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Does the PH really matter?

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#1 HeadPawthead


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Posted 11 February 2008 - 04:58 PM

Nutrient availability is pH dependent so it definitely matters. Cannabis can tolerate a wide pH range, but does best between 6.2 & 6.8 in soil/less media and 5.8 - 6.0 in hydroponic media. There are two pH's that matter; that of the water going into the system and that of the root zone. Measuring the pH of the root zone can be difficult in soil/less media since the probes often require that a media-water slurry be made. The pH of the runoff water is often measured as an estimate of the actual acidity/alkalinity of the medium. In my experience, this estimate is good enough.

How do I measure pH?
The best way is a wateproof digital pen. These are typically accurate to 0.1 or better and cost anywhere from $20 to several hundred dollars. Hydroponic growers need this accuracy as the pH range is much more narrow than that of soil/less media.

Dipsticks can be had that change color for comparison against a chart. They typically have an accuracy of about 0.5. pH paper, also sold as nitrazine paper measures by color change as well and is also accurate to about 0.5. They are sold for around $10.

Kits that use a sample of water into which a couple drops of "developer" are added are the least desirable devices. They depend upon comparing the color of the water sample to a chart and are often only accurate to 1.0. If the water sample is not colorless before adding the developer, the kit's accuracy will be skewed from the beginning. These are available for about $5.

Do I measure pH before or after adding nutrients, etc to the water?
Add everything to the water and mix it well, then measure the pH. Nutrients, growth stimulants, etc will alter the pH, so it's best to adjust it right before the water is applied to the plants.

I've added all the stuff to my water and the pH is x.x, what now?
If growing in a soil/less medium the pH should be approximately 6.5 and in hydroponic media the pH should be about 6.0 before application.

To lower the pH an acid must be added to the water and to raise the pH a base must be added. There are products available on the shelf called "pH up" & "pH down". They work well. There are also household products that will accomplish the same goal.

Household "pH downs"
Vinegar (white distilled, apple cider, wine...)
Lemon juice
Household "pH ups"
Tap water (my preference) Municipal water is often pH'd above 7.0 to improve palatability (taste).
Baking Soda (NOT baking powder)

Baking soda is sodium bicarbonate. Its use can cause problems so I advise against it. It works well in a pinch AND when used infrequently. Increased sodium concentration in the medium displaces K and may cause a condition appearing as K deficiency. There is a maxim in chemistry which states "water follows sodium"; if the sodium concentration in the medium is high, water can actually be wicked AWAY from the roots causing wilt.

How much pH up or pH down do I add to change the pH of my water by 1.0?
That all depends upon the quality of the initial water sample. Quality is determined by ppm/tds/ec measurement. Water samples with lower ppm/tds/ec measurements will require LESS acid or base to change the pH of the sample as compared to those with higher tds/ppm/ec numbers. "Hard water" will have high tds/ppm/ec.

Regardless of what a product's label states, some trial and error has to occur. The manufacturer can't know the purity of any given water sample.

Add a little of the product, mix well, and measure the pH. Once you get the pH in the proper range note the amount of product you added. Next time add that amount to your water and measure to see if the pH comes out in the same range. Once you've reproduced the pH several times you can safely put the pH meter away until the plants shows signs of distress. That said, any time the source of water changes OR the additives change (different brand, new additive, etc) the same procedure will need to be performed.

I wouldn’t recommend using a probe that you place in the soil. The pH pens are much more accurate.

Here are the methods of testing your soil pH. I’ve put both methods here, but the one I use and the one you’ll most likely want to use is the runoff method because it’s much faster and easier to do. After you get a handle on what’s happening with your pH you can dial in your soil and nutrient regiment so you won’t need to test the pH very often.

Both methods below will work. I put the “collecting soil sample” tutorial because it’s considered to be the only real scientific way of testing the soil pH. I’ve found the runoff method to be very reliable if done correctly, and again, it’s the one I use.

Collecting soil and mixing with distilled water method:

Soil Sample Preparation

1. Calibrate the pH pen.

2. Scoop up loose soil samples with a clean, dry plastic
spoon. Avoid touching the soil with your hands to prevent contaminating the sample. Take soil sample as deep as possible without affecting roots.

3. Remove any stones and crush any clumps of soil.

4. Fill up your sample soil up to 3/4 and add distilled water to the jar. Cap the jar tight and shake it vigorously a few times. Let the mixed sample stand for 5-10 minutes to dissolve the salts in the soil.

5. Dip the pH pen electrode into the wet soil slurry. Take the reading when it stabilizes.

6. Rinse your pH pen thoroughly in clean water between each use.

Soil pH Data
The pH test value in this procedure is accurate to ±0.5 pH or better (usually ±0.2 pH). The soil sample preparation and test procedure is adapted from accepted laboratory methods. Most soil pH measurement cannot achieve ±0.1 pH accuracy, even with elaborate laboratory procedures and expensive pH instruments.

Recommendations for Best Results
Prepare and run at least three tests of the same soil sample to confirm results. Minor (< ±0.2 pH) or no differences between readings indicate good technique and high confidence in results. Larger differences (> ±0.5 pH) require more testing.

Testing runoff after watering method:

1. Use distilled or reverse-osmosis water. This is inert water that will readily take on the active soil pH.

2. Do not adjust the pH of the water being used for the test, as that will buffer the results in the direction of the pH adjustment.

3. Scuff the surface of the soil to allow for uniform wetting and drainage of the soil.

4. Poor water in slowly until the soil is saturated with water. You may get a small amount of runoff, but it’s best to just saturate the soil.

5. Wait 30 minutes. Chill out and smoke one.

6. Add the amount of water that will give you a small concentrated amount of runoff.

7. Test the runoff.

I like to see that the runoff, for my soil anyway, is rust colored. That lets me know that there are a lot of constituents of the soil present in the sample as opposed to a more clear runoff that would be indicative of the water running down the side of the pot or not filtering nicely through the medium. In other words, get that water dirty.
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#2 Tokecrazy


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Posted 12 February 2008 - 09:34 AM

HeyHSIC,Great post there. It's always good to test your run off water before transplanting and all through the grow.I picked up this method a few years back when I grew in soil.You may know the lab rat,{stinkyattic},showed me this, take a large strainer and put a coffee filter in it and fill it with the soil you are testing.Use the same ph water that you are going to water with for your test.Pour in the water filling the strainer up.Let the water drain into bowl or what you have, take a paper towel and push down on the watery soil and force it out a little.Then take your PH reading and tds if you like.The readings will tell you what you will do ,raise the PH or lower the PH.Just like HSIC said,don't use baking soda to ph with,I did and I was having big problems with mag and calcium in the flower cycle and ph swings all through the veg cycle.It was locking out a lot of the nutr needed.In hydro baking soda will not hold the PH stable very long and vinegar will lose it stability about as fast as soda.I don't use any thing but PH up & PH down from General Hydroponics.Don't buy PH up and down from {homedepot or lowes and fish stores} they didn't work and you have to use a lot of it too.Well that's all I got.Great post HSIC. PEACE
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#3 spun420



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Posted 13 December 2009 - 09:06 PM

'I wouldn’t recommend using a probe that you place in the soil" why not are they not as consistent and for arguemnt sake say if U where to use do thee same methods apply ?
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#4 Lumix


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Posted 13 December 2009 - 09:59 PM

Yes a balanced Ph help regulate the uptake of nutes

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#5 Guest_johnface51_*

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Posted 13 December 2009 - 11:30 PM

can i use pool acid to drop the ph ?

#6 Messmore3



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Posted 13 December 2009 - 11:33 PM

Sure does, if it is too high your plants will suffer from what I hear. Insects and mold can come from a ph that is too high or too low. For example if PH is too high it can cause plant deficiencies of important micronutrients such as zinc and manganese which are still in the soil but "tied-up" and unavailable to the plant
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Posted 13 December 2009 - 11:56 PM

can i use pool acid to drop the ph ?

What is its main ingredients???

#8 CaliWildViolet420


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Posted 20 February 2010 - 12:18 AM

Great Post...thanks...Was searching high and low for soil PH...found it here...
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#9 Dr. Trees

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Posted 20 February 2010 - 12:59 AM

Great post HSIC. I can't help but wonder how much this would have helped me in my early days of growing. I'm sure it'll come in handy for a lot of growers to come.

'I wouldn’t recommend using a probe that you place in the soil" why not are they not as consistent and for arguemnt sake say if U where to use do thee same methods apply ?

As far as soil probes go, they're pretty much pieces of junk. The only way to really know if you're measurement was accurate, would be to test it using some other method, and at that point the soil probe becomes obsolete!

Thanks again for this post.

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#10 Bonniesicker


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Posted 18 August 2010 - 04:56 AM

Hi all! Excellent and very vital dicussion. As one new to cultivation, I have learned that obtaining a pH report is the first step in treating any disorder. Oftentimes the disorder can be traced directly to the pH being out of wack. Thanks
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#11 Bong


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Posted 06 August 2012 - 02:14 AM

hey dude great info on the ph, is it true u can white vinnegar to bring ur ph up? bonghead
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#12 Tokecrazy


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Posted 06 August 2012 - 05:30 AM

Yes it will work. But it doesn't stay in the mix very long. It will lower the PH .Baking soda raises the Ph.
This what I use to lower PH. https://www.greenpas.../7871-ph-minus/
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#13 Bong


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Posted 06 August 2012 - 11:06 AM

so do u use acid or ph down for fish tanks toke? bonghead
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#14 PoeticLife


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Posted 06 August 2012 - 03:49 PM

nice to see a thread resurface.... now gotta remember what can be used as a substitute for up/down!!!
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#15 Blue61


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Posted 07 August 2012 - 12:51 PM

It only costs about 10 bucks per liter of ph up and down and it lasts for ever.My bottle of ph down is still full after 3 years. My bottle of ph down is about 1/3 full after the same amount of time.I would splurge and get the stuff that stays stable and doesn't wear off quick. Peace, Blue

#16 flyinglow



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Posted 03 December 2015 - 12:13 AM


 This is a great topic. I love to hear people talk about ph, seems so many people

dont understand it. I love the chart posted above by Lumix, which clearly shows

which nutes get locked out at different ph ranges.


I read headpawtheads post closely and am confused by it.

Why in the world would you test your water and nutes and adjust the ph too?

and than test your runoff? This makes no sense to me.


1-test water and nutes, write down ph value for later reference <value for arguments sake is ten>

place plant on a plate or something simular and catch your run off....

test your run off....<value for arguments sake is five>


so in this instance, my soil has a runoff ph of 5 and my water and nutes are 10

This means I need to raise the ph of my water and nutes to 11.5 (not lower it)

That extra 1.5 we just added will bring our run off up to an acceptable ph of 6.5

Peace out

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