A good potting soil provides all necessary nutrients for healthy plant growth. While some people may prefer starting with new soil each year, in most cases there's really no reason to do so.
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Characteristics of Good Potting Soil
Good potting soil may or may not have any real soil in it. However, it has organic material such as peat moss and a product for good drainage such as vermiculite or perlite. Other ingredients, such as sand, may vary depending on plant type.
Reusing Potting Soil
While potting soil can be reused immediately after removing the plant that lived in it, some experts suggest growing an entirely different type of plant in it and others suggest replacing some of it with new potting mix or adding fertilizer. However, as long as the plant previously growing in it was healthy, no other additions are necessary. If the plant shows signs of decline, nutrients may be exhausted -- add organic compost or synthetic fertilizer.
When Not To Use
Eventually every potting soil breaks down into very fine particles which compact and does not allow good drainage. At this point, replace the potting soil totally. Also replace it if you find any mold or insects in it, This is also true for unused soils left over from previous growing seasons.