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NPK and tomatoes

Posted by catlover_gardener sfl10miami (My Page) on
Wed, Jun 24, 09 at 18:01

Somewhere on this forum (I think) I read where someone said that plants use the N (in NPK) faster than the P & K and that if you add more fert then you will be adding more N than the plant needs. If I am wrong please correct but I am trying to remember it as best as I can.

Anyway, am I correct in saying that if that is the case, then it would be ok to add just the PK part in order to get the plant to fruit. That is after the tom plant has passed the stage where the N has done its part, which is stimulating leafy growth and stems, etc.

I have been trying to find the PK part alone so as not to add more N esp since I have observed that my toms are leafy and sturdy looking.

Also my beans and squash. They have good green leaves, but now I am hesitant to add more MG for toms (18-18-21)bz I am afraid that I will be addding more nitrogen now the plants are big.

Please set me straight anyone...


Follow-Up Postings:

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RE: NPK and tomatoes

Tissue analysis of the entire plant shows that tomatos use NPK in roughly a 3:1:3 ratio. This holds true for their entire life cycle.

With that said, let's get to your questions:

Somewhere on this forum (I think) I read where someone said that plants use the N (in NPK) faster than the P & K and that if you add more fert then you will be adding more N than the plant needs. If I am wrong please correct but I am trying to remember it as best as I can.

Adding an *excessive* amount of N results in plants directing their energy into vegetative growth at the expense of reproduction (making tomatos in this case). Adding N at the same rate as potassium isn't excessive. If you are growing in the ground (as opposed to containers) and the ground already has a decent amount of available potassium and the other nutrients you could fertilize with *only* N and be fine since the soil is supplying the rest. You still have to be careful to not add too much in terms of absolute quantity.

Anyway, am I correct in saying that if that is the case, then it would be ok to add just the PK part in order to get the plant to fruit. That is after the tom plant has passed the stage where the N has done its part, which is stimulating leafy growth and stems, etc.

N is required in significant amounts (relative to most of the other nutrients) the entire life of the plant. N isn't just for new growth. Plant cells are constantly dying, just like human cells and need to be repaired/replaced.

Also my beans and squash. They have good green leaves, but now I am hesitant to add more MG for toms (18-18-21)bz I am afraid that I will be addding more nitrogen now the plants are big.

Beans and squash are very different from one another. Beans sometimes are able to form a symbiotic relationship with soil bacteria that allow them to use N from the air. Squash (and tomatos) can't do this.

If you are growing in the ground and wish to use synthetic fertilizers I highly recommend a professional soil test to find out what your soil actually needs instead of guessing.

This approach generally pays for itself quickly as you avoid adding things plants simply don't need.


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RE: NPK and tomatoes

Thanks for such a good answer. With that I am going to assume that each time I water I can add a weak solution of the MG 18-18-21 fert to my toms and squash that I have in containers. THe beans would benefit also I am thinking. I feel better.

It has been raining a lot here in Miami so that's not going to be very often right now bz pots are wet. Overcast afternoons competing with the sun.


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RE: NPK and tomatoes

I'll just add that contrary to what many say tomatoes are NOT heavy feeders.

I've grown tomatoes for several decades and at one time or anonother have used 5/10/5 or triple 10 granular fertilizers, Espoma, and can't remember the NPK and also fish and or seaweed fertilizers with low NPK but with abundant minerals.

I fertilize once about two weeks after the plants go out and then once more when frujits are setting and that's it unless the season has been wet and then later in the summer I may use a fish or seaweed spray.

Of course as was mentioned above, it's always good to know the composition of your soil as well as the pH.

Carolyn


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RE: NPK and tomatoes

  • Posted by squonk z 5/6 Indiana (My Page) on
    Thu, Jun 25, 09 at 11:46

I agree with you Carolyn. I'm not babying my tomatoes like I did last year and they are fine. And, I think I found the cure for the minor leaf curl I had last year by not watering near as much.

Don't baby your plants, they will become bratty if you do.


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RE: NPK and tomatoes

Hi catlover,
One more concurring with Carolyn.
Tomatoes decidedly are reducing the proportion of Nitrogen (N) they will actually take up, relative to the dried weight mass the plant makes.
Of course the plant grows & increases it's potential dry matter, so the bigger plant uses more total NO3 but at a lower rate. In other words, the ratio of N in a fertilizer formula can be adjusted downward.
Technically it works out like this (figures are for mEq/Lt. per 100 grams dry matter weight, or "milliequivalent" of solute/liter of solution per 100 grams D.W.):
At seedling stage, with 8 true leaves, N use is +/- 115 mEq/lt. per 100 gr. D.W. during the day & 41 mEq/lt. per 100 gr. D.W. during the night ....
At first flowering, with flower truss, N use is +/- 75 mEq/lt. per 100 gr. D.W. during the day & 41 mEq/lt. per 100 gr. D.W. during the night ....
At fruiting, growing phase not ripe harvest (no data for cropping), N use is +/- 15 mEq/lt. per 100 gr. D.W. during the day & 29 mEq/lt. per 100 gr. D.W. during the night ....
I have discussed Phosphorus elsewhere, without giving statistical data ( see G.W. Growing Tomatoes 2009 forum post "Bloom Time"). Sorry, inept at linking directly to it.


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RE: NPK and tomatoes

I was told by my nursery 'expert' not to use a fertlizer higher than N of 5 because exceeding that would drive leafy growth vs blooms. Is this accurate? (Some of the info above is a bit over my head! - as Denzel Washington's character said in "Philadelphia": 'talk to me like I'm a five year old'....)

She also said (the nursery horticulturist), my plants need to be fed because due to all the rain the nutrients run off and need to be replenished (raised beds and containers). This sounds reasonable based on the recent rainy weather, agree? So she sold me some 0-10-10 liquid and I also side-dress with Tomato Tone. When necessary to water (rare!) I sometimes used compost tea (store bought, brewed in tea bags).

BTW, more rain coming tomorow! But we have had a good week of no rain and sun/partly cloudy, so this time I am happy for the rain cos it's just a little too hot!!!

Any thoughts? Am I doing things that make sense?


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RE: NPK and tomatoes

ditcn, I think the advise from the nursery is sound. One thing people might try if they are not getting the flowers they would like is to dilute one table spoon of Epsom Salt per gallon of water when watering. Epsom Salt makes the plant more able to pick up Potassium from the soil. It also promotes flower development. I found this on the net a little while ago:"Just as "Milk does a body good," Epsom Salt may be one of the most perfect nutrients for flowers and plants. And mid-to-late spring is the ideal time to nourish the soils and roots of your favorite foliage and flowers with this inexpensive and easy-to-use compound. According to the Epsom Salt Council, research indicates Epsom Salt can help seeds germinate; make plants grow bushier; produce more flowers; increase chlorophyll production; improve phosphorus and nitrogen uptake; and deter pests, including slugs and voles." Sounds good to me.

Don't be concerned about the late spring comment. This is a good thing to do every few weeks or so.

Tom


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RE: NPK and tomatoes

I was told by my nursery 'expert' not to use a fertlizer higher than N of 5 because exceeding that would drive leafy growth vs blooms. Is this accurate?

No.

Many growers use 10-10-10 or 20-20-20 to fertilize their entire gardens with, including tomatos. It's important to understand that every plant on earth has very similar nutrient requirements. It's only the fertilizer industry that wants you to believe that one plant needs high N, another high P another low N etc. It's all myth. Unfortunately most gardeners haven't the first clue as to sound plant nutrition, but instead have their ways of doing things they will confidently talk about. This helps perpetuate the myths.

If you get a soil test done for your ground gardens you may notice that the results will recommend adding various nutrients at various rates to get the levels of each in the soil to a particular level. You will notice these results aren't for any specific plant. There is a reason for that. All plants are very similar in their nutrient requirements. Likewise you will never find a soil science entity recommending boosting P levels in soil above K, instead it will much lower than K - always. There is a reason for that. There isn't a plant on earth that uses more P than K. Not one.

Here is a link to the CEO of DynaGrow (maker of fertilizers) answering my question of why they market a high P fertilizer when there isn't a plant on earth that benefits from it. You will find his answer informative and shockingly honest.

With that I am going to assume that each time I water I can add a weak solution of the MG 18-18-21 fert to my toms and squash that I have in containers.

Sure, the key is to dilute it. It's also key that these are container plants, not in ground plants. If you fertilize every time you water at 1/4th strength you have a very sound nutrient program. When growth is slow, plants won't need to be watered as often. This means they are also getting less nutrients which is appropriate. When growth is rapid they require more frequent watering and thus get more nutrients.

As long as the dilution rate isn't very high (1/4th strength is fine) you won't see a problem when using water soluble nutrients as the water leaving the drainage hole is removing some of those nutrients.

I will add it is EXTREMELY important to water until water comes out the drain hole EVERY TIME if you wish to fertilize at every watering. If you don't do this you risk salt build up in the container.


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RE-: NPK and tomatoes

According to the Epsom Salt Council, research indicates Epsom Salt can help seeds germinate; make plants grow bushier; produce more flowers; increase chlorophyll production; improve phosphorus and nitrogen uptake; and deter pests, including slugs and voles." Sounds good to me.

This is another myth widely believed and propagated by gardeners. It's odd that the source for these claims is an industry group who benefits from people buying epsom salt ;) Normally folks get suspicious about claims when they come from concerns who profit from the product they are recommending. Anyway...

Epsom salt is simply magnesium sulfate. In other words, it's magnesium and sulfur. Both are plant nutrients.

If one's soil has an adequate amount of both nutrients, adding more will have no benefit. If one's soil is lacking in either, then it will provide benefit.

For containers it's not uncommon for fertilizers to lack one or both of these so it can be used to good benefit in such a case.


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RE: NPK and tomatoes

Because I work with our local extension office, I try to rely on university based advice for most gardening questions. A bit of research on the web, using only .edu sites, brings a wide variety of recommendations on fertilizing tomatoes. Here are just a few I picked at random to illustrate the point.

OSU
Tomatoes respond well to fertilizer applications, especially phosphorus. Excess nitrogen fertilizer can result in plants with extremely vigorous vine growth but little fruit production. Apply 2-1/2 to 3 pounds of a complete fertilizer, such as 5-10-10, 5-20-20, or 8-16-16 per 100 square feet of garden area. Work the fertilizer into the soil about 2 weeks before planting. An additional sidedressing of a nitrogen fertilizer may be desirable after the first cluster of flowers have set fruit.
(And for container plants:) Once a week give each plant a good watering with a water soluble fertilizer such as Peters 20-20-20 or Miracle Grow 15-30-15 at the recommended rate.

NCSU
(Soil prep) If no soil test has been taken, apply 3/4 cup of lime and 1/2 cup of 8-8-8 fertilizer for each plant.

To make a starter solution, mix one pound of a complete fertilizer such as 8-8-8 in 10 gal of water. If small quantities are desired, 3 to 4 Tbsp. of fertilizer can be mixed per gal of water. The high phosphorus content in commercial starter solutions make them the preferred choice over home mixes. Never use more than one cup of fertilizer solution per transplant.

Sidedress tomato plants with 2 to 3 Tbsp. per plant of a complete fertilizer such as 8-8-8 or 10-10-10 after the plants have started to set fruit and 4 to 6 weeks thereafter throughout the growing season.

UofMissouri
Add a complete garden fertilizer at the time the soil is prepared. For tomatoes, use a fertilizer low in nitrogen (N), high in phosphorous (P) and medium to high in potassium (K). Among the best analyses for tomatoes are 8-32-16 and 6-24-24. Avoid using ammonia fertilizers such as urea or ammonium nitrate for tomato fertilization.
Use a maintenance rate of 1 pound per 100 square feet after the proper fertility level has been developed from previous soil tests and fertilizations. If only 5-10-5, 5-10-10, or similar analyses are available, apply 2 pounds per 100 square feet.

UofArizona
Heavy feeder. Use starter solution for transplants. Sidedress 1 to 2 weeks before the first tomato ripens with 1-1/2 ounces 33-0-0 per 10-foot row. Sidedress again 2 weeks after the first ripe tomato with a balanced fertilizer such as 5-10-5; repeat 1 month later.

TAMU
Add 2-3 pounds of fertilizer such as 10-10-10 for every 100 square feet of garden area.

My point in this is that there are differing opinions on this topic, even among experts. Not an unusual occurrence at all! And a lot of this advice is very area specific, meaning the soils in one area of the country differ in nutrients from other areas and the advice is for the particular state the university serves.

If you have NOT had a soil test done and therefore don't know what your soil lacks or has in abundance, then a fertilizer specifically formulated for tomatoes might be the best bet. If you have spent the few dollars for the test, preferrably at a soil lab, then follow the advice of the experts who did your test.

Hope this helps.
Sandy


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RE: NPK and tomatoes

My point in this is that there are differing opinions on this topic, even among experts. Not an unusual occurrence at all! And a lot of this advice is very area specific, meaning the soils in one area of the country differ in nutrients from other areas and the advice is for the particular state the university serves.

The problem with relying upon university extension services for plant nutrition information is one you identified yourself.

They make recommendations based on knowledge of what local soils are likely to contain. Therefore one extension service will recommend high phosphorus (because the local soils are low in phosphorus) and another one won't (because local soils are abundant in phosphorus).

Unfortunately this tells us *nothing* about the ratio or total amount of nutrients any plant actually uses ;)

It is not that there is disagreement among experts. There isn't. Anyone with an education in plant nutrition understands the topic and there is very little to disagree about.

The appearance of disagreement is easily cleared up when one understands each extension service is providing information relevant to local soil conditions and is not making *any* statements about actual plant nutrient usage.

To get *that* information one looks to tissue analysis of the entire plant as well as field studies that test the nutrient levels in a field before and after a crop to determine what was actually removed from the soil by the crop.


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RE: NPK and tomatoes

Wow Justaguy2. I followed your link to other posts by you and Al. What a great education. Thank you thank you for taking the time to spell it out so clearly. I loved the response from the CEO. For years I have hit my tomatoes with a super bloom high P product when I see flowers. I "just" bought a substantial supply and used some! I even recommended it to a neighbor admiring my mass tomato plantings. Grr marketing.

Ah well, Thanks again. Now what the heck do I do with this stuff? :)


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RE: NPK and tomatoes

Thank you thank you for taking the time to spell it out so clearly

You are quite welcome and I am thrilled you have found benefit in what I have offered. Thank you for saying so.

For years I have hit my tomatoes with a super bloom high P product when I see flowers. I "just" bought a substantial supply and used some! I even recommended it to a neighbor admiring my mass tomato plantings. Grr marketing.

Ah well, Thanks again. Now what the heck do I do with this stuff? :)

Are we talking about container growing or soil growing? If soil growing you would need a soil test to determine if the P levels are low-high. If low then the high P ferts are of use. The plants will use what they need and the rest will get stored in the soil (unless it's predominately sand) and thus elevate the P levels over time which is a good thing for soils deficient in P.

In containers they aren't so great and can lead to nutrient problems for a few of the minor nutrients. That's what happens when the thin watery area around the roots contains too much P. This induces nutrient deficiencies which oddly, often result in stress induced flowering. It's not healthy for the plant long term, but can induce flowering in many plants.

Ultimately what you do with the high P fert is your call, but I would ask you to please consider the environment. Overuse of phosphorus has lead to eutrophication of our water bodies and some places, including the entirety of my state (Wisconsin) have made it illegal to use fertilizers on the soil that have any phosphorus unless one has a soil test result indicating the need due to how serious the problem has become.


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RE: NPK and tomatoes

JaG, you hit the nail on the head that I obviously missed. My closing line should have been, "Have a laboratory soil test done and follow the advice given."

Most home gardeners just want a decent tomato crop and should be able to achieve that with a reasonable fertilizer program, based on the results of the soil test. But instead of spending around $10 for a reliable test, they will guess or listen to a neighbor or let the local garden center sell them something whether it's the best for their soil or not.

Tissue analysis is primarily for commercial growers whose livelihood depends on getting the most bang for their fertilizer buck. I'd like to have one done, solely for curiousity sake, but never felt it was worth it for my few plants.

(The powers that be in Wisconsin have really gotten serious about the phosphorus problem, haven't they?)

Sandy


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RE: NPK and tomatoes

I have not seemed to be able to find this question answered, but once fruit are set, then all other things being equal (i.e. assuming soil is not an issue, because I probably won't be testing it at this point), what fert would likely be best to promote the growth of fruits? It is repeatedly noted that lots of N promotes leaf growth, lots of P promotes flowering and fruit set, but I cannot recall anyone making suggestions on what promotes fruit growth once it has set. My assumption would be that once fruit is set, I really don't care about excess N resulting in blossom drop, or a lack of P resulting is less flowers. If photosynthesis produces sugar that causes fruit to grow, wouldn't a higher N fert produce more suface area of foliage which in turn would have more photosynthesis to feed and grow existing fruits?


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RE: NPK and tomatoes

Extra K for cell wall development... That's probably why most tomato fertilizers are K heavy.


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RE: NPK and tomatoes

I have not seemed to be able to find this question answered, but once fruit are set, then all other things being equal (i.e. assuming soil is not an issue, because I probably won't be testing it at this point), what fert would likely be best to promote the growth of fruits?

None ;)

There is no single element that is responsible for the growth of the fruit.

In this regard plants are like people. We/they require proper nutrition and if getting it can be expected to grow and develop as best they/we can limited by our genetics and environment.

If you want rapid development/ripening of fruits, pray for warm, but not hot days and nights. This will have more of an affect that any fertilizer combo will.

When growing in most soils, the soil is a storehouse of nutrients so changing from fert a to fert b mid season will have very little affect. In containers or hydroponics it's a different story, but in soils your best bet is to get a soil test, follow the instructions to get nutrient levels in range and let that be that.


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RE: NPK and tomatoes

Great thread here, I feel like I'm learning a lot. Question for the experts: I have a 4x8' raised bed garden, filled about 8" deep with 20 bags of store bought bagged garden soil, Miracle Gro brand I think even. Below that is the normal soil that I turned over a few times before adding the garden soil. Is it a safe assumption that this contains all the nutrients I need for my tomatoes, and I don't need to add fertilizer? Obviously I haven't had the soil tested, but I'm guessing it's not a big secret what kind of nutrients/soil is in the bags I bought. I bought some Miracle Gro tomato fertilizer (18-18-21), but I've only used it once because after reading this forum I decided I probably don't need it anyway. My plants are all coming along very well, but just like everyone else on this forum, I'm looking to learn anything I can to get more, bigger, and better tomatoes.


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RE: NPK and tomatoes

Hey justaguy2 (or others) any thoughts?


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RE: NPK and tomatoes

The fertilizer added to bagged products is usually of the controlled release type (think Osmocote) and they are rated to release nutrients over a certain period of time assuming an average soil temperature of some amount (usually 70F).

Whether or not this will be adequate is something you will have to determine by looking at your plants. You have to keep in mind the roots will likely grow into the soil below and that likely has nutrients as well.

If your plants are growing, flowering and the leaves all look a fairly uniform green then likely everything is OK.

There will be zero nutrients (practically speaking) in the raised bed soil (the bagged stuff you added) come next season though. This would be a good time to determine how you wish to meet plant nutrient needs for the next crop. By that I mean organic/synthetic/hybrid etc.

The MG Tom fert you have will be fine in the event your plants start to show signs of nutrient stress such as lower leaves yellowing. Keep in mind it's pretty normal for the lowest leaves to yellow as the plant ages, but if the yellowing continues up the plant then take it as a sign they require fertilization.


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RE: NPK and tomatoes

Cool - thanks for the quick reply! My plants do look really good so far, with one of them (Beefmaster) over 5' tall already and well over its cage. (Deciding what to do about that now) Sounds like I don't really need to add anything unless it looks like it needs it, though it probably wouldn't hurt either. I've heard from some people here who say they add a diluted MG mixture every week or two with good results. Next year I'll probably add a few more bags of garden soil as well as some fertilizer.


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