There are major differences between potting soils and your typical garden
soil found outdoors. However, garden soils are not all the same; there are
sandy, heavy, rich or clay soils, and garden soils are rarely as consistent
as potting soils. Potting soils are usually mixed commercially and are
designed for planting in containers to give plants good drainage, moisture
retention and aeration. Don't use garden soil to pot houseplants or
container gardens without some amendments or screening first.
Density
Potting mix or potting soil is often much less dense than a typical garden
soil. In fact, many potting mixes commercially available are even "soilless"
mixes consisting of sand, moss, mulch and vermiculite all mixed together.
In any potting soil, the mixture has been screened and aerated and is looser
and lighter than a garden soil, with plenty of amendments like those above
mixed in to provide good aeration and drainage.
Sterility
In general, potting mixes are more sterile than garden soil and shouldn't
carry weed seeds, fungi or other diseases or pests. However, do not rely on
this quality unless the soil was labeled as sterile on its original
packaging, as it still sometimes carries weed seeds or mold strains. Garden
soil can have all kinds of active biological agents but has the benefit of
providing good biological additions to the soil, like earthworms and
beneficial bacteria.
Drainage
One of the biggest reasons for the difference in composition between potting
soil and garden soil is drainage ability. A plant in a pot can't extend its
roots out to catch water or nutrients from far away, so it must be able to
retain and reach more water nearby. Moisture-retaining amendments to the
soil like vermiculite, peat moss and mulch help with this problem and are
present in higher amounts in potting soil than garden soil.
Fertility
A major disadvantage for plants in potting soil is that potting mixes lack
the nutrients and natural fertility in garden soils, so you must nearly
always use fertilizers. In gardens, the natural cycle of decay and growth
keeps soils richer, and the interaction of many plants and insects balances
the soil. In a pot, the plant has only what you give it, so a balanced
fertilizer, often liquid, is necessary.
