Four weeks ago, I bought a bag of soil with the label “For In-Door Plants”.
However, I noticed the formation of mold very quickly after I used it:
Is it the soil? The watering? Or something else I am doing wrong?
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The conditions for growing mold are warmth, soil consistently wet, little air movement and undisturbed soil, and the presence of spores to feed on the organic matter present. It is natural to find some spores in potting compost, not a big deal. The plant in the pot (probably Chlorophytum) has fleshy roots and can be allowed to dry down between waterings. So one suggestion is to hold off on the water somewhat, reduce the temperature a bit, expose to gentle breezes and, as you are doing already, keep the surface loose.
This is totally common to have happen with new soil. It must not be sterilized very well or they added organic matter and fungal spores later. Not a problem. Use your fingers to 'fruff' up the top of the soil, might have to do that a couple of times but this fungal growth will stop. Very common when organic matter is added to the soil.
How did you plant your plants in this new soil? Is that pot full of soil or did you put rocks or gravel below the soil and above the drainage hole? If so, you need to redo by turning over, get rocks or gravel out of the bottom completely and re-potting adding more soil. You should only have 1" from the rim to the top of the soil. Do not water too much. Allow to dry a bit and as this is new soil for this plant make sure it gets some water around the roots until it is well established then you can start soaking the soil, allowing it to drain, wait until the top 1" is dry before watering again. Could you take a picture of the ingredients and send?