Is Nutrient PPM 360 too low for Tomatoes

skoka123November 29, 2009


I bought dry nutrient variety pack from ebay as importing nutrient solutions or concentrates to India is very expensive.

I am not sure if it is ok to link to an ebay item. Therefore, please search for "AeroGarden Herbs Orchids Flowers Nutrient Variety Pack" for the exact item I am referring to.

When I mixed the pack with 1 US Gallon of water, I got a PPM of 360. As per the instructions that came with the shipment I am supposed to dilute it to 1/4 or 1/2 the concentration if using on seeds or full strength if using for grown up plants.

Wherever I search on the net I find that the Nutrient PPM should be in the range of 1500 PPM. According to the How To Hydroponics book by Keith Roberto, it should vary between 1400 and 3500 PPM.

I am confused! Should it be 1400-3500 PPM or 360 PPM as the seller implies?

Please Help.



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Hi Srikanth,

Let me try to explain you how a tomato formula is composed in the first place and how the actual concentration needed can be understood in respect of it. There are a few contradictory charts that give ranges of concentrations for various vegetables. But they actually don't tell how these charts were done or on what they were based in the first place. For my understanding, a recommended concentration without a formula is in fact useless. Why? Because it's the other way round, the total concentration is actually the result and total of each element needed and present in a specific nutrient.

Have a look at the elemental ppm content of a classical formula for tomatoes, and you'll understand what I am talking about:

N: 140 ppm
P: 50 ppm
K: 350 ppm
Mg: 50 ppm
Ca: 150 ppm
S: 150 ppm

You could also use 150 ppm of N and less K (some 280-320 would be sufficient) But let's stick with this formula which is quite generous in K.
Anyway this formula has EVERYTHING you need to grow tomatoes and it equals a total of 890 (894 if we include some 4 ppm of trace elements).

So the actual recommended ELEMENTAL concentration for tomatoes (not seedlings but maturing plants) is roughly between 900 and 1000 ppm with this formula. Much less of any element would probably not be sufficient and anything more would actually be a waste of resources and money.

Why? Because if you do raise the concentration of the solution, you'll raise the content of every macro nutrient (and T.E. as well) gradually. More than 150 ppm in N would most probably induce too vegetative growth, over 50 ppm of Phosphorus wouldn't be of any use or good, as it's sufficient anyways. Same for Mg an Sulfur. More than 350 ppm of potassium (K) would just be a waste as well!

I could give you an alternative formula, just for fun (and for those who might think that 140 ppm of N isn't sufficient):

N: 150 ppm
P: 55 ppm
K: 320 ppm
Mg: 48 ppm
Ca: 142 ppm
S: 99 ppm

For my understanding of things, in both formulas there is EVERYTHING a tomato plant needs. This-one has slightly more N, a bit more P, less K, less Ca, just enough Mg and obviously less (but enough S). The total is only 814 ppm and there isn't much change anyway (except notably lower K).

As for the Orchid formula of a total of 360 ppm:
I truly don't know how to deliver sufficient NPK (+Ca,Mg,S, etc) with 360 ppm for a tomato formula. No way...

But In case you blow it up to some 800+ ppm (instead of the recommended 360) you'll end up with some random- but most probably unbalanced (disproportional) -formula anyway! Most probably to high in N, "overdosed" in P, lack of calcium and Mg and S, and no idea how much P. I wouldn't do that it any case ...

As for other recommendations: I wonder what could possibly be added to the formulas I've listed, to reach 1400 or even those 3500 pm!?

PS: For those who have followed recent threads about the difference between elemental ppm content of a formula and ACTUAL total EC/TDS reading when measured with an instrument->

The actual TDS/EC reading of both formulas shown here would be around 975 and 1090 ppm, respectively EC 1.9 and 2.2. Not based on LEC calculations but simple empirical tests I did perform to get the difference between elemental content of a formula in use and actual reading of my instrument.

In simple words, when dissolved in water, in an actual nutrient solution they would actually give a reading of EC 1.95 and 2.18 - or the equivalent in ppm when converted 500/1.

    Bookmark   November 30, 2009 at 1:57AM
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OK. I am now beginning to think to that there is no sense in trying to attain a particular total PPM. Instead I should be focussing on the PPM of each nutrient component (N,P,K, Ca, Bo ...).

I have two questions:
1) Is there a list where I can get the nutrient requirement per nutrient component for common hydroponic plants?

2) Is there a list/chart/table that gives the PPM for each of the nutrients per gram?

Example: If I use 10 grams of 20:20:20 NPK then I know that 20% of it is Nitrogen i.e., 2 grams. I know that the molecular weight of N is around 14. Now how do I translate it to PPM. To rephrase it: How many PPM in 2Gms of Nitrogen?

As it is not possible to get readily available Nutrient Solutions in India, I am planning to use Water Soluble raw ingredients (Calcium Nitrate, Potasium Nitrate, Sulphate of Potash, Monopotassium Phospate, Magnesium Phosphate and Trace elements).


    Bookmark   November 30, 2009 at 6:01AM
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OK. I did some more internet research. Can you confirm if this understanding is correct?

If I dissolve 2 gms of Nitrogen in 1000 liter of water then it would be 2 PPM. Hence it should be 2000 PPM per liter or 528 PPM per US Gallon.

In order to attain a PPM of 150 per gallon I should be using 0.57 grams of Nitrogen per Gallon of water.

Is this correct? If so, then it is much simpler than what I thought it is.


    Bookmark   November 30, 2009 at 6:24AM
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greystoke(South Africa(11))

I still think your life would become a lot less complicated if you adopted the EC-scale as an indication of concentration.
"ppm" can be very confusing in multi element formulas, because the ppm of the individual elements dont add-up to a final ppm (or tds).
However, the EC-scale does. There is a linear relation between the EC values of the individual constituents and the EC value of the final mix.

It will take a bit of getting used to, but in the end you'll be glad you did.

    Bookmark   November 30, 2009 at 11:01PM
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Anyone who is going to calculate formulas can't avoiding to adopt ppm as a calculation unit. Hence content in single elements or trace elements, as well as the total content of any formula has to be done in ppm in the first place. All nutrient calculators I know of use ppm and not conductivity.

As for the actual practice of measuring and controlling (final) concentration, it really doesn't matter what scale you use AS LONG AS you are aware of the difference between elemental ppm content of your formula and the actual reading you'll measure. Actually, you could get as confused when using EC and converting it to (theoretical) ppm, as if you use ppm as a scale. In fact people often tend to do that very mistake when using EC-scale for final concentration. They erroneously think that the EC-reading, converted from elemental ppm/500 will give them a correct final concentration. You could even say, that In case people use an instrument that is able to switch from EC (ms/cm) to ppm, the error is even more likely to happen.

But as we know, it's not the hight or the kind of fall that causes death, but the sudden impact. As long as you understand and include the DIFFERENCE between elemental ppm of any formula and the actual reading of the final mix, (wether in EC or TDS/ppm scale or else), you're just doing perfectly fine!

    Bookmark   December 1, 2009 at 2:11AM
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greystoke(South Africa(11))

I'm trying to make a contribution by convincing people that the ppm scale is - in fact - grossly inappropriate when working with salt mixtures. There is no standard relationship between the ppm of an arbitrary salt mixture and its EC value - there never was - and your invention of "elemental contributions" (or whatever you called it) is unproffesional, confusing, and - in fact - entirely unnecessary. Conversely, I can't recommend anyone to adopt it.
Once you use the EC-scale there is no need to invent "fiddle factors". We're doing fine without them, while using scientific methods that can be reviewed and compared by anyone.

    Bookmark   December 1, 2009 at 5:45AM
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I always calculate my nutes in ppm. Not sure how you'd do it with a condcutivity meter. Then after it's all mixed up, I take an EC reading. I can then use the EC readings to judge whether to add water or to add additional nutrient solution in between nutrient changes.
Also, since conductivity is linear proportional, I can somewhat get a feel of the concentration at various EC readings. Though I can not tell from the meter (in EC or ppm) which elements have been used up the most.
Interstingly my concentration of 751 correlates to an EC of 1.5 which happens to be concentration/500. I never use the stick to 'observe' the meters EC though. I use the old math standard: current EC reading x initial concentration / initial EC reading = current concentration
The reason for using EC is that apparently the conversion factor used on EC meters is not always the same. sometimes it 500, sometimes its 750. Since we might not all use the same meter, we can't accurately discuss your nutrient levels.
Also, ALL EC METERS measure conductivity in, I believe, semens/cm. EC is in fact a conversion of this measurement multiplied times 10. ppm is this measurement multiplied by some other number dependent upon what meter you use. That is why you should state EC numbers as either EC or µsemens/cm (CF) or actual ppm as you have calculated yourself and not simply the ppm your meter spits out.

    Bookmark   December 1, 2009 at 7:57AM
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greystoke(South Africa(11))

The way I do this, is to first work out how much of the individual salts will be equired to make up the formula, taking into account possible variations in purity.
I routinely check the conductivity of a diluted sample of a salt, to verify it.
However, in the case of MgSO4 and KNO3, I keep saturated solutions, which are very constant at a fixed temperature.
MgSO4 = 26.20% w/w, density of the saturated solution = 1.3005
KNO3 = 24.24% w/w, density of the saturated solution = 1.1646

Next, I work-out the conductivity contributions of each salt to the final mix, using LEC values. This also gives me the conductivity of the final mixture.

Finally, I mix it all together and voila !

    Bookmark   December 1, 2009 at 10:21PM
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