Never make the mistake of placing regular garden topsoil into a plant container. You'll find its riddled with ants, worms or other critters and after a couple weeks, it compacts and dries into an inhospitable, hard "brick" in the pot. Luckily, horticulturists learned from their mistakes early on and developed various formulas for potting soil mixes, which allow any container-grown plants to develop healthy roots and robust leaves, stems, and flowers.
Potting soils need to both retain adequate amounts of water for roots, while also allowing good drainage so these roots are submerged. While organic matter like peat is used to provide the substantial volume of the soil, peat will hold tremendous amounts of water like a sponge. To improve drainage, sand, white perlite granules and beige vermiculite may be added to allow for water to percolate through the soil and out the container drainage holes without being fully absorbed and retained by the peat.
Closely related to the issue of drainage is porosity and aeration. Plant roots exchange gases from their surfaces, just like leaves. Water-saturated soils prevent the absorption of oxygen by roots in soils. Most plants need a soil blend that retains moisture with water while allowing air to penetrate in small amounts. Perlite, vermiculite, coarse grit or bark takes up space and prevents water and peat from fusing into mucky masses.
Organic matter like peat and compost incorporated into a potting mix carry only small, but important, amounts of nutrition to plants. Some mixtures include slow-release fertilizer granules to provide needed nitrogen, phosphorus or potassium to plants. These fertilizers degrade over one to six months and must be replaced just like peat or compost particles need to be added and recombined to refresh old potting mix.
Like garden topsoil, potting soil also has a pH that is based on the materials in the mixture. Acidic potting soils (pH lower than 7.0) result when acidic organic matter is the parent substance, such as oak leaf mold, coffee grounds or peat moss. More neutral-pH potting mixes develop when acidic organic matter leaches out. Alkaline potting mixes (pH Higher than 7.0) often add calcium-rich sands or lime powder in amounts to counteract the pH initially supplied by the peat moss.
Not all potting mixes demonstrate the same qualities. Plants that need drier, fast-draining soils need a potting mix more heavily made up of coarse aggregates like sand or grit, while still having some organic matter. Conversely, other plants require a dense, but moist soil in order for seeds to germinate; less aggregate and more finely-sifted peat particles make up that mixture. Gardeners may create their own potting mixtures, too. If a commercially fabricated potting mix is too expensive or doesn't meet the needs of the gardener, he may mix his own recipe of sand, compost, perlite or pebbles.