The Relationship Between Water pH And Soil pH

This is my fourth harvest and I am still only just learning to appreciate the vast repository of knowledge and experience required to grow great herb. And that is my long-term ambition. I live on the periphery of Europe where there isn't really any tradition of growing so I've been learning the hard way, making mistakes all along but I've still grown some great herb and opened my mates' eyes up to what they should be doing - which I'm sure we'll all agree is a good thing.

I've had one recurring problem with all of my efforts, from the earliest harvest to the latest experiment - my ladies' leaves tend to droop down near the tips and the older, lower leaves yellow out and become brittle much too quickly. From what I've read recently I am almost 100% sure that this is a pH-related issue causing nutrient lock-out in the plant, specifically nitrogen.

This has opened my eyes to the crucial part that pH plays in nutrient uptake in soil. I really want to understand more on this subject as I would like to move to organic growing as soon as possible.

When I started pH balancing for this grow, (first time I've ever bothered pH balancing) I began adjusting water to 5.5 wrongly assuming that this value suited both Hydro and soil. Noob mistake I know. So I'm left with very acidic soil after 3 and a half weeks and nute deficiency problems.

I know now that the ideal soil pH is 6.8, ensuring optimal uptake of nutrients. My question is this, given that most peat-based commercial soils will have a low pH value, (I'm growing with BioBizz Light-Mix, pH 6.2), wouldn't a neutral-ish pH water-feed end up contributing to a soil pH well below our target of 6.8?

I'm not just speculating, I've been conducting some admittedly basic experiments designed to get a better understanding of my soil's pH and even when adding water with a slightly basic value of 7.2 pH, soil samples and run-off tend to read in the low sixes. For example one experiment I conducted involved taking 10gm of soil and making a dilution with 50ml of pH adjusted water, measured at 7.2 pH. After shaking and leaving to settle for ten minutes I used my electronic meter to take a reading. Result? 6.4

I conducted a similar experiment with run-off from the plants. Now given that they're still recovering from being fed pH 5.5 water I'm loathe to saturate them in order to get a good dark run-off. Still I watered with a pH around 7.2 and the resulting run-off read 6.0 pH Okay, maybe I ended up flushing out some more acidic moisture but still...

I've read somewhere that 6.5 is the ideal pH value to adjust to when watering but when added to a slightly acidic soil (in the low sixes), wouldn't that just end up with a soil pH around the roots that stays in the low sixes?

Given the information above if we're growing in soil and aiming for a soil pH of 6.8 wouldn't it be best to water with a high pH feed? Something like 7.6 at the least?

Hey Man. Heres my 2 cents worth....

Yea, good on ya...you were certainly suffering a pH lockout.

Anywhere between 6.3 and 6.8 is great for soil/soilless grows. Some plants are more picky about their pH....and some, you will notice differences if you grow the same strain over time and adjust the pH from 6.3-6.8 over many grows observing if you see any improvements in things or not.

I pH my nute solution and water to 6.5 every time.

I played with runoffs for awhile. Seems they can be errouneous sometimes....but if you check them regularly (pain in the arse IMO) and take notes, you certainly do build data by trend analysis.

Sound,

Should you notice run-offs if you're giving plants the correct amount of water though? I'm assuming that avoiding run-offs helps avoid over-watering.

I suppose my argument is hypothetical and I appreciate that it's difficult to get an accurate reading of the soil's pH around the roots. I'm basing the information on a post by MynameStitch that states fairly convincingly that 6.8 is the best soil pH to aim for, http://www.icmag.com/ic/showthread.php?t=11688. Well, if you're using a spaghnum peat based medium with a pH around 6.2 surely feeding plants with water pH'd down to 6.5 isn't going to give you a soil pH close to 6.8? From the experiments I've done, feeding plants water pH'd at 6.5 would probably give you a soil pH of 6.2 I'm guessing.

Is it dangerous to feed with a regimen of 7.4 water for example? For my next grow I'm definitely going to be adding dolomitic lime to the soil-mix, even if I don't go organic as that seems to be the easiest way of keeping the soil mix close to the magic number of 6.8.

However, does anyone believe that a feeding regime of 7.4 pH water could bring a pH 6.2 soil close to a pH of 6.8 or is it too risky to attempt?

My ph is 7.8 in my tap water. I have used FF Ocean Forest soil and Botanicare Pure Blend nutes with good results never worrying about PH. No water treatment. I wonder what my soil ph works out to...I may check next grow.

If you have Dolomite lime in your soil, pH should not be a concern.

-Funk

Here is a thread you should read.
https://www.icmag.com/ic/showthread.php?t=53792

soil and nute mixes for the organic grower.

FunkBomb said:

If you have Dolomite lime in your soil, pH should not be a concern.

-Funk

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I second that.

Thanks for the advice everyone. Does anyone know a good ratio of lime to soil? I'm assuming that gardeners' lime which can be picked up in most garden centres does the trick just as well.

I live in the land of Metric so if I was filling out 20 ltr pots for a greenhouse grow in the spring what would be the ideal amount of lime to add to the mix?

bcell said:

My ph is 7.8 in my tap water. I have used FF Ocean Forest soil and Botanicare Pure Blend nutes with good results never worrying about PH. No water treatment. I wonder what my soil ph works out to...I may check next grow.

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pH should be detailed on the bag along with NPK ratings and other info. Nitrogen rich nutrients typically bring your pH down so if you were adding say 4ml of nutes to a litre of water your pH could drop to 7.5. Also, if you're leaving water standing over-night to allow chlorine to evaporate the pH value will rise by a couple of decimal points.

My tap-water is pH 7.1 but when I leave it stand it usually rises to 7.4. If following the orthodox regimen for watering soil then I suppose I should be pHing down to 6.5 or 6.7 depending on how the plant seems to be doing.

Googan said:

Is it dangerous to feed with a regimen of 7.4 water for example? For my next grow I'm definitely going to be adding dolomitic lime to the soil-mix, even if I don't go organic as that seems to be the easiest way of keeping the soil mix close to the magic number of 6.8.

However, does anyone believe that a feeding regime of 7.4 pH water could bring a pH 6.2 soil close to a pH of 6.8 or is it too risky to attempt?

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Googan,

Very good question...I have wondered the same... I get conflicting answers when i ask this question....Some people say, dont worry about run-off in soil, just PH the water going in and thats all....and others say, its all about the run off.....So i dont really know the answer to that question.... But generally, I add "about" 1 cup of powdered dolomite lime to 1 cubic foot of soil...OR 2 table spoons to 1 gallon....So i would say for a liter, i would add about 2 TEASPOONS...As long as you mix it well, you dont have to be precise with the measurements...Even though i use lime, i still PH my water/nutes to 6.5 to 6.8 before i feed the girls...alot of folks say that is not necessary, but it sure won't hurt....Sounds like you are doing a really good job of figuring things out as you go...If your not more than a couple weeks in 12/12, maybe transplant into bigger containers and mix up some lime in your soil... That will help you out....Just make sure you get the right lime.. There is alot of info about "lime" on here (icmag)...I got a 40 pound bag of Powdered Dolomite Lime from LOWES, you should be able to find that at any garden store or home improvement chain...Hope all this helps man... Keep it green..Take care..

Googan said:

Well, if you're using a spaghnum peat based medium with a pH around 6.2 surely feeding plants with water pH'd down to 6.5 isn't going to give you a soil pH close to 6.8? From the experiments I've done, feeding plants water pH'd at 6.5 would probably give you a soil pH of 6.2 I'm guessing.

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Even though with some water sources there is sometimes a connection between high pH (from the source) and a high level of alkalinity, altering the pH of the irrigation water has little or no impact on the pH of soil or soilless substrates. The pH of the water impacts the chemical solubility of nutrients in the soil / soilless substrate solution.

High levels of alkalinity in the irrigation water can cause the pH of soil and soilless substrates to rise.

many, many credible sources that have been made available to everyone first in libraries and now on the internet can confirm this. it is accepted crop science.

examples:

http://ohioline.osu.edu/hyg-fact/1000/1249.html

How pH and Alkalinity Affect Fertility

Water pH, water alkalinity, and the pH of the growing mix are closely connected. Over time, all three factors affect fertility. For example, high or low soil pH values usually produce minor element deficiencies or toxicities. Of the two, water alkalinity is more important than water pH in influencing media pH. Water with high pH but low alkalinity will have little effect on media pH over time. However, highly alkaline irrigation water, regardless of its pH, may have a profound effect on media pH. Water pH is a poor indicator of the capacity of water to modify the pH of the growing mix; alkalinity, however, is an excellent indicator. Growers who intend to manage media pH must know the alkalinity of their water. Water pH can also affect the stability of some pesticides and growth regulators. Acidifying the spray water is advisable when water pH is 7 or higher and alkalinity is 150 ppm or more.

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and

http://www.umass.edu/umext/floriculture/fact_sheets/greenhouse_management/waterph.html

High pH and High Alkalinity Effects on Plant Nutrition

Potential adverse effects. In most cases irrigating with water having a "high pH" ( 7) causes no problems as long as the alkalinity is low. This water will probably have little effect on growing medium pH because it has little ability to neutralize acidity. This situation is typical for many growers using municipal water in Massachusetts, including water originating from the Quabbin Reservoir.

Of greater concern is the case where water having both high pH and high alkalinity is used for irrigation. In Massachusetts this situation is most common in Berkshire county. One result is that the pH of the growing medium may increase significantly with time. This increase may be so large that normal lime rates must be reduced by as much as 50%. In effect the water acts as a dilute solution of limestone! The problem is most serious when plants are grown in small containers because small volumes of soil are poorly buffered to pH change. Therefore, the combination of high pH and high alkalinity is of particular concern in plug seedling trays. Trace element deficiencies and imbalances of calcium (Ca) and magnesium (Mg) can result from irrigating with high alkalinity water.

It is much more difficult to predict the effects of irrigating outdoor flower crops, gardens, and landscape plants with water having high pH and high alkalinity. On the one hand, in some parts of the United States, long-term irrigation of crops with water high in bicarbonates and carbonates has led to yield-limiting trace element deficiencies which must be corrected with special fertilizers. On the other hand, in New England, several factors probably act together to partially offset the effects of high alkalinity water. First, rainfall levels are relatively high and historically this has caused Ca and Mg ions to leach from the soil. These are replaced with H+ and the result is acidic soil. Second, this acidification may be helped along by the rather acidic rainfall common in this region in more recent times. Third, acid-forming fertilizers also help counteract high pH and alkalinity.

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It is confusing still, because sometimes in literature about farming and gardening, it is assumed by the writer that high water and pH and high levels of alkalinity go hand in hand, but this is not always true. It's doubly confusing because of the similarity of the word alkalinity to the word alkaline.

Alkalinity and Water Quality

Alkalinity refers to the capability of water to neutralize acid. This is really an expression of buffering capacity. A buffer is a solution to which an acid can be added without changing the concentration of available H+ ions (without changing the pH) appreciably. It essentially absorbs the excess H+ ions and protects the water body from fluctuations in pH. In most natural water bodies in Kentucky the buffering system is carbonate-bicarbonate (CO2HCO3 CO32-). The presence of calcium carbonate or other compounds such as magnesium carbonate contribute carbonate ions to the buffering system. Alkalinity is often related to hardness because the main source of alkalinity is usually from carbonate rocks (limestone) which are mostly CaCO3. If CaCO3 actually accounts for most of the alkalinity, hardness in CaCO3 is equal to alkalinity. Since hard water contains metal carbonates (mostly CaCO3) it is high in alkalinity. Conversely, unless carbonate is associated with sodium or potassium which don't contribute to hardness, soft water usually has low alkalinity and little buffering capacity. So, generally, soft water is much more susceptible to fluctuations in pH from acid rains or acid contamination.

Methodology: Alkalinity is an electrometric measurement which is performed by the computer aided titrimeter (CAT) and the pH electrode. A potentiometric titration is taken to an end-point reading of pH 4.5. The amount of acid required to reach a pH of 4.5 is expressed in milliliters. The calcium ions (CO3) neutralize the acid in this reaction, and show the buffering capacity of the sample. From the amount of acid used, a calculation will indicate the amount of carbonate (CO3) involved in the reaction. This then is expressed as mg of CaCO3/L even though actually part of the alkalinity may be contributed by MgCO3 , Na2CO3 or K2CO3.

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I have always adjusted my pH of my nutes to 6.5. I like that number b/c its right smack dab in the middle of the 6.3-6.8 which is your target zone. I like to use a pH balanced soil when growing (7.0) and b4 I put any plant in it I hit it with a good watering of 5.8-6.0. This helps to set the pH of my soil around where I want it. After that I always water with a 6.5 mix and use the water catching trays under the pots to catch the run off. But don't be too hasty in removing and emptying the trays, I find that the soil with soak up the excess water usually, and bottom watering is ideal a buddy told me once, but too much of a pain in the ass to bother with for me anyway. Shoot for 6.5, and then adjust it once you become familiar with your strain. Take lots of notes and you'll learn whats best for you and your family.

If there's enough humus via worm castings or compost, the PH is irrelevant. Humus buffers PH, so the plant still feeds, regardless of PH.

If yer plants are yellowing out to soon in flower, try feeding em grow nutes through the first 2 weeks in flower before switching over to flower nutes. Some strains ( and even different phenos ) are worse than others about this sorta thing. Good luck! BC

I agree with Carl Carlson about the alkilinity. I used to clean pools for a job and as long as the alkilinity of the water was in the ideal range, the water would not only hold chemicals longer, but it would have a much more stable Ph over time when compared to a pool with alkilinity readings that were not in range. I don't know how much that related to growing, but it's something I observed over time.

Ive found that the quality of my water source dictates how well my plants uptake nutrients from my organic soil mix. Fresh stream water from a pure source will obviously give better results than a municipal water source

Lisdexik

Googan said:

This is my fourth harvest and I am still only just learning to appreciate the vast repository of knowledge and experience required to grow great herb. And that is my long-term ambition. I live on the periphery of Europe where there isn't really any tradition of growing so I've been learning the hard way, making mistakes all along but I've still grown some great herb and opened my mates' eyes up to what they should be doing - which I'm sure we'll all agree is a good thing.

Click to expand...

certainly there is a tradition of farming though.

FunkBomb said:

If you have Dolomite lime in your soil, pH should not be a concern.

-Funk

Click to expand...

My experience exactly. I use 1 1/2 tablespoons per gallon of medium.

heres a couple good links.
i use promix more like a hydro medium and get the best results at 5.8-6.2.
http://www.biology-online.org/11/1_plant_cells_vs_animal_cells.htm
http://en.wikipedia.org/wiki/Soil_pH

i cant find the good one tho. if i do, ill post it

Carl Carlson said:

certainly there is a tradition of farming though.

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Yeah, but you can't smoke potatoes. We've had it bad here for a long, long time... soapbar
I'm going to wean them onto pH6.7 for a few weeks and see how they'll respond, I won't push too hard towards 7 just to compensate for the ph 5.5 they were getting originally... stoopid me.

Thanks for all the advice again. Great little community. They've survived to their first week of 12/12 and they aint' looking too shabby.

Keep it green.

Googan said:

Great little community.

Keep it green.

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Yeah... I agree totally... ICMAG is a daily read for me...Its like crack for MJ growers....Nice smiley by the way..lol...i was changing my 2 year olds diaper the other day and i ralphed all over the little dude and the changing table....worm poop is fine, hell even bat shit is ok...but human poop is no good....i dont see how any of this pertains to PH fluctuations, but its still relevant, because it happened...atleast to me...On a side note.....I would advise to add 1-2 tablespoons of dolomite lime to your soil mix..i use 2:1 FFOF soil to FF Light warrior and add lime and worm castings to every batch...I still PH my water because one of my bloom ferts will drop the PH to the 4's....i may not need to PH my water but it makes me sleep better at night, and my buds are nice and dense and rock hard, (depending on the strain)....i do have hard water though, and from the above readings, i guess my PH is a little more stable for it, so maybe i dont have to worry about PH at all....Who knows, im a skivvy stoner...Later folks....grow BIG FAT SWEET/PUNGENT globs of buds....good luck and be safe.

Vinegar to lower PH

Vinegar to lower PH

Anyone use vinegar to lower pH? A 1/4 tsp per gallon lowered my water ph from 7.8 to 6.6. I have heard it being used with good results - no salts like some commercial ph lower.