Whether you need soil, perlite or a combination of the two depends on what you are trying to do with gardening.
Perlite is sterile, meaning it will not have any potentially harmful bacteria or fungus. Soil, especially soil brought in from outdoors, has many naturally occurring microbes, including bacteria and fungus that could be harmful to some stages of gardening. Soils, however, can be sterilized by placing them in a 180-degree to 200-degree oven until the center of the soil reaches 180 degrees. Keep the soil at that temperature for 30 minutes to sterilize the soil.
Perlite is a good medium for starting seeds and seedlings. Its lightness and ability to hold water makes germination much easier. Soils, even if sterilized, can be too dense for good seed germination. Mixing 50 percent perlite with 50 percent peat moss makes a good, sterile homemade sprouting medium. However, you might also have some success with a sterilized soil and vermiculite mix, if your soil is light and loamy.
As a growing medium, perlite has no inherent nutrients. Therefore, it must be mixed with soil to allow for healthy plant growth. Some soils, on the other hand, can be too dense. Adding perlite to these soils can help make a growing medium that will hold adequate water, drain well, and help keep adequate air around the roots. The exact percentage of soil and perlite will vary, depending on the type of soil you are starting with.
Hydroponic gardeners can use perlite as the sterile rooting medium in mesh pots. Perlite will wick up the nutrient solution to allow its absorption by the roots. Soil, however, even if sterilized can easily foul the hydroponic solution, making it unsuitable as a hydroponic growing medium.
Perlite is made by taking a dense volcanic ore that contains a small percentage of water and heating it to around 1,400 degrees Fahrenheit. This causes the water to expand, which forces the perlite to expand to a light, airy rock that can hold water and air. Many soils, however, are very dense. Clay soils, for example, are so dense that water moves very slowly down to plant roots. Clay soils also do not have space for the air that is required for root health.