Soilless Media

• Ideal potting soils consist of organic materials but no actual soil. Soil can contain diseases or weed seeds that prevent your plants from growing as well as they should. Soil also contains heavy particles that seedling roots have trouble penetrating and that can hold on to too much water.
  
  When selecting a potting soil, University of Nebraska-Lincoln extension educator Don Janssen recommends lifting the bag and squeezing the mix through the bag. The bag should be lightweight, and the mix should not hold a shape when you squeeze it.
  
  

Water Retention and Drainage

• Good potting mix maintains a fine balance between holding enough water to supply plants with what they need while letting excess drain away so that it doesn't drown plant roots. Master gardener Linda L. Reddick, of the Arizona Cooperative Extension, recommends adding 2 cups of water to a quart of potting mix. After a few minutes, 1 cup of liquid should have drained through. More than 1 cup indicates that the mixture might not retain enough moisture for your plants. Less than a cup means that water is collecting in the soil, causing poor aeration that can damage plants.
  
  

Ingredients

• Potting mixes can contain myriad soilless ingredients. Understanding the role of these ingredients can help you to choose a potting mix that best meets your needs.
  
  Peat, sphagnum peat moss and pine bark are lightweight bulking materials that also help a potting mixture to retain moisture. Vermiculite, sand and perlite improve drainage and aeration of the potting mixture. Of the three, vermiculite performs best in retaining water and nutrients. For succulents and other plants that need a well-drained, dry potting mix, calcined clay allows for excellent drainage and aeration.
  
  Compost provides a source of nutrients, as well as containing beneficial microbes that prevent diseases. The best compost for potting mixes should age at least 6 months. Use 20 to 50 percent compost in your potting mix.
  
  

Fertilizers

• Some commercial potting mixes also contain a slow-release fertilizer, and if you make your own potting mix, you'll have to decide which fertilizers to use. Compost provides a minor source of nutrients, but plants will eventually need additional nutrients for healthy growth. Poultry litter, bloodmeal, alfalfa meal and crab meal provide organic sources of nitrogen, or you can use a synthetic nitrogen fertilizer. If you use a fast-release fertilizer, bloodmeal or poultry litter, wet down the potting mix a week before use to allow potentially damaging ammonia to dissipate from the mixture.
  
  Bone meal, rock phosphate, sul-po-mag, greensand, kelp meal and synthetic fertilizers provide plants with phosphorus and potassium. Compost generally contains adequate trace minerals for plant growth.