Answer: Potting soils are formulated specifically to meet the needs of certain plants such as improved drainage for cacti or a particular pH for say, acid lovers, or a high level of cleanliness as for seedlings. Soilless mixes such as Miracle-Gro are based on peat and vermiculite and perlite. They are relatively sterile and as such reduce the risk of importing pathogens. They may also be formulated to contain specific fertilizers or special water holding polymers. In contrast, outdoor soil varies widely in both texture as well as nutrient analysis; it may also contain insects or diseases which could affect your plants. In some cases gardeners will make their own potting soil and include some loam, but container grown plants have such specific needs, particularly for drainage, that most often average outdoor soil simply will not be satisfactory.
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