How to calculate NPK needs for container tomatoes?

[outsiders71]

I was wondering if anyone could explain how you would go about calculating the amount (pounds) of NPK that is needed for a 10 gallon container?

Thanks.

[mdvpc]

That is a great question and I dont know the answer, but I am looking forward to some of our folks here answering. Also, can I add to your question not only the macronutrients, but how would one calculate the micros also?

[Wi-sunflower]

The question I would have is this -- is the container a "closed system" like the earthboxes or is it a pot with drain holes that gets watered daily and the excess runs out ???

If the former, you could calculate much like the needs for field grown. Possibly a bit more if you have heavy production.

If the later, you should use a "constant feed" weak soluable solution in your water every time. Otherwise you risk flushing out what nutrients are there with the daily watering.

Carol

[mdvpc]

Carol-Good post-I assumed it was not an earth box. I have used many different types of soluble and pelleted fert-so are better than others. But I have never heard if there is a formula to calculate. I just thought of someone that has a protocal-T&J.

Here is the link:

http://www.tandjenterprises.com/tand..._gardening.htm

[outsiders71]

My question was regarding an open system (ie: containers that are not self-watering that have drain holes).

Do you think top-dressing with organic amendments could imitate routine weak liquid fertilizer?

Every bag of fertilizer I've read gives application rates in square footage. I assume that this would be the wrong approach since the depths of our containers are nowhere near the depth of the ground.

[dice]

When using a bagged, dry fertilizer, I calculate per plant rather
than per size of container, like "a handful mixed into the soil
around each transplant, a second handful top-dressed under
the plant when they start to set fruit." (If I had a long enough
season, I might add another handful about a month after
first fruit set.)

Top-dressing and watering in will not give quite the same
effect as soil-drenching with a dilute, soluble fertilizer.
With top-dressing, there will be more nutrients concentrated
in the top few inches of soil, while the water-soluble stuff will
tend to be distributed more evenly from top to bottom
(although the container mix will tend to have a filtering effect
on solids in suspension in any liquid fertilizer, so there will be
some accumulation near the top of the container, just not as
much as with top-dressed dry fertilizers watered in).

I do not know which way works better. If one got a lot of rain,
one would not have as many opportunities to add the soluble
fertilizer ("container definitely does not need more water"),
while the top-dressed fertilizer would be soaking in with
every rain. A dry summer would be better for comparison,
where one is having to water the container regularly either
way.

[outsiders71]

Interesting dice. However I would think that top dressing with fertilizer, if watered regularly, should disperse through the container. Part of the top dressing would become water soluble when the container gets watered and it would work its way down through the soil. A dilute liquid fertilizer would disperse throughout the container faster, as the top dressing would take a few waterings to get down there. I could be wrong though. Time for experimentation.

[dice]

I mainly mix in initially and top-dress at fruit set, because I am
too lazy to go mix up soluble fertilizer for every container plant
every two weeks for the whole summer. I do add an occasional
watering with soluble fish+kelp to a plant that seems to be
underperforming (just sitting there while everything else around
it is growing).

[jcmorse33]

From my own experience this year using a 3-1-2 ratio water soluable fertilizer plus liquid kelp (listed as 0-0-1) once a week at half the "monthly" strength listed on the box directions provides plants that are in the same league of health / productivity to what I get from a plant in an earthbox like closed system container.

Well, at least it seemed to be equal up to the point where disease and then deer stepped in.

--Justin

[sprtsguy76]

"Tissue analysis of plants will nearly always show NPK to be in the ratio of approximately 10:1.5:7. If we assign N the constant of 100, P and K will range from 13-19 and 45-70 respectively. All we need to do is supply nutrients in approximately the same ratio as plants use them, and in adequate amounts to keep them in the adequate to luxury range at all times."

I use Foilage Pro and Pro-TeKt which is 9-3-6 (3-1-2) + 0-0-3 (3-1-3) respectively. I keep my closed growing systems reservoir such as an Earth Box at roughly 1/4 strength at all time. With draining systems like pots or buckets I use 1/4 strength at almost every watering.


Damon