Evaluating your plots soils: Testing your PH

Evaluating soil PH
Soil PH is important, you can have all the best fertilisers in the soil but if the PH is far enough off the ideal, the nutrients may not be available to your plants, this is called "Nutrient lockout". Too high or low a PH changes the properties of the nutrients, and makes them unusable by your plants. A good PH helps keep your plants healthy and growing strong, because the nutrients they need are available to them.

The scale for measuring pH runs from 0 to 14, with 7 assigned as neutral. A pH below 7 is acid; a pH above 7 is alkaline.

Here is a link explaining soil PH- Link to Wikpedia explaining soil PH

Marijuana grows in soils with a pH range from 5 to 8.5, but it thrives in nearly neutral soils. A PH of 6.5 to 7.5 is ideal.

Taking a soil sample
The first thing you need is a soil sample to work with, tests run on this sample will tell the tale of your soils potential.

To collect an sample, scrape away the first two inches of soil and dig a hole 6 inches down. Try to take a sample at this depth that contains as little topsoil or litter(leaves, twigs) as possible, take about 2 gallons or so of soil. Be sure to label it, so you know which plot it came from. Now head home to run some tests.

If you will be adding organic materials like peat or compost to loosen or otherwise improve your soil texture, you will need to prepare a soil sample for PH testing that has the same ratios of native soils and the organic materials.

A example: You have clay soil that needs texture improvement, you have decided you will be replacing 25% of the soil with Peat moss. Take a 1 cup measure, fill it to 3/4 cup with the native soil sample, now add peat moss until you have 1 cup. Now moisten it, and mix it very well. This will now serve as your soil sample for PH testing.

How to test your soils PH

Litmus paper-

To test your soils PH using Litmus paper you need to make a water sample with the plots soil. Fill a glass jar about 1/3 full with your soil sample, fill the rest with distilled water. Shake the jar vigorously and then allow it to settle out for 24 hours. The clear (or semi-clear) water on top contains your sample.

When a strip of Litmus paper is dipped in your sample, its color will change in shade, in reaction to the PH of your sample. Comparing the color of the PH paper with a chart supplied with it will show your PH.
Litmus paper can be hard to find, places to look: Beer and wine making stores, Drugstores, Healthfood stores, Medical supply, Lab supply, or online.
You need two sets of Litmus, one that reads Acid(4-7), and one that reads alkiline (7-9).

Rapid test soil test kit-

This another good way to test your soil PH, it also comes with tests for the big 3 plant nutrients, Nitrogen, Phosphorus, and Potassium(NPK), which may also come in handy. With this test, you make a sample similar to how I described for the Litmus paper, a chemical dye is added to the water sample in a vial, the dye causes the water to change a certain shade of color, which indicates its PH, the tests for NPK are similar. A chart showing the PH scale with their representitive shades are convenently built into each vile for comparison.
This is the method I personally use. There are other brands available, your local growshop should have them.

Hand held PH meter

These are PH meters like indoor hydro growers use, if you can afford one they are the easiest way to test PH. You use them to test soil the same way you use Litmus paper, by making a sample with your soil and distilled water. Your local grow shop should have them.

Rapidtest PH meter
These meters are frequently found to be inaccurate, and I don't advise using one. Here is alink to a test comparing it to other methods- A comparison of soil PH test kits

Your soils PH and what to do with it

Your soils PH and what to do with it

As stated above, Cannabis will grow in soils from PH 5 to 8.5, many growers feel that 6.5 to 7.5 is ideal. If your soil falls into this range, you should be in good shape! If not, move on to the next step, ajusting your soil PH.

Acid soils(Below PH 6.5)
If you soil is below PH 6.5, raising your PH is advised, this is most commonly done with either Dolomite lime or Hydrated lime.

Dolomite lime is the one most used because it also ammends your soil with Calcium and Magnesium, micro-nutrients that Cannabis needs. Also it is gentler than Hydrated lime, and it is harder to hurt your soil with over application.

Rather than haul all your supplies to the plot, fill you holes and test them, we will make a test batch of the soil at home. With this test batch we will find out how much Lime will be needed to treat each hole.

If you are going to be mixing organic ammendments like Peat moss or Compost into your holes to improve texture, you will need to make a soil sample with the same ratios of native soil and introduced materials(If you will be using Perlite, it has neutral PH, and doesn't need to included in this test).

A example: Your soil contains 50% Clay, you will be replacing half of the native soil with Peat moss. To make a soil sample, fill a gallon container half full with the native soil sample, now fill it to 1 gallon with Peat moss, (dump in a larger container for mixing) moisten the soil and mix it very well. Now you have your soil sample, move on to the next step.

Into a container larger than a gallon, dump 1 gallon of your soil sample. Now add two Tablespoons of Fine Dolomite lime, moisten the soil and mix very well. Let the mixture sit for 1 hour, and do a PH test.

If the test puts your soil at, or above 6.3, this will be good, by the time the Dolomite is done treating the soil months later, its PH should rise to 7, you can move on to the next step. If your test came up short, add one more Table spoon of Dolomite, mix and repeat test, this should fix it.

Now you know how much dolomite you will need to treat each gallon of soil in your plots holes.

If you aren't sure how to estimate your holes size, read this:
First determine how large your holes will be in gallons. Convert the number of gallons into Cubic feet, there are 7.5 gallons in a cubic foot, so number of gallons divided by 7.5 equals needed cubic footage of hole.

A cubic foot is a cube that measures 1 foot on all sides. A hole measuring 1 foot by 1 foot wide, and 2 feet deep would have 2 cubic feet of capacity 1'x1'x2'=2 cubic feet. A hole measuring 1.5 feet by 1.5 feet wide and 2 feet deep would have 4.5 cubic feet of capacity, 1.5'x1.5'x2'=4.5 cubic feet.

Once you know how large your hole will be, multiply the number of Tablespoons of Dolomite it took to treat your 1 gallon soil sample, by the number of gallons each hole will contain.

A example: I want 15 gallon holes, so 15 gallons divided by 7.5 equals 2 cubic feet, I dig my holes 1 foot by 1 foot wide, and 2 feet deep.
My 1 gallon soil sample PH reached 6.5 with 2 tablespoons of Dolomite lime, so I will multiply 2 tablespoons by 15 gallons, for a total of 30 TBS of Dolomite per hole.

Alkaline soils(Above PH 7.5)
Very rarely growers may encounter soils that are too alkaline, the procedure for ajusting is similar to ajusting a low PH, insted of Dolomite use pure Sulfur in 1/2 Table spoon increments to find your needed ammounts.

Great stuff, Backcountry! Thanks for posting!

Somthing I have been meaning to do, should have collected a sample before the ground frize though lol.

thanks alot for the informative posts man.

they also sell litmus paper at pool stores, no need to go to a warehouse or anything, if pools rnt the norm where u live jsut hit up a grow shop or home depot/lowes

its kurmit said:

they also sell litmus paper at pool stores, no need to go to a warehouse or anything, if pools rnt the norm where u live jsut hit up a grow shop or home depot/lowes

Click to expand...

Yes, I considered listing pool ph test kits and strips but decided not to based on the fact that Pool PH testers usually only measure the Alkaline end of spectrum which would be PH 7 and above, at best some measure down to 6, we need to be able to measure from 5 to 8, considering that chances are you will encounter mostly acid and neutral soils in most regions.

Also consider that most Lowes and Home depots will carry the Rapidtest soil testing kits in the gardening sections as well, I actually have found them to be easier and more positive to use than Litmus paper and Pool testing strips(yes, I tried using them a few years back).

Hey buuuuudy

Just another day at the office for BACKCOUNTRY.....truely a fine OGer, ICer, proliferator of cannabis.

Ladies and gents, you have no idea how lucky you are to have this fella here spreading the knowledge. You are learning from one of the best in my opinion.


Readers.....let me tell ya somethin'....pH is one of the most important factors for healthy plant life. The expensive(but accurate) pH meters are great and will help you keep pH at a steady and healthy level(which depends on your medium).Litmus paper is not very accurate because it'll only tell you if the solution is acidic or basic....but not the concentration of hydronium ions. So it will only give you a general idea of where your pH is.

A good pH pen will prove to be one of the best investments for your grow. Big fat, juicy, healthy, trichome covered nugs usually get that way by having all the nutrients they need. And when your pH is out of the proper range...nutrients get locked out. Which is not good for a marijuana plant you are trying to help thrive.

So get a pH meter and get some dolomite lime and help your plants grow fat and frosty buds.

Thanks My soil is a little high and everything I have read said to add sulfer to lower the PH. Gypsum powder sounds like it may be more enviroment friendly? I have also heard or read somewhere that added organic fertilizers will also lower the PH is this true? Thanks

yep.....ferts are acidic and will lower pH. I have yet to run into one that brought the pH up.

Sorry to steal your action, BC. But I was here first....so....

BELOW FRIGID- You have brought up a good point, after reseaching a bit more, it turns out Gypsum is not as good for lowering PH as other sources have claimed. At one time it was common knowlege that Gypsum was the best ammendment for ajusting high PH, but now this has been proven wrong, looks like my sources of info on that subject were a bit too old.

Pure elemental Sulfur is the best way to lower soil PH, keep in mind that it is more than 4 times as powerful at lowering PH as Dolomite is at raising PH.

So in making test batches of soil, add it in 1/2 TBS increments to test its effects on your soil. I will edit my post on lowering PH to reflect this.

Thankyou so much for helping me find this mistake! What region of the world do you live in, if I may ask?

The use of large ammounts of weak organic fertilisers like Steer manure, and weak composts like leaf mold can lower PH, although it can be hard to be sure how much a given fertiliser will affect your soil, I would not rely on it exclusivley, but it can help!

Not to Go off the Subject BC

Not to Go off the Subject BC

thats beautiful BC, must be a trip to grow in the hills/mountains, you must really have to wear stuff to blend in there, as people can easily see ya moving from a many different places & angles ! Shheww, ive been scoutin the woods alot lately, (flat & sandy here) and not wanting someone seeing me, just so if i truelly like a place/plot area (sun, water a must !) i can feel confident about working it, oh yeah i dont prefer camo most of the time, just something that doesn't stand out. Peace

*bump*

Great post.
Should be a sticky.

hey another great thread by back country. haha no surprise here. does soil PH vary by city or state or maybe just particular location?

Hey Boris!
You can find trends in PH from region to region, here are a few.

The Pacific Northwest tends to have a low PH, due to our high rainfall, which has washed the Cal-Mag content out of our soils.

The Mid-west is typically thought of as having a neutral or slightly high PH, due to the fact most of the region is based on ancient seabeds, its high Cal-Mag content is due to large deposites of shellfish based limestone. This naturally balanced PH is part of the reason the region is so naturally fertile.
Parts of Central and southern California apparently also are based on Limestone, as are parts of the Atlantic seaboard, if you have heard your area has a "Limestone soil", chances are your local soil PH is neutral.

Dry desert regions with mineral based soils usually are neutral or slightly high in PH, unless the soils are Volcanic based, where the Sulfur content may lower the PH.

Wet, swampy,and marshy areas are often low in PH, again due to the moisture washing away much of the minerals that balance the PH.

Great info BC!

Being from the midwest I think of our clay as having buffering capacity for soil acidity..... Usually I dig down to subsoil clay, breaking it up well, layer it with organic amendments on top of the refill dirt so the organics are covered in clay, and then mix....

Used lime last year too, so I don't want to get carried away over here and end up with too high pH .... Think we'll do a test this fall....

awesome thanks alot BC i learn something new and interesintg everytime you write. do you think a lil dolomite lime would be sufficient to raise my native soil up to neutral... im guessing its in the 6.0-6.5 range... im in the seattle area so your righ the cal mag is all washed out.... fuckin rain haha anyways i got my outdoor plot cleaneed up and my gorgeous alibis and cascadias are all a couple inches tall

Thanks guys!

Boris-If I was you, I'd go ahead and test the soil as I showed earlier in the thread, you may be surprised at what it actually is.

If you just don't want to go to the trouble of testing the PH and how much Dolomite lime it will take to fix, then just add about 2 TBS of Dolomite per gallon of hole, and cross your fingers. It may or may not be enough, but it won't hurt.

haha yea im kinda lazy when it comes to being exact.. i will be more precise with my indoor grow once i get my own place for sure. thanks for the advice i will do just that and hope for best.