University of Maryland Extension

"Soil" for Containers

Choose a Good Growing Medium for Container Gardens

The material that your plants grow in is called the “growing medium or media” never dirt. Dozens of different ingredients are used in varying combinations to create homemade or commercial growing media.  By understanding the functions of growing media, you can evaluate the qualities of individual types and select which ones might work best for your container garden. The choice is very important because your plants are dependent on a relatively small volume of growing medium.  Unlike their cousins growing in garden soil, containerized plant roots cannot grow around obstacles or mine the soil far and wide for nutrients and water.

  • Growing medium has three main functions- 1) supply roots with nutrients, air, and water, 2) allow for maximum root growth, and 3) physically support the plant.
  • Roots grow in the spaces between individual particles of soil.  Air and water also travel through these pore spaces.  Water is the medium that carries nutrients that plants need to fuel their growth, and air is needed for root growth and the health of soil microorganisms that help supply plants with nutrients.
  • Irrigation water moves through the pore spaces, pushing out the air.  If excess water cannot drain away, fresh air cannot enter and roots will suffocate. 
  • Select light and fluffy growing media for good aeration and root growth.

Qualities of Different Types of Growing Media

  • Garden Soil— soils are too dense to allow for good air and water movement when added to a container garden. Soils hold water very well in their small pore spaces and can drown roots- especially in shallow containers. Topsoil should only be added to very large containers and not exced 10% of the volume.
  • Commercial Soilless Mixes— these are an excellent choice for containers.  They are lightweight, drain well, hold water and nutrients, and are generally free of weeds, insects, and diseases. They have a pH of about 6.2 and are typically comprised of ingredients such as sphagnum peat moss, perlite, vermiculite, composted bark, compost, and coconut coir.  plus small amounts of lime and fertilizer. (To produce “organic” soil-less mixes, suppliers omit chemical wetting agents and substitute organic for synthetic fertilizers.) Soilles mixes tend to be hydrophobic- they repel water. Work water into the media with your hands until it is thoroughly wetted.
  • Other Types of Commercial Mixes— are advertised as “topsoil”, “planting soil”, “planting mix”, or “potting soil”.  They vary a great deal in composition and quality. Avoid mixes that contain sedge peat, feel heavy or gritty, have very fine particles, or appear clumped.
  • Sharp Sand— use only coarse builder sand, not play sand.  Sand increases porosity because of the large particles.  It is relatively inexpensive and heavy.
  • Bark Fines and Wood Mulch— these are high in carbon and low in nutrients and not recommended for container vegetables.

Compost:  In a Class By Itself

Compost is the dark, crumbly, earthy-smelling product of organic matter decomposition.  Leaves, grass clippings, wood waste, and farm animal manures are some of the common ingredients that are combined with water in piles or windrows and digested by huge populations of oxygen-loving microorganisms.  LeafGro™ is a well-known commercially available yard waste compost in Central Maryland.  It’s highly recommended to include some compost in the growing media for your containers.

  • Compost contains all the major and minor nutrients that plants need for good growth. For gardeners, this makes it an excellent substitute for sphagnum peat moss, which has very few nutrients (although it does hold water better than compost). Composting effectively recycles the nutrients from gardens, landscapes, and farms thereby reducing nutrient pollution of waterways. However, fertilizing is still necessary because the nutrients in compost are released slowly and are usually not sufficient for an entire season.
  • Vegetables, herbs and flower plants can be successfully grown in 100% compost or leaf mold. Baltimore City community gardeners have been doing this for decades!
  • Vegetable plants generally grow best when soil pH is in the 5.5-7.0 range. Many composts have a pH over 7.0 but research has shown that there is no benefit in reducing the pH to a more desirable level because nutrients in compost are available over a wide range of pH values.
  • Properly made compost is turned multiple times and reaches temperatures that kill weed seeds and plant and human pathogens.   

Some good media mixtures for container vegetables

  • 100% compost
  • 100% soilless mix
  • 50% soilless mix + 50% compost
  • Topsoil should only be added to very large containers and not exced 5-10% of the volume
  • To save money- empty the growing media from container gardens. Remove all plant residues, plant tags, etc. Store the media in a trash can or heavy-duty trash bags. (Don't save the media if root diseases were a problem). Soil-less growing media and compost lose nutrients and break down physically over time. Mix last year’s growing media 50:50 with fresh growing media and/or compost next year. 
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