Apologetic note: In the creation of this "Beginners Guide", I photographed my own "newly purchased" cacti. So the cactus in the pictures below will appear amongst these photographs on a kind of a random basis, depending on which picture was best for the topic at the time or maybe even just the one that was in best focus at the time. Please forgive the fact that this is not a professionally produced instruction manual.
I went to Lowe's to pick up a couple of inexpensive plastic pots for cacti that needed a new pot, as they were rootbound. And wound up coming home with 9 cacti in addition to the pots. As I examined these cacti, I noticed that the grower had used peat as a growing medium, and the root balls of the tiny root bound cacti were just as hard as stone. They actually felt like a a baseball they were so hard.
Not wanting for my new cacti to continue their existence in this extremely hard dried peat, as peat is difficult to re-wet once dried, I decided to try and use water to soften the root balls. So I stuck the new cacti in their new pots and watered them. This did a couple of things... 1) It wet them so that the root ball might soften up some, and 2) gave them their first drink of spring.
Two days later I decided they had softened enough, so I de-potted them yet once again and began to rinse the root balls in a neutral temperature water stream, catching the detritus in a kitchen strainer. I did this in my kitchen sink without hesitation as my source of water is a well. It's not city tap water.
Despite having been wet and softening for two days, it was still quite difficult to rinse the peat from the root balls. And quite time-consuming also, as I wanted to spare the roots as much trauma as possible by being as gentle as i could. Unless the damage you do to your roots is severe, they will regenerate, and quite handily. But I prefer to avoid as much damage to the roots, which provide water and nutrients to the cactus, as is possible. But despite my care, some breakage was inevitable, and on one of the cacti, I actually used a sharp knife and sliced the last eighth of an inch from the end of the very thick-with-roots root ball in order to gain access to the peat underneath it.
As you can see from the photo of what was left below, I managed to do this "peat removal" without breaking off too many roots. You can see from the next two pictures that I had mixed results in rinsing the peat from the root balls of my cacti. Some had very thick roots that were not so extensive as some of the others, and some had very fine roots that literally invaded the peat and held onto it so tightly that it was nearly impossible to rinse more than a small percentage of the peat from the root balls. I laid the cacti on a thick layer of paper towels and allowed them to dry all that day and all of the following night. The place where I live has central heat and air and so the air here is very dry as the A/C runs nearly constantly. So by the time I got to them this morning, the roots were already very dry. But take note here that I am careful that the roots were dry before I proceeded any further.
So now it's time to repot. For this batch of repotting I'm using somewhere between 10% and 20% potting soil depending on which measurement you use, weight or volume or number of scoops. Anyway, it was a scant percentage of organic material. Of the remaining percentage of the mix, i used equal amounts of pumice, and turface.
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A great many arguments can be made on all sides of a discussion regarding the construction of a potting medium. Many cacti will not only grow, but will thrive in a wide variety of growing media. But one thing I'm afraid I'll have to insist is true. That cacti do not like to have their feet wet for long periods of time. The soil that they live in must be quick draining, and while it needs to retain some degree of moisture, must not retain it for too long. The longer I grow cacti, the more I am convinced that a high percentage of inorganic material is just absolutely necessary. Please note that I labeled this topic as "A beginner's guide" as that is certainly what I am, and what this topic is directed toward. Also note that while I insist on this being true, it is a general rule, and that your cacti may well indeed have very specific requirements regarding this particular facet of its environment, and that it is strictly up to you to do the research necessary to discover whether or not your specific cactus has specific requirements that supersede this general rule.
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First I select some of the larger pieces of the pumice, as a bottom layer in the pot for use as an aid to drainage. A very thin layer, just to insure the drain holes remain unclogged. Some pots have large drainage holes, or a large number of drainage holes and so I like to use screen in the bottom of my pots to keep the soil from washing out the bottom every time I water. Care must be exerted here to ensure that the screen does not clog from salt deposits that will form from bottom watering or that the screen does not clog from very small particles in the soil. In both cases, this will raise the water level in the pot closer to the roots which in all cases is to be avoided with all due diligence! And then put my layer of pumice in the bottom of the pot. ElieEstephane, moderator, and one of the more knowledgeable members here reminds me that this drainage layer should not be applied if the growing media that you use is less gritty then what I have chosen as it has a tendency to raise the water level closer to the roots. And as we all know, this is to be assiduously avoided. I then pick the cactus up, and turning it over spread the roots out as far as possible. Hold the cactus high up off the bottom of the pot so that the cactus is not resting on the bottom, and using the utensil of your choice fill the pot up right to the very Brim using your selected potting medium. I have never been able to repot a plant without making a big mess. But as an aid in keeping my mess as limited as possible, I try and fill my pots while they are sitting in the bowl of potting media, and fill it completely to the brim. Using some non-sharp item (to avoid damaging the roots), like a dowel or a chopstick... I use a stainless steel chopstick... poke your Implement of choice down into the soil as far as it will go with a gentle inserting motion. Do this continually around the edge of the pot until you can no longer insert your implement any further than a quarter of an inch. This will have removed air pockets from your potting medium. You will at this point note that the level of the soil has dropped a quarter of an inch or so completely around the edges of the pot, leaving you some room to water your cactus. Some people like to use top-dressing in their pots. And in other cases, some cacti like to keep their root neck up away from the wet soil by the addition of a little grit. In either case the addition of some material is at this point called for. Remove a little bit of soil, just enough for your top dressing or grit and a little room for adding water, and then insert enough of your selected material to support the cactus all the way around.
In this case, I found the root neck of the cactus was quite soft, and so suspecting that it might be susceptible to rot, I have elected to use very small pieces of both black and red volcanic rock to isolate the root neck from the soil, keeping it dry. While volcanic rock is extremely light and very airy, thus not blocking airflow, the black and red combination was made for aesthetic purposes. And now that the repotting job is done I'm going to set these in a bright window, NOT direct sun, and I'm going to wait a full 7 days before I water them, in order to not encourage any pathogen from introducing itself through roots that might have been damaged during repotting. A FULL week to heal before watering after repotting.