It is best not to use ordinary garden soil in containers to grow plants. Garden soil compacts around the roots of plants when it is placed in containers. Once the soil is compacted, it will not hold water well, and will stunt the root development of your plants. Instead of garden soil, you can use a soilless potting mix. Although you can purchase a commercial soilless mix, you can also easily mix your own.
Sand or Perlite
Like dirt, soilless mixes require a soil medium that will facilitate good soil drainage and aeration. You can achieve this in your soil by including sand or perlite. Perlite is a volcanic glass that pops like popcorn when heated, while sand is composed of miniscule rock particles. Perlite and sand are coarse materials that bond loosely with the surrounding soil and hold air close to them. This makes soil fluffier and provides water with a way to drain from soil. For plants that need little water, such as cacti, mix two parts sand or perlite and one part peat moss or compost.
Peat Moss and Compost
Peat moss and compost are great soil additives to rejuvenate a garden before planting. Peat moss is made up of plant material that has decomposed in peat bogs. Compost may be made up of garden scraps, animal manure, kitchen remains or anything organic that rots. The compost should be finished, which means that all large particles should be removed from the compost. This ensures that there are no microbes in the compost that can kill the plants in the potting soil. Peat moss is slightly acidic in pH, while compost has a neutral pH. For a good general potting soil, mix one part peat moss, one part compost, one part sand and one-half part well-rotted manure.
If you open a bag of commercial potting soil, you may find tiny beads mixed in with the soil. These beads are bits of granulated fertilizer. Granulated fertilizer is designed to release nutrients into the soil over time as your container plants are watered. Granulated fertilizers may be purchased in any concentration, but most plants will do well with a balanced (10-10-10) fertilizer. Choose a fertilizer formulated for container plants and add the fertilizer at the rate recommended on the package. Fertilizer application rates will vary between manufacturers. Adding too much fertilizer can burn a plant, which will quickly kill it.