I live in Canada, and have several cacti and large agave plants. It's soon going to be time to take them outside as the weather starts to warm up. They are potted in a mix of coir, sand, fast draining cactus soil and a small amount of regular potting soil.
Does anyone have advice on particular types of fertilizers/fertilizer ratios that would be best suited for these types of plants?
Don't use fertilizer regularly, on potted desert succulents. Their natural habitat is high in minerals, low in major nutrients and moisture. Feeding too much can cause excessive growth and structural collapse due to overly weak new tissue.
Usually, a regular repotting on these plants, using fresh potting mix, is sufficient, as long as a proper medium is used. Small amounts of fertilizer can help, during the active growing season. Use a diluted liquid feed, low in nitrogen (such as a fertilizer suitable for root vegetables). Only use it once or twice during the active growing season, if you see signs of nutrient depletion.
One of the most experienced and successful cactus growers I know uses "tomato" fertilizer, half strength with every watering. The tomato fertilizer is 5-10-5, generally. I also use a dilute fertilizer for every watering during the sunny months. If your water is alkaline, use an acid (vinegar, citric acid) to acidulate the water to about 5.0 - 5.5 pH.
The key is the plant must have plenty of sun in order to effectively metabolize the fertilizer. Too much fertilizer and not enough sun will cause your plants to become etiolated (stretched, weak, unhealthy growth, usually associated with low light)
Let the potting mix dry between waterings. Not completely dry, but the top inch or two. If the plant body starts to shrivel, it's definitely past time to water.
Your potting mix sounds fine except for the sand. Sand will clog the pores between the potting mix particles and inhibit drainage. Use pumice, pearlite or calcined clay. All of these things will create air spaces in your mix as well as attenuate water evaporation. Do not use peat or peat-based soils. It compacts and is difficult to re-wet once dry.
If someone tells you not to fertilize your cactus, ask them to show you their ten best cactus. There are a lot of myths and misconceptions about cactus culture with no basis in fact or experience.
