Everything eats. This is a good first understanding to have when thinking about whether or not to feed a particular plant. If the question is "should I fertilize this plant", the answer is always "yes". And it's "yes" no matter the variety of plant that the question is being asked of. Everything eats. Grass, trees, fungi, tropicals, annual flowers, perennials, cacti, succulents... If it's alive, it eats.
But after this question is answered, there are two more important questions to ask before heading to the store (or to Flyleaf) for your fertilizer. These questions are: "with what kind or "analysis"" and "how often".
The analysis of a fertilizer refers to the specific blend or ratio of minerals and nutrients that a particular fertilizer contains. Some will contain more nitrogen (lawn fertilizer), some will contain more phosphorus and/or potassium (tomatoes and perennial flowers), some will contain more of the micronutrients, and some less. For any commercially available fertilizer, the fertilizer analysis is listed on the label as a series of numbers with dashes between them. And the numbers always represent the percentage of nitrogen, phosphorus, and potassium (in that order: N-P-K) that a specific blend contains. So, a 10-10-10 analysis listed on the fertilizer label states that the fertilizer consists of 10% nitrogen, 10% phosphorus, and 10% potassium.
The fertilizer analysis is important when determining what kind of fertilizer to use on your cacti and succulents (or any other plant) because too much may kill them pretty quickly and too little can lead to a poorly performing and unhealthy plant.
This is due to the fact that different plants require different diets. Some plants want very little of one mineral or another, while another plant needs quite a bit of this, that, and the other to perform its best. A tomato plant that receives too much nitrogen, for example, will usually produce big, beautiful leaves but no fruit. Same with hydrangeas. Too much nitrogen leads to great leaves, but no flowers. A lawn fertilizer formulation used on a cactus will likely only produce a sad memory of a cactus that once was, but is no more. So, again, the analysis is important.
So how do I know what to use? Check the label. The label on the fertilizer packaging will offer enough information to help you decide, in most cases. If the label says that the fertilizer is good for roses, for example, it may well be. Follow the measurements and application instructions provided and you'll soon find out if the fertilizer works.
Another way to go about choosing is to first research the various formulations or analyses that professional growers use for the plant you're interested in fertilizing. The right professional grower, or production grower, will have years worth of experience with the plant in question. They've grown them, they've observed the different responses to different analyses, and they've done it all, in most cases, tens or even hundreds of thousands of times. Production growers have already done the work as far as fertilization goes, so it makes good sense to follow their lead.
And how do they lead when it comes to cacti and succulents? 150-200 parts per million nitrogen, continual feeding. That's it. If you Google "production guidelines for succulents", you will find (in time) that most production growers around the world have a single focus when it comes to fertilizing cacti and succulents: parts per million nitrogen (ppm). The other macronutrients will be present, but nearly irrelevant. It's all about the nitrogen. And it's all about having a precise dose.
You'll find that some propagators will use just 50 ppm nitrogen prior to root initiation, and then bump it up as the plant begins to mature. Some growers move to 200 ppm, many use 150. At Flyleaf, we've experimented within that range and found that 200ppm is a little high for some of the species that we grow, so the formulation that we use and sell contains 150ppm nitrogen (I don't recall the N-P-K off the top of my head because they are in the decimals - it's on the label, though).
So, how often do you fertilize? What's the schedule? As with many or most production growers, we feed at each watering, so every plant that we grow has been fertilized several times before it goes out for sale. This isn't to say that anyone else necessarily needs to follow this schedule - we do it because we want our plants to be as vibrant and full as we can possibly get them as quickly as we can - but it's best to remember that everything eats. If the question is "should I fertilize this plant", the answer is always "yes". Everything eats. Just make sure that you're providing the right diet.