A COMMON SENSE APPROACH TO GROWING CACTI AND OTHER SUCCULENTS
I am appalled and horrified at the really bad information you see on the computer nets and chat rooms. These are living plants just as you are a living animal! They need everything you do, so if you wouldn't do something to yourself, DON'T do it to your plants!
SOIL MIX: It can be as simple as two ingredients or as complicated as you wish. The only important thing is that it allows food, water and air to get to the roots, supports the upper part of the plant and is porous or loose enough to let water drain through. We use humus (Super Soil, low peat planter mix, coir (a coconut fiber) or a good oak leaf mold), 50/50 with large (") washed pumice. You need to adapt local materials to your use. In a hot climate a little sand will hold water in the mix a little longer. In a wet or cold climate a little small gravel in the mix will hold less water. A small pot dries out faster than a large pot does. So adjust the soil mix for that size pot to be almost dry in a week. Experiment!
ROOTS: What you see above the soil line is unimportant! The roots below the soil line do all the work. If the roots are healthy, your plant is happy, and so are you. A plant drinks - it does not eat. Nutrients dissolved in water are what a plant drinks. No water - no food reaches the plant. Only the white (or yellow) tips of the roots absorb water and feed the plant. The other roots are for producing the feeder roots and anchoring the plant in the soil. The feeder roots die when the soil is too dry or the pot gets too hot or when they are exposed to air when re-potting. When conditions are right they are rapidly replaced to feed the plant. In the case of Agaves and Aloes it's a good idea to trim back the many unneeded brown roots so new ones can be grown. In fact, when re-potting plants with lots of small rootlets we often trim them, let them dry a day and then repot so lots of feeder roots can be produced.
WATER: Succulent plants evolved because the water supply was erratic, not because it seldom rains. In fact other sources of water are probably more important to these plants than rain is. In the wild almost all these plants have their roots in some water source. Dew dribbling off a rock, fog or clouds condensing on leaves that drip onto the soil or some such conditions. It's only a little bit almost every day, but they have evolved to take advantage of every drop! So you lock the roots in a pot. They cannot travel to a water source, you are the only one who can deliver the goods, so, a good watering once a week in hot weather and about ¼ that amount weekly in the dormant period or colder weather makes a lot of sense. Remember, no water, no food can be absorbed by the plant.
SUN AND LIGHT: The two are not the same! Succulent plants need light but they really grow better if they don't bake in the midday sun. Wild seed seldom sprouts in an area in the full sun. You find small plants tucked under a bush, or on the afternoon shade-side of a rock or under some protection. The big ones have survived to a size and bulk that can take the adverse conditions in their environment. Remember, these plants breath at night, unlike regular plants. When it gets above about 850F they just shut down and wait until it's cooler. Protection from hot midday sun will give you a better plant and not let the roots in the pot cook to death. If the new growth on a plant is pale green and elongated, it is not getting enough light! If the side of the plant nearest the light source is yellow, tan or indented, it is being burned by too much light.
FERTILIZER: If you repot your plants every year in a good potting soil you don't need fertilizer. In fact too much fertilizer is worse than none at all. Most succulent plants have some kind of woody skeleton or some kind of rod or thick wall to support the water holding core of the plant. If you push it with fertilizer the skeleton doesn't grow and become hard and you end up with droopy branches or a soft plant. If you don't repot yearly (as most of us don't) then use fertilizer at about ¼ strength every time you water in the growing season. A simple syphon device and a bucket is all you need to add food to the water. Grow the plant slowly and you will have a healthier, better looking plant.
THE POT: What kind of pot you use is rather irrelevant and up to personal choice. It's what you put in the pot that matters. I use all three kinds: porous clay, plastic and glazed ceramic.
If you are putting the pot out in the sun or live in a hot climate, porous clay is a good choice. The water evaporating through the wall keeps the roots cool. Just be sure to water more often.
Plastic and glazed pots do not allow water to evaporate and cool the roots. The soil in these pots also dries out slower so it needs coarse material (like small gravel) in the larger sizes so too much water does not hang up in the soil mix.
The size of the pot should be in proportion to the plant with some space to grow. For plants with shallow roots a pan or shallow pot is good. For columnar or tall plants a deep pot is needed. Some kinds of plants have a huge root system (ie: Sansevieria or Eulophias). If the pot is too small, they will not flower or grow well.
TOPPING ROCK: Placing crushed granite or small pebbles on top of your planting mix not only looks good, there is a practical purpose as well. It keeps the top of the soil mix from caking and drying out faster than the rest of the soil. It also allows water to penetrate the top of the soil better, rather than run down between the soil and the pot.
BOTANICAL BOOKS, BOTANISTS and NAMES:
It's a botanist's job to classify, arrange, rearrange, separate, combine, name and rename plants. That's what they do. They seldom grow, or care to grow, plants. That's the horticulturist's job. So botanists write botanical books giving their opinion as to what a plant is and where it should go in the classification system set up by humans for plants. If they cannot get out into the field to study, they use dried herbarium specimens to come to their conclusions. Each botanist comes to a separate decision based on what is studied and what is known. Even botanists don't always use "legal" names that may be botanically correct, but confuse the ability to communicate. A "good" botanist will always give a reason for their opinion as to why they change something. So it's okay to use any published name for a plant if you don't agree with a botanist that changes one and doesn't give you a good reason for doing so. Names are for communication! Both parties have to understand what plant is being discussed. Botanical books should be used for guidelines not as a way to grow plants under cultivation. Your plant is not "in the wild", it is in a pot. The information in this type of book is valuable to you so you become familiar with the "nature" of your plant, but you control the "culture" of that plant.
HORTICULTURE BOOKS: The author of a horticulture book can only tell you how they grow plants. It might not work for you. No one can write a book for the materials available to you, the climate you have or the conditions (outside, greenhouse or window sill) under which you grow plants. Only you know the specifics! A book written in England isn't much help to a grower in Arizona and a book written in California is not very good for a grower in Michigan. This kind of book can only give you ideas about different ways to grow your plants.
Your common sense, mixed with the knowledge of your plants, plus ideas gathered from other horticulturists will make growing cacti and other succulents not only fun, but give you great satisfaction.