is about 5.6g). If a teapoon weighs 6g then that is equal to 6000mg. Since N is 21% by weight that would be 1260mg N. Since ppm equals mg/L, 1260mg/L = 1260ppm. Typically fertilizer recommendations have people dilute a tsp in a gallon (3.78L) which would be about 333ppm which is a pretty standard 'full strength' concentration.

Hydroponic tomato nutrient solution recommendations are for about a 4-1-4 ratio fertilizer with a N concentration of around 200ppm.

Just diluting to 2L is going to be really hot and risks burning roots. 630ppm N.
fungus
8 years ago
Old plants are not easy to burn, but with liquid ferts is better to add more often and less. So split in two, add once a week. You can dilute it in as much water as your plants need.

I would dilute 0.5g/l for a good watering at least every week, and not care about other things. I think 30-40 plants for one season is rather optimistic for tomato plants even if it's concentrated.