Liming & pH

Nan's Nook : Archives : Botanicals : Cactus : San Pedro : Growing San Pedro

Posted by: Nanook Jun 30 03, 08:13 PM GMT

My outdoor work was cut short today by an early evening pop-up thunderstorm and nothing beats lime into pots like rain does, and I wanted to add a tip.

The weeds were let go all spring in the cactus pots, crabgrass especially loves the sweeter soil and has very intrusive roots. However letting them get started in the tops of the pot in the spring, then pulling them when they get established, breaks any crust on the tops of the pots, and opens channels in the soil mix for water and lime to get down to the roots wink.gif

The pots were weeded out yesterday, today with a heavy downpour I tossed powdered limestone rock on everything and ran inside before the rain started. These pots have not yet been limed, so we ran around tossing handfuls of powdered rock ad lib.

user posted image

These cacti above are in commercial peat based soil and had not been limed previously. Base rot had already started in one rooted cutting due to the pH drop and resulting acidic soil conditions which San Pedro cannot tolerate.

These Peyote and ornamental cacti are potted in the correct soil mix and get 1/4 cup per pot.
user posted image

Larger pots, or pots that have not gotten lime previously should get 1/2 cup of powered rock.
user posted image


Posted by: Toad Jul 10 03, 05:56 PM GMT
When you said lime liberally I never knew, it looks like a 1/2 cup give or take per pot,

That explains some of my color problems.


Posted by: Nanook Jul 11 03, 11:55 PM GMT
Exactly Toad, if they have not been limed in awhile, 1/2 cup of powdered rock per pot is about right. Then use lime water: About one heaping cup powdered rock swirled in 5 gallons water and dipped into the pots... Use this about once a week or so if you are fertilizing to "sweeten" and "cool" the pots, especially if you are using a high nitrogen summer feed for watering every day or two. If/when the powdered stone dissolves away (it vanishes with time) you need to dump another 1/2 cup on there.

Powdered limestone or powdered dolomite (you want lots of magnesium in cactus culture) is the best for the soil mix additive and for top dressings.  Pelleted lime may be used for both, be sure to mix well and water in as well as possible if using pellets.

In a sense, this is a stone ablation tek. The interaction of water, growing roots, fertilizer, and organic matter in the pots produces acids which then dissolve limestone.  The dissolved stone provides essential nutrients (calcium and magnesium) at the correct pH for absorption.  This also happens to be the best pH for uptake of the primary N-K-P nutrients... But there has to be limestone present for the soil chemistry to function properly, and even limestone gravel as part of the soil base is not enough stone to handle daily summer watering and frequent fertilizations... You need to keep a top dressing of powdered lime on the tops of the pots in summer for intensive grows.

If the pots are metabolizing correctly it is not uncommon for cultures of blue-green algae to begin growing on the wet powdered lime and the sides of the clay pots. It's a good sign indicating correct pH and proper summer moisture conditions. Keep em well limed, and keep em wet

Posted by: Fungusmaximus Aug 01 03, 12:05 PM GMT
Does PH solve every cactus problem?
I know its important but damn!
I need to ask you more experienced growers to help draw some conclusions.

OK you know the black spots from insects problem I was having?
Well I checked the PH with a quality meter and found the PH was low, below 6 ohmy.gif
I have since limed the soil weekly getting it as high as 9+, the cactus has really improved. Its fattened the tip and the black spots seemed to have vanished. THey are there but the numbers are far less. And no new bug bites either.
Ive also noticed some of the cacti yellowing w/ spots.
Even when in full sun all day. THis too was solved by liming a great deal.
And last some of the cactus were starting to shrivel like they weren't getting water.
They were getting plenty, they just needed some lime.
A few days after I limed them, there was a noticeable difference and the shriveling was starting to reverse.

Is it just me or is a PH imbalance the cure all for most cacti ailments?
But how does it keep the cactus from sustaining bug bites?
Bugs just don't screw with them if they have a high ph, its sorta odd?...


Posted by: Nanook Aug 01 03, 02:17 PM GMT
When they are not in chemically correct soil, the little chemical factories in the cacti that make the natural insecticides don't get supplied with nutes... The cacti go wanting.

Healthy cactus roots in good soil will push the tops, get them so healthy and bitter... Most of the pests go for weak plants, and low pH = weak plants.

They rot easier, suffer more insect damage, do not grow as fast (or at all), refuse to green up...

I good healthy dose of lime is the cure wink.gif Sick plants I top dress with lime, and I use hardwood ashes to make a tea. I soak em well, working the lime and ashwater into the pots... Takes a few days, but they come back.

Most of the sick cacti I have seen were in poor soil mixes... The sickest ones are always potted in peat based soil bulb.gif

Posted by: Driador Aug 01 03, 02:25 PM GMT
Speaking of pH, can either of you guys (or perhaps anyone else), recommend a good electronic pH meter that's not overly expensive?

Posted by: ion Aug 01 03, 03:22 PM GMT
Define "overly expensive"... good pH meters are difficult to keep good, too, so you have to spend some more cash on buffer and calibration solutions... if you have a wet-bulb type. Dry probes are either stupid cheap (like the ones for soil testing) or crazy expensive. If you want it for analytical chem, it's gonna be pricy. If you want it for soil, just use a cheap test kit or meter from the store. wink.gif

I posted it a long time ago, and Nan's been saying it for years, max... pH is extremely important! It has to be right, or the cactus has nothing to grow, defend itself, or make goodies with. smile.gif

Most problems can be solved by correcting pH for the specific type of plant you are growing. Petes and pedros (and most other cacti, in fact) need a pH around 7.5 - 8.5.

Other plants like a slightly acid soil... that's the major reason most plants (not cacti) do so well in store-bought potting soil (peat-based)... most folks think it's because the stuff is nice and fluffy, but that's only a small part of the picture. wink.gif


Posted by: ion Aug 01 03, 03:25 PM GMT
BTW, I won't use ashes anymore... the fine dust clogs my soil and it annoys me. tongue.gif

But, I will filter water through ashes with a coffee filter or just use some dissolved KOH in the water after this winter is over. biggrin.gif


Posted by: Fungusmaximus Aug 01 03, 04:28 PM GMT
I just read some info that stated if your cacti are yellowing the soil is too alkaline.
Is this so?

Posted by: Nanook Aug 01 03, 10:31 PM GMT
Not in my experience... They yellow from not enough lime. Lime does not really hurt them.

Posted by: Dr. Bombay Aug 01 03, 11:01 PM GMT
can you take some soil, mix it with distilled water, then test with ph paper?

Posted by: Nanook Aug 01 03, 11:07 PM GMT
What I have done is set the pot up on some bricks so you can collect water from the drain hole. Pour distilled through the pot and test your sample. I like this method because it gives you a better average pH wink.gif

Posted by: ion Aug 02 03, 05:37 AM GMT
Yes, Dr., that's the way most folks do it. Nan's method scares me... I'd have to fertilize my pots immediately after leaching with distilled like that. ohmy.gif

Just stir some soil barely covered in distilled (ph 7... test it first to be sure) water and let it settle... then test the thin top layer of water with the hydrion strips. You don't want solid particles to touch the paper, as they can give you spurious results (imagine a chunk of lime touching the wet paper). wink.gif


Posted by: Nanook Aug 02 03, 08:52 AM GMT
I would think there are enough minerals in the soil to salt buffer the water. Rain water is pretty pure (used to be). But I usually only do this once (if at all) in the spring, when the plants are still pretty dormant. That gives me a starting pH so I know how to work the pots...

But that was years ago. I can look at a cactus and soil... They talk to me, I can tell the pH without a test, I know what they need when I am tending them. All add lib at this point, I just look at em and ask em what they need.

Posted by: CockyMandrill Aug 02 03, 09:47 AM GMT
Most rain water around cities and stuff is usually acid rain though now, its not uncommon for it to be as low as pH4 at times. Luckily the streams around here are on limestone or else it would cause a lot more problems.


Posted by: Bob Roberts Aug 02 03, 08:34 PM GMT

Hello, I've decided to post this under a new topic of it's own for easier viewing. I have been seeing alot of talk about pH and it just made sense for me to post some info on how to test for pH. I test the pH of various crops that I grow once a week and would like to present two easy, standardized methods for effectively measuring the pH of your soil. If followed correctly, the data can be used to ensure optimum growth and disease-preventative conditions for whatever you may grow. In addition, graphically tracking the data will help you to understand the effects of different conditions/inputs on your crop.

In horticulture, when we do pH tests, we typically use 1 of 2 methods.

The first is a

Saturated Media Extract/Dilution Variation

1. Obtain 50 mL of soil from the media to be tested
2. Dry soil on a screen until there is no moisture left
3. Measure 100 mL of Distilled water
4. Add 100 mL of Distilled water to 50 mL of Soil
5. Stir gently and let sit for at least 30 minutes until complete saturation
6. Set up a funnel and obtain some filter paper (coffee filter will work -- must be pH neutral)
Note: If you use a coffee filter, you may need to put a cotton swab underneath so the filter will not have
a blowout in the next step.
7. Pour Saturated Media into coffee filter and let drain into a container. In this step you are only wanting
the liquid to come through. If you mess up, then get a new filter and try again.
8. Measure with a pH meter. You may also do an Electrical Conductivity (EC) test now or a Total Dissolved
Solids Test (TDS) now.
9. This method relies on a 1:2 ratio of soil:water so you may use that for obtaining amounts of soil. 50 mL
should be the minimum amount of soil in my opinion.

The next process is gaining acceptance in Greenhouse Management and I can personally verify it's accuracy in comparison to the Modified Dilution.

1. Make sure the pots to be tested are all of consistent moisture (saturated is best). If not, then it's
better to water all of them until saturated using your normal water that you use
2. Wait 30-50 minutes if you watered.
3. Place a clean saucer underneath the pot(s) to be tested.
4. Add enough distilled water to the media to obtain 50-100 mL in the saucer.
5. Test pH/EC now.

Below is the style of pH/EC/TDS tester that I'd recommend. It has an electrode on the bottom that does the measuring. Using these methods, you may also use other measuring devices, but I've found this type to be the easiest and is highly accurate when calibrated correctly.

user posted image

Posted by: Fungusmaximus Aug 02 03, 08:09 PM GMT
I hate those meters!
That damn wet bulb mad.gif

They are fairly accurate but you must use them in a water test.
Use em in soil and you get a meter with dirt all stuck in the end, then you break the diode in the end trying to clean it out, I did blush.gif

Posted by: Bob Roberts Aug 02 03, 08:16 PM GMT
Exactly, use them in a water test, which is what they are made for wink.gif. Follow the steps I outlined above and you'll be about as accurate as you need to be or even could be without using a vacuum filter. They are definitely not made for soil!

Agricultural Lime