Mixes for rooting cuttings should be a sterile medium,without any humus or fertilizer. An initially sterile potting soil for cuttings will get contaminated with pathogens more quickly overtime. If you have plants that root quickly, a humusy soil will work.

However, slow to root plants need to be in a less reactive medium. I've tried inadvertent experiments with slow cuttings in vermiculite that were starting to root. When I saw new, light green tips on about a third of my cuttings, I fertilized the baby flat with a little plant starter liquid fertilizer, and the cuttings with the new, light tip growth took off as expected. However, the cuttings that had not yet rooted mostly took a turn for the worse and eventually died off, with only a few rooting.

Cuttings need a sparse, airy medium to force them to develop the organs they need to take in not only water, but nutrients. When they develop the critical mass of new roots, then they can absorb nutrients as well. Exposed cambium and/or callus can only work through osmosis, drawing in enough water to keep the cutting turgid until it finds the resources to form new roots.

Thank you for your response. I didn't think anyone had replied as I didn't receive an email. Just checked on the off chance. So are you saying I need to repot my little salvias into a more nutritious mix asap?

Thank you for your help.

Actually, a sterile and less nutritious mix is called for. The cuttings need fertilizer only after new roots develop. Check the underground parts of the cuttings for health. There may be knobs of callus tissue forming, which is OK. Check for shriveled, rotting, or darkened stems.