Bloom Boosters - How Much P is Enough?

tapla (mid-MI z5b-6a)April 22, 2008

Lets first look at the role of fertilizers in general. There are 6 factors that affect plant growth and yield; they are: air, water, light, temperature, soil or media, and nutrients. Liebig's Law of Limiting Factors states the most deficient factor limits plant growth and increasing the supply of non-limiting factors will not increase plant growth. Only by increasing most deficient factor will the plant growth increase. There is also an optimum combination of the factors and increasing them, individually or in various combinations, can lead to toxicity for the plant.

From the above, we can say that when any nutritional element is deficient in the soil, plant growth slows. We have a term for this occurrence: environmental dormancy. When the deficient element is restored to adequacy levels the environmental constraint caused by the deficient element is eliminated and plant growth can resumes at a normal rate, as long as there are not additional limiting factors. Continuing to increase the element beyond the adequacy range offers no benefits and can deleteriously affect the plant - often in several ways, depending on the element.

Somewhere along the way, we curiously began to look at fertilizers as miraculous assemblages of growth drugs, and started interpreting the restorative (of normal growth) effect of fertilizer as stimulation beyond what a normal growth rate would be if all nutrients were adequately present in soils. ItÂs no small wonder that we come away with the idea that there are Âmiracle concoctions out there and often end up placing more hope than is reasonable in them. In couplet with the hope for the Âmiracle tonic is Âmore must be betterÂ. IÂll use the latter idea as the lead-in for my thoughts on high-phosphorous fertilizer blends.

Among container growers you often find common belief that high-phosphorus (P) content fertilizers are a requirement for promotion of root growth and/or flowering. Fertilizer blends like 15-30-15, and even 10-52-10 are sold under names that imply that you actually NEED these formulas for plants to bloom well and to produce strong roots. Lets examine that idea in a little more depth.

While anecdotal evidence abounds, there is very little scientific evidence to show any need for such products. IÂve mentioned in other posts that high-P fertilizers are a historical carry-over from when it was most common for plants to be started in outdoor soil beds, the soil in which was usually still quite cold at sowing time. Both the solubility of P and plants ability to take it up are reduced in cold soils, so it was reasoned that fertilizing with high levels of P insured that at least some would be available during periods of growth in chilled soils.

We know that tissue analysis of leaves, roots, flowers - any of the live tissues of healthy plants will reveal that P is present in tissues at an average of 1/6 that of nitrogen (N) and about 1/4 that of potassium (K). Many plants even contain as much calcium as P. If we know that we cannot expect P to be found in higher concentrations in the roots and blooms than we find in foliage, how can we justify the belief that massive doses of P are important to their formation?

It is well known among experienced growers that withholding N when all other nutrients are available at adequate levels induces bloom production, even on smaller and younger plants. Though plants USE nutrients at approximately a 3:.5:2 ratio (note that N is 6 times the level of P, and K is 4 times the level of P), most greenhouse operations purposely fertilize with something very near a 2:1:2 ratio to limit vegetative growth so they can sell a compact plant sporting pretty blooms to tempt you.

Simply limiting N limits vegetative growth, but it does nothing to limit photosynthesis. The plant keeps making food, but it cannot use it to grow leaves and extend stems because of the lack of N. To where should we imagine the energy goes? It goes into producing blooms and fruit.

What harm might there be in a little extra P in our soils? First consider that the popular 10-52-10 has almost 32 times more P than a huge percentage of plants could ever use. Even 1:1:1 fertilizer formulas like the popular 20-20-20 are already high P formulas because they have 6.25 times more P (in relation to N) than plants require to grow robustly and normally.

Evidence of phosphate overfertilizing usually always includes some degree of leaf chlorosis. P competes with iron (Fe) and manganese (Mn) ions for attachment sites and causes antagonistic deficiencies of these micronutrients. Unfortunately, the deficiency of these elements causes interveinal chlorosis (yellowing), and the first thing we normally consider as a fix for yellow leaves is more fertilizer, so we give the plants a good dose of our favorite bloom-bomb which causes, no surprise - worsening of the condition.

IÂll close with an anecdote of how I used to fertilize plants with showy blooms before I had a better understanding of the overall picture. I would fertilize with a "bloom-boosting" fertilizer as long as foliage was bright green. As foliage inevitably yellowed, I would then switch to a high N formula until the color returned and start the cycle over again. I THOUGHT that the P was helping produce blooms and the yellowing was caused by a lack of N, which I quickly jumped to correct at the first evidence of yellow. I now understand that the high levels of P were what were causing the yellowing and it wasnÂt my returning to a high N formula that greened the plant up again, it was the reduction in the level of P in the soil when I stopped using the high-P formulation.

Al Fassezke

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filix

Al I'm just starting to understand fertilizer. But still confused. I'm going to ask a few questions at the risk of sounding dumb. :)You said Though plants use nutients at approximately 3.5.2 ratio. note that n is six times the level of p. Doesn't The 5 mean more? I know I'm missing something obvious.
I'm getting ready to feed my clematis supper phosphate is this not good? filix

    Bookmark   April 23, 2008 at 8:08AM
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tapla (mid-MI z5b-6a)

that's '.5', not 5.

3:1/2:2 is another way to write it. ;o)

Al

    Bookmark   April 23, 2008 at 8:54AM
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justaguy2(5)

3 .5 2

    Bookmark   April 23, 2008 at 9:08AM
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wyndell

I hope you don't cringe when you see me posting again. I thought I would ask a question I asked in another post but this would give you a place to respond in this thread where it belongs.

If I'm using the MG 24:8:16 and I wish to lower the N to increase fruiting or blooming. Assuming I am using one scoop per gallon. If I understand correctly I would use a lesser amount. I would add something that raises K. Could you explain what I could add to that and if I am understanding correctly how to reduced N.

Also, if I didn't have anything to add, what I might look for (5:10:10 for example, even though the P is high) that we could use. I notice on the earthbox forum many suggest using anything with a lower N such as the example and many on here are using SWC's.

Thank you so much for your helping.

    Bookmark   April 23, 2008 at 9:42AM
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filix

Thankyou. I have got to watch those decimals! filix

    Bookmark   April 23, 2008 at 11:08AM
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puglvr1(9b central FL)

Hi Al,

Thanks so much for posting this very informative thread. I learn so much from you. :o)

    Bookmark   April 23, 2008 at 2:01PM
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filix

I understand that that we dont want to raise the P but lower the N. How about elimamting N and K like bone meal.

Also when you have a 20 20 20. What is the other 80%? What do they add to fertilizer. what is the carrier? I mean if you have 20% of 100 Whats the other 80%. In a liquid fert is it water? Dumb novice question. But I have wondered this. filix

    Bookmark   April 25, 2008 at 8:08PM
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tapla (mid-MI z5b-6a)

Thanks again, N. ;o)

Hi, Filix. I'm not sure what you're asking me about the elimination of N & K & the bone meal thing, but bone meal acts soo slowly that it's very ineffective in containers.

20-20-20 is 20% by weight of each - N, P, & K for a 60% total. Fertilizers are not pure NPK, Ca, Mg, etc. These elements are found in compounds with other elements like potassium nitrate, ammonium nitrate, monoammonium phosphate, potassium sulfate ..... you get the picture. In dry fertilizers, 'ballast', which can be a variety of things like smelting slag and limestone, make up a portion of the blend and in the liquid fertilizers, water is a big part of the product.

    Bookmark   April 25, 2008 at 11:34PM
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tapla (mid-MI z5b-6a)

Thanks again, N. ;o)

Hi, Filix. I'm not sure what you're asking me about the elimination of N & K & the bone meal thing, but bone meal acts soo slowly that it's very ineffective in containers.

20-20-20 is 20% by weight of each - N, P, & K for a 60% total. Fertilizers are not pure NPK, Ca, Mg, etc. These elements are found in compounds with other elements like potassium nitrate, ammonium nitrate, monoammonium phosphate, potassium sulfate ..... you get the picture. In dry fertilizers, 'ballast', which can be a variety of things like smelting slag and limestone, make up a portion of the blend and in the liquid fertilizers, water is a big part of the product.

Al

    Bookmark   April 25, 2008 at 11:35PM
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filix

Hi Al.And thanks. Yes I've learned that stuff like bone meal is not for containers, thanks to you:) How about a chemical fert like 0-15-0 just for an example. Would something like that benefit plants? Or 0-.5-0 ? filix.

    Bookmark   April 26, 2008 at 7:05AM
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tapla (mid-MI z5b-6a)

0-any number you choose-0 is only appropriate when you have a known P deficiency and all other nutrients are already in the soil in the adequacy range; so for the purpose of container culture we can say, pratically speaking, never.

Al

    Bookmark   April 26, 2008 at 10:54AM
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filix

Thankyou very much Al. I'm learning slowly. This is a good subject for me. I need to learn more about fertilizer. filix.

    Bookmark   April 26, 2008 at 8:30PM
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citrusnut

Wow! This is great stuff. Thank you so much for your explanations. You are helping me to become a better grower.

Theresa

    Bookmark   April 27, 2008 at 7:55PM
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tapla (mid-MI z5b-6a)

...... very kind of you guys to say so. Thank you very much - I'm glad you were able to take something useful from the effort.

Al

    Bookmark   April 27, 2008 at 8:59PM
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tapla (mid-MI z5b-6a)

I never bump my own posts, but in this case I'm making a small exception. It's been a while since this has been on the front page of the container forum, so rather than repost it, I thought I'd bump it in case there is fresh interest, and in the hope some may find it helpful.

Take care.

Al

    Bookmark   March 10, 2011 at 11:03PM
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jojosplants(9/ Tucson, Az.)

Great Article to have handy with spring around the corner. ;-)

JJ

    Bookmark   March 11, 2011 at 6:32PM
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jojosplants(9/ Tucson, Az.)

bump....

    Bookmark   April 4, 2011 at 1:53AM
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derbyka

This was very useful and I learned a lot.

    Bookmark   April 15, 2011 at 9:13PM
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