Follow up question: If I should replace the soil, is it OK to dig the cactus up considering I planted them about 3 months ago.

There's nothing wrong with using a soilless mix. A lot of people if given the choice, would use a soilless mix. The reason behind using a soilless mix is to remove peat from the mix.

Your source is correct in that you will have to feed more with a soilless mix. That's the nature of the mix, but that doesn't mean there's something wrong with it.

As to how much and how often you should be fertilizing, I can't say. I don't use a soilless mix myself (cost related issues).

Hopefully someone else will chime in and tell you about fertilizing.

But, with that being said, your mix is not one I've heard of. I don't know if your mix is best, I'll let someone else address that (people who make their own soilless mixes use other "materials").

Planto

All prepared potting soils, regardless of purpose, are 'soil-less' mixes.....all that means is they have no real 'dirt'(soil) in them. And that's a good thing, as soil - like garden soil, loam, topsoil, etc. - is too fine a particle size and impedes drainage for things grown in containers. And fast drainage and aeration is key in a container.

Most all-purpose potting soils are peat or coir based. Then various things are added to enhance/improve drainage, add texture and retain moisture - bark fines, perlite, pumice, Turface, poultry or granite grit, coarse sand, etc. Some may even have organics like compost, worm castings, guano or chicken manure added but these should be a very small percentage of the mix, if present at all.

One of the best soils for cacti or succulents is the gritty mix referred to on the Container gardening forum:

1 part uncomposted screened pine or fir bark (1/8-1/4")
1 part screened Turface
1 part crushed Gran-I-Grit (grower size) or #2 cherrystone

Depending on the how the coir is introduced (shredded or granular), the OP's mix is not too bad - I would add some bark and coarse sand as well if not switching directly to the gritty mix.

FWIW, there are never many nutrients in potting soil. The nature of container gardening is that the grower needs to provide all the nutrients the plants need, as well as to closely monitor the watering. The Schultz is probably ok but I prefer a liquid fert that provides all the trace elements as well, Foliage Pro by Dyna Grow.

I too use the shultz cactus fertilizer and have had decent results with it, but many people here use foliage pro by dyna grow. I believe the schultz brand is lacking in a couple elements (magnesium?? and something else). I might be wrong though, so dont quote me on that haha :x