Q. Can I combine different packaged soils, or use thawed frozen soil for houseplants?
If your potting soils are both regular soil mixes for indoor plants, with no allowances for special genera, that is, no soils indicated on the packaging as being formulated special plant needs, such as “cactus” or “African violet” soil mixtures, you should have no trouble combining them. Plants can often be more adaptable than we expect them to be, and two similar soils from different producers might in some cases actually provide an enhanced environment for a plant. If both soils are healthy, mixing them will at least not harm your plants.
Soil can freeze and thaw over a number of cycles without killing the microorganisms in it. Although extreme circumstances might reduce the number of microorganisms in a specific soil sample over time, these situations are not apt to occur in these latitudes. Canadian studies reported by the American Society for Microbiology have shown that although the population complexity in soils exposed to more than four dozen freeze cycles can be reduced, the soil is still viable. Other cold-weather studies reported by Elsevier and the Cary Institute of Ecosystem Studies show seasonal changes in soil composition, especially concerning plant nutrients, but these have been conducted over large areas, not indoors, where correction is easy. If you are looking at commercially produced potting soil from a seller who hasn’t left it exposed to a climate like New York’s for a number of freeze cycles, (a year or so, for example) it should be just fine. Since you will be using it under controlled conditions, you will probably be feeding your plant regularly, which should help keep it robust, as well.
Hope this helps.