Low P High N grow results

Alright guys, it's been a few months of me running around here talking about 'low P', dishing out the theories and research I've been tracking, etc. Finally wanted to post some results. I'm about 2 weeks from harvest, and 90% of this grow so far has been on a custom low P high N diet the entire grow. The only thing I changed later in flowering is adding more epsom salt (MgSO4) than I used during veg, maybe 10-20% more.


Phosphorus- everything I've read on the subject in published papers, plus some good anecdotal evidence and forum claims, seems to show that low P levels are very beneficial as far as plant health, and overall health of the chemical solution itself. Phosphates have a very interesting solution chemistry that involves ph-dependent speciation (its not always a phosphate), very similar to carbonates. Carbonates and phosphates also both tend to form low-solubility compounds with metal cations (Mg, Ca, Mo, etc). The variable speciation of these ions makes them both valuable buffers, because when ph changes the speciation starts balancing itself against certain factors such as solubility (dissociation), and additionally in the carbonate case, dissolved CO2. This is why you'll find most pH Downs are phosphate based, and pH Ups are carbonate based.


These solubility rules are also mostly what governs the "nutrient availability at a certain pH" chart. It's not that X element or Y element is less available at a certain pH, it's more that "at a certain pH, a less-dissociative compound may form". So at a certain pH where... Calcium is less available, it's actually more like: "at this pH, calcium phosphate may be less available" or "calcium sulfate may be less available". So it's a matter of 2 ions, at a strong concentration, with the right pH, then it may take on a less 'available' (dissociative) form. This also becomes more dependent on the concentration of various ions in your recipe. So a low P mix may actually have a higher Ca/Mg availability at a low pH than something with high P.

So, what does this really mean? Well, it means that every solubility/availability issues involves two parts. A low pH will make calcium less available, provided there is a strong phosphate or carbonate concentration. Well shit. So these nice buffered GH pH ups and downs are actually making that situation worse.

Remember when I mentioned the ph-dependent speciation makes them good buffers? Well, it also makes them really slow reactions. Carbonate buffers will eventually equalize with available dissolved CO2 from the air. Phosphates are also slow, and not bound by dissolved gasses, but... shit. They're also a super in-demand nutrient for the plants and microbes. As in, most bio-life will eat phosphorus as fast as it possibly can, the plants too. This means the added phosphates are a big target for any algae, bacteria, fungus, you name it. Once a microbial bloom dies, most of the phosphorus is released back into solution- giving you rather large swings of phosphorus, and opening you up to many pathogen and root issues.

So anyways, most of the papers I read stated that available soluble phosphate in soil really never gets above 30 ppm, and thats during the peak of the phosphorus cycle in soil.

Here's some P ppms for various formulas:

GrowWeedEasy DWC: 10ppm - 53ppm - 33ppm
Lucas light (5/10): 67ppm
Lucas heavy (8/16): 107ppm
GH recommend (half strength): 16ppm - 40ppm - 51ppm

Besides lucas, most of the schedules tip-toe around the 30ppm mark. I designed my formula to keep a steady 30ppm. It's similar to week 3 of the GH recommend recirculating schedule, but with less micro and more epsom. But it's basically 4ml micro, 8ml grow, 3ml bloom per gal + 0.1g epsom (all per gallon) + tap water. I kept this steady for most of the entire grow (first few weeks was something a bit different), but with more epsom salt during late flowering.

All of flowering so far as has been 1 rez change, I've only been adding water or nutrient mix (at same ratios) when ppm drops. I've been using a phosphate test kit to check for phosphate levels specifically, but I've honestly not seen em drop below 20ppm yet (highest my kit tests for). I've been treating with Great White, and dosing with Floralicious+ every few weeks (sparingly, and only with a good dose of great white).

As far as 'high N', my mix pretty much just keeps a steady 120ppm N. It's not high for veg, but it is a bit high for flowering normally.

So the main way I fixed my pH was by ditching the store-bought GH pH Up and Down. I switched to sulfuric acid and potassium hydroxide.

Potassium Hydroxide you can get like... 2 pounds of dry salts for less than $20 on Amazon. This will make about 10L of pH up at similar strength. Potassium is another super soluble ion with a high electrical-conductivity that the plants can deal with very easily. It's used internally as an 'ion-exchange' currency, it's the main electrolyte used by plant cells. Be careful when mixing this shit up, you can melt your hand off.

For the sulfuric acid, I personally just bought some reagent grade sulfuric. If you have a chemical supply house near you, you can grab some pre-made 10%. Nitric acid would be even better because nitrates have really strong solubility characteristics. Plants can deal with up to 300ppm of nitrates in solution before it's definitely toxic. Most of the N toxicity you see with premade nutrients is actually Ammonium toxicity. Ammonium can only be tolerated up to about 20ppm, so when you see nitrogen sick plants, its usually the ammonium.

Strong acids like nitric or sulfuric are also great at breaking carbonates in hard water. If you crash your ph (to like 4) with a strong acid, then bring it back up with a strong base (like potassium hydroxide) you'll have destroyed most of the carbonates in the water, and it should behave a bit more like RO (you convert the calcium carbonate into calcium nitrate/sulfate).

So, I can now officially confirm that this at least hasn't killed my plants lol. They're almost done flowering, and everything seems to be going normally. My pH doesn't creep up at all, only down. This is really the only natural way the ph should shift ever. If the plants drink more water than nutrients, the pH will drop because the solution is getting stronger. If the solution is too week, the pH should still drop. The plants natural reaction to most (except N and K i think) nutrient deficiencies is to drop the pH by exuding organic acids. This is the plants main mechanism for extracting nutrients from alkaline soil. The roots however normally rely on the ph of the soil to 'bounce' back up, so this is really the main place where manual intervention is needed with DWC/water-culture.

I've got sources for 90% of this stuff, if something looks fishy I can 90% for sure reference the source. I actually found a bunch of this info reading around about Mych research. They're very involved in the phosphorus cycle, ph management, and root exudates.

Here's what my last week of pH looked like. All the bumps up are me adding potassium hydroxide or tap water. I honestly haven't had to use any acid/down in a long while. The rez will stay stable for a few days before it drops off again. I only have 10 gal for 4 plants, so I think the exudates end up swinging the ph far more than normal.



and here's the buds so far. These are both top buds really close to the light, so they're curling a bit, but all the lower fan leaves look 100% (click to enlarge).




The second one has had reallly dark leaves since flowering. It's also easily my most aggressive plant, so I really don't think its N toxicity. The others are also a more tame color. But besides the crazy dark color, I have seen 0 nutritional oddities out of the plants since... 2 weeks after 100% switching to these feed schedule/procedure. Before i switched over I had a rough 2 weeks of flushing trying to get rid of damn phosphate precipitates caused by using too much GH pH Down (like 250ml of it in 4 weeks).

hi, your plant definitely is showing high N signs and possibly low PH as well, the clawing leaves are a very clear sign to high N. I am researching this topic and have done many trials on MJ and veggies like tomatoes cukes and peppers, and up until now, the research is showing that MJ specifically likes higher levels of P than most veggies. I have found that growing with high N and low P can leave you with a better looking plant like a plastic plant in flowering but with smaller buds. I find my plants asking for P in flowering and it seems as if they like it balanced to the same levels or just below the K levels. I currently have 4 flowering res's, 3 with self mixed ferts, and 1 GH ferts, and I can say that I find myself hovering around 2-3-4 NPK during flowering. GH plants are looking perfect but their bud growth is the least of all 4 recipes NPK of GH res is around 3-2-4 and it is proving too much N especially with that Ammonium Nitrate in it which is very fast to be absorbed. I have one res with low P and higher NK and it is also showing very little signs of deficiency with less bud development. A little bit deficiency is normal in flowering ime, the plant is practically preparing to die, and the main goal must be to produce the most buds and not the least deficiencies

zem said:

hi, your plant definitely is showing high N signs and possibly low PH as well, the clawing leaves are a very clear sign to high N. I am researching this topic and have done many trials on MJ and veggies like tomatoes cukes and peppers, and up until now, the research is showing that MJ specifically likes higher levels of P than most veggies. I have found that growing with high N and low P can leave you with a better looking plant like a plastic plant in flowering but with smaller buds. I find my plants asking for P in flowering and it seems as if they like it balanced to the same levels or just below the K levels. I currently have 4 flowering res's, 3 with self mixed ferts, and 1 GH ferts, and I can say that I find myself hovering around 2-3-4 NPK during flowering. GH plants are looking perfect but their bud growth is the least of all 4 recipes NPK of GH res is around 3-2-4 and it is proving too much N especially with that Ammonium Nitrate in it which is very fast to be absorbed. I have one res with low P and higher NK and it is also showing very little signs of deficiency with less bud development. A little bit deficiency is normal in flowering ime, the plant is practically preparing to die, and the main goal must be to produce the most buds and not the least deficiencies

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Attaching a larger group photo, there's almost no clawing lower down and no tip burn. Not sure about bud size yet tho! That's definitely a factor I've been wondering about, so far I have at least one 3" wide nug on the far right plant. I'm getting a lot of popcorn buds below, but I think that's more about my shit trimming and light availability in this cramped 2x4.

So honestly, I'm not really that big on the NPK ratios in general- plants can regulate nitrate (pure nitrate, not ammonium) and potassium up to almost 300ppms each. So a 2-2-4 5ml/gal and a 1-1-3 at 10ml/gal still give you the same total P ppms, but different N/K levels. Provided you swap the solution out regularly, it really shouldn't matter, plants shouldn't become N or K deficient. That's why I'm really sold on N and K based pH control, almost nutritively benign to the plant. I honestly feel like... micros, P, and ammonium are really the only things you need to "precisely control". The 3(3.5)-1-4 elemental (would be 3-2-4 standard NPK) is just based on those chromatography reports that AN published, that's just the ratio of the elements in the plant tissue at the time of flowering. I'm not totally sure how accurate that is, and might vary greatly between genetics, but it's still a decent figure to deal in if you want the plant to try and consume the nutrients at the same rate. Then I just took that NPK ratio and grounded it around 30ppm P.

EDIT: and I agree about the ammonium in the pre-mixed solutions. Next grow is going to be Peters Professional based (no ammonium) and I'll just use ammonium nitrate to dial in the ammonium ppms exactly without having to worry about my micro mix.

Also, here's a group pic, they're all pretty dark, but they're half blueberry which i guess can get pretty dark. The top buds are only inches away from the light =\ So the curling is definitely more near the top. The tent is also half outdoors- so they suffer 40 F nights pretty often the last few weeks:

I'm also not disagreeing that the plants may become more P hungry during flowering- but eventually they do match that chromatography reading (roughly), and this is where im really relying on my phosphate test kit. If they get really P hungry, I can check it and bring the levels back up to 30ppm at will. While with the N/K, I'm just trying to 'preload' the predicted amount into the solution, because it really doesnt effect availability of other nutrients.

I think that 30 ppm is way too low, that is less than the magnesium and sulfur ppm. I also experience increased growth with the plants that are more fed with the potassium phosphate salt which is 28%P and 23% K. I want to ask you a question i think that i did not grasp it perfectly, when you say that only micros, P and ammonia must be closely regulated, why is that and what exactly happens to the P? Do you have to keep adding P regularly to dose it at your preferred 30ppm while you can load the solution with N and K one time and almost forget about them? So when i am adding a lot more P than you are, is it precipitating the Calcium and magnesium as well? I would like a more comprehensive reading about phosphate and how it acts in hydroponic solutions if you have a link, thanks.

Can you post a pic of the roots?

Those leaves look like they're plastic. they oughtta be loaded with trichs, but they're not.

zem said:

I think that 30 ppm is way too low, that is less than the magnesium and sulfur ppm. I also experience increased growth with the plants that are more fed with the potassium phosphate salt which is 28%P and 23% K. I want to ask you a question i think that i did not grasp it perfectly, when you say that only micros, P and ammonia must be closely regulated, why is that and what exactly happens to the P? Do you have to keep adding P regularly to dose it at your preferred 30ppm while you can load the solution with N and K one time and almost forget about them? So when i am adding a lot more P than you are, is it precipitating the Calcium and magnesium as well? I would like a more comprehensive reading about phosphate and how it acts in hydroponic solutions if you have a link, thanks.

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Let me know if you can't find the PDF for this paper, I can PM you the file (I got it from JSTOR i think). The paper talks about both soil and hydroponics, so it kinda flips between them, but is really good at describing the differences (basically cation exchange and ability to hold insoluble phosphate compounds).

Effectiveness of Phosphorus Fertilizers in Hydroponics and Glasshouse Settings with Moderate and High Organic Mater Soils
Je rey Sean Christian Summerhays
Brigham Young University - Provo

Providing adequate food, fiber, and fuel for society requires efficient P fertilization (Hopkins et al., 200. Phosphorus is needed in relatively large quantities for crop production. Phosphorus is among the top four mineral nutrients in regard to plant concentration (Havlin et al., 2005) but is often applied at rates nearly equal to N and K due to poor solubility in soils. Unfortunately, P is so poorly soluble that uptake by plant roots can be problematic (Foth and Ellis, 1996; Hopkins et al., 200. Therefore, plant root growth and diffusion promote P uptake. Consequently, P deficiencies are relatively more common in the early part of the growing season when plant roots are developing and soil temperatures are relatively cool (Hopkins et al., 200.

...

Although N is usually the limiting factor for plant growth in soil-based systems, P is generally the limiting factor in fresh water systems (Mueller and Dennis, 1996; Sharpley et al., 2003).

...

Recovery of P from fertilizer is impacted by soil constituents (cations) and pH (Sposito, 200. For example, solubility of P minerals is dramatically less in strongly alkaline and acidic soils compared to soils of slightly acidic to neutral pH. In acidic soil, the negatively charged phosphate bonds with Al, Fe, and Mn and forms poorly soluble mineral precipitates. A similar reaction occurs in alkaline soil, as phosphate bonds with Ca and Mg.

...

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Then the paper goes on to talk about a few different fertilizers, and how they get around P solubility issues by chelating most of the cations. They then move on to a hydroponics experiment with corn, which eventually shows that P concentration stops being beneficial after about 100ppm.

http://www.plantphysiol.org/content/116/2/447.full
This paper is reallly reallllly wordy and deep, but it covers pretty much the entire lifespan of P in the plant. Most important, it points out the concentration of available phosphate that would normally be in soil. Points out that you see a max of about 50ppm available P in the soil at any point. Usually about 10 (100 um).

http://extension.psu.edu/plants/nutrient-management/educational/soil-fertility/managing-phosphorus-for-crop-production
Also points out some common soil P availability, no more than 50ppm. Does point out that soil has far more unavailable P, but it's locked in the soil as an insoluble or organic form. Also points out:

Below 30 ppm phosphorus, additional phosphorus must be applied to build up the soil for optimum crop production. Above 50 ppm phosphorus, there will be no benefit to adding additional phosphorus.

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This indicates that you should *heavily* amend soil so that it produces the available 30-50ppm P, even though you may be adding a solution with 100ppm P. Totally different from hydro! We just want to target the 30-50, as everything we add in solution is available phosphate (Pi).

So according to @Airwalker16, dirt plants will always have inferior trichome counts =\

This nice Cornell paper that describes a few recipes, all under 50ppm P, and no change for flowering/veg.
http://www.greenhouse.cornell.edu/crops/factsheets/hydroponic-recipes.pdf

http://cpl.usu.edu/files/publications/publication/pub__9984184.pdf

Plants quickly remove their daily ration of some nutrients while other nutrients accumulate in the solution. This means that the concentrations of nitrogen, phosphorous, and potassium can be at low levels in the solution (0.1 mM or a few ppm) because these nutrients are in the plant, where we want them. Maintaining a high concentrations of nutrients in the solution can result in excessive uptake that can lead to nutrient imbalances. For example, the water removed from solution through transpiration must be replaced and it is necessary to have about 0.5 mM phosphorous in the refill solution. If the refill solution was added once each day, the phosphorous would be absorbed by the plant in a few hours and the solution phosphorous concentration would be close to zero. This does not indicate a deficiency, rather it indicates a healthy plant with rapid nutrient uptake. If phosphorous was maintained at 0.5 mM in the recirculating solution, the phosphorous concentration in the plant could increase to 1% of the dry mass, which is 3 times higher than the optimum in most plants. This high phosphorous level can induce iron and zinc deficiency (Chaney and Coulombe, 1982). Feeding plants in this way is like the daily feeding of a pet dog, some dogs would be badly overweight if their food bowls were kept continuously full.

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Basically all these papers describe that the plant is REALLLY good at utilizing low P ppm in solution, and maintaining far higher concentration within the plant cells.

Now for the less scholarly sources:
http://www.420magazine.com/forums/how-to-grow-marijuana/71652-never-ending-abuse-phosphorous-enhance-flowering.html
http://www.growersunderground.com/PhosphorusMyth.pdf
https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/phosphate.pdf
http://www.gardenmyths.com/bloom-booster-fertilizer-nonsense/
http://blog.brightagrotech.com/phosphorus-in-aquaponics/

Airwalker16 said:

Those leaves look like they're plastic. they oughtta be loaded with trichs, but they're not.

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That could be from anything tho- trichmore count seems about 3x denser on some of these phenos, if there truly was a P deficiency then the overall trichome count would be limited, and not allowed to express itself in varied concentrations throughout the different phenos (unless one pheno was just some kind of power-house monster at uptaking P).

Also, the amount of P I'm using is almost the exact same as the GWE recirculating schedule you use, just 30ppm instead of 50ppm for flowering. So really not much different than what you do =\

zem said:

I think that 30 ppm is way too low, that is less than the magnesium and sulfur ppm. I also experience increased growth with the plants that are more fed with the potassium phosphate salt which is 28%P and 23% K. I want to ask you a question i think that i did not grasp it perfectly, when you say that only micros, P and ammonia must be closely regulated, why is that and what exactly happens to the P? Do you have to keep adding P regularly to dose it at your preferred 30ppm while you can load the solution with N and K one time and almost forget about them? So when i am adding a lot more P than you are, is it precipitating the Calcium and magnesium as well? I would like a more comprehensive reading about phosphate and how it acts in hydroponic solutions if you have a link, thanks.

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I also forgot to answer any of your questions lol

So when I'm saying to closely regulate micros (basically everything besides N/K), it's basically because the micros have relatively sensitive mechanics in solution, especially around some of the other ions needed for the nutrients. By that, I mean they tend to form less-dissociative compounds in solution (https://en.wikipedia.org/wiki/Dissociation_(chemistry)). Basically, if you add like... Calcium Nitrate, and Phosphoric Acid to some plain water, you now have 3 types of 'ions' floating around... So that's not totally true. You'll have mostly free calcium, nitrate, and phosphate ions floating around in the solution, but some of it stays in water as the Calcium Nitrate and Hydrogen Phosphate (acid). Every soluble compound dissociates over time, and never quite completely. The non-dissociated compounds may just stay suspend in solution, form colloids (semi-soluble), or will crystalize (precipitation).

Now taking that above example, this is where shit gets interesting. Now that we have those 3 types of ions floating around, brand new compounds can start to randomly form, like Calcium Phosphate in this example. Calcium Phosphate happens to be less dissociative at this pH (lets say its neutral 7). So even tho we never added that compound, it starts to form in the solution because its "stickier" (less-dissociated) than the other compounds or ionic form. Even if it doesn't precipitate out, the fact that the ions stuck together like that makes it impossible for the roots to use, and if the concentration is high enough it might start to precipitate, or scale (like phosphates tend to). But in all cases, a compound is more highly dissociated in lower concentrations. Most compounds formed with nitrate or potassium are highly dissociative in water. This is why they are strong acids and strong bases. The strong an acid or base compound of an ion, generally the more dissociative the compounds formed with that ion. There's exceptions there, but with the particular ions found in hydroponics solutions, nitrate and potassium are mostly 'spectators'.

Phosphates get even more complicated because of 'speciation'. They like to change form depending on pH, and tend to reach an equilibrium. Plants can use phosphates (PO3) and hydrogen phosphates (HPO4), then the other forms are more soluble, but not available to the plant.

So while precipitation is the ultimate worst-case scenario, there's actually issues that happen in the chemistry before then that can make the nutrients unavailable to the plant as a normal ion.

I do have to monitor my P levels, but I actually haven't seem then dip down below 20ppm yet. I check the PPM/EC first always, add nutes, then next day check if I need more P and it's been ok so far. It probably does dip down low between feedings, and my NPK ratio is probably a bit weird after this long without a rez change, but so far keeping up with PPMs has kept me up with P.

So I haven't fed the plants in 2 weeks, been adding almost a gallon of water every day slowly starting to dilute it more in preparation for flush. PPMs read (on my pen) at about 800, a new full strength mix read in at about 950 for me.

Has been about 2 weeks since i checked, just did another phosphate test, and its still off the charts high. I did double check the color scale and I guess it only goes to 10ppm (not 20), so I guess I've technically never seen it dip below 10ppm, which is about the soil range for free phosphates. So assuming the plants can effectively feed on P below 10 ppm (which they should, its what most of the soil papers point to), then my plants are not P hungry.



also, here's a pic of the roots. They're not as pearly white as they once were, but the last feed was pretty heavy, i think it dyed em.

@Yesdog ,

To get a more accurate result with this API test, you can cut the original sample with distilled water, then multiply the end result with the fraction of the cut.

Yesdog said:

That could be from anything tho- trichmore count seems about 3x denser on some of these phenos, if there truly was a P deficiency then the overall trichome count would be limited, and not allowed to express itself in varied concentrations throughout the different phenos (unless one pheno was just some kind of power-house monster at uptaking P).

Also, the amount of P I'm using is almost the exact same as the GWE recirculating schedule you use, just 30ppm instead of 50ppm for flowering. So really not much different than what you do =\

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In flower, I'm using quite a bit more of the bloom actually.

Airwalker16 said:

In flower, I'm using quite a bit more of the bloom actually.

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What feed schedule you using these days?

Dumme said:

@Yesdog ,

To get a more accurate result with this API test, you can cut the original sample with distilled water, then multiply the end result with the fraction of the cut.

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I'm a complete moron for not figuring that out lol. You're a life saver.

Ok, so followed @Dumme advice (still kicking myself for not thinking of that) and now I'm getting a reading below the max. Looks like maybe 50-60ppm phosphate. Also, I have been using this stupid test wrong- it measures ppm phosphate, which is 3x elemental phosphorus. So I have about 16-20ppm elemental P. My EC/TDS meter also read in at about 750ppm (new mix is about 950).

So, it's about where it should be, a tad low maybe. Nutrient strength is 75%, I targeted 30ppm, so I'd expect about 22ppm elemental P. Of course, I haven't tested a fresh batch of solution with this test kit so I'm not sure where it reads exactly in when its 'hot', but the tests are supposed to be accurate to within +/- 0.2.

Honestly pretty surprising to me, considering 90% of the flowering time has been without a res change (about 42 days). I've mostly been using the test just to make sure I haven't totally depleted the phosphates, really cool to actually get some meaningful values out of it now.

I'd do it again, with a different amount of distilled water, to cross check. When using multiples, the numbers tend to wonder off quite a bit.

The more you cut your solution with distilled water, the more extreme your numbers could be off.

...just a heads up

Dumme said:

I'd do it again, with a different amount of distilled water, to cross check. When using multiples, the numbers tend to wonder off quite a bit.

The more you cut your solution with distilled water, the more extreme your numbers could be off.

...just a heads up

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That's fine, I'm honestly ok with dealing up to like... 15-20ppm variance. I mostly just got the test to check for almost-total depletion, so this is even better than i could have hoped.

Yesdog, I read the sources that you linked, and i have been researching this for a while, I still think that 30 ppm is way too low, i would not go below 3-1-3 ratio, the ratio you are suggesting is something like 8-1-8 or so depending on the final ppm.

Hack & Lamott also make high range testers that go to 100ppm (4408-01)
Hanna makes the HC - HI717 colourimeter that goes to 30ppms.

..not sure on how into testing you are.

I might as well add my target ppm for Phosphate is 30ppm in my Aquaponic system, for Cannabis in full flower.

Dumme said:

Hack & Lamott also make high range testers that go to 100ppm (4408-01)
Hanna makes the HC - HI717 colourimeter that goes to 30ppms.

..not sure on how into testing you are.

I might as well add my target ppm for Phosphate is 30ppm in my Aquaponic system, for Cannabis in full flower.

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How into testing am I? Pretty into it lol. These Hanna tests all look pretty awesome, going to have to add it to my wish list for sure.

Awesome, 30ppm phosphate, so about 10ppm elemental P. Everything still flower ok? I was initially going to target 20ppm but decided to preload a bit up to 30ppm P.