The material that your plants grow in is called the “growing medium”. 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 vegetable 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— never use this by itself for container gardens. Soils hold water and nutrients very well and can drown roots growing in a container. Diseases and weed seeds can also be a problem. And soil is heavy which is an advantage if you are trying to anchor top-heavy plants and pots, but a disadvantage if you want to move pots.
- Commercial Soil-Less 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 sphagnum peat moss, perlite, vermiculite and small amounts of lime and fertilizer. Examples of soil-less mixes are ProMix™, ReddiEarth™, Jiffy Mix™, and Sunshine Mix™. (To produce “organic” soil-less mixes, suppliers omit chemical wetting agents and substitute organic for chemical fertilizers.)
- Other Types of Commercial Mixes— are advertised as “top soil”, “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. 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 mould. 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% soil-less mix
- 25% garden soil + 75% compost
- 25% soil-less mix + 25% garden soil + 50% compost
- 25% garden soil + 75% soil-less mix
- 50% soil-less mix + 50% compost