Stephen Albert
Soil delivers nutrients and water to plant roots and anchors plants in place. Because the planting medium in containers is limited its composition is very important to plants growing there.
A container planting mix must be nutrient rich and hold moisture for several days. It must be soft and friable and resist compaction so that roots develop properly. A potting mix must be free of soil-borne fungus pathogens and predatory pests such as grubs, cutworms, and larvae that can destroy plants.
Commonly, backyard soil is not suitable for container planting. While backyard soil may be fertile it often contains too much clay or too much sand or can be too acidic or too alkaline. Such concentrations in the limited space of a container can be detrimental to plant growth.
To ensure healthy plant development, the best course is to either buy a planting mix for container growing or blend your own. There are many brand name potting mixes available at garden centers and nurseries. Ask your nursery person what she recommends. Two commercial light mixes suitable for balcony and rooftop gardens are Supersoil and Jiffy Mix. Other commercial potting mixes are Pro Mix and Redi-earth.
Blending your own potting mix is an alternative to buying ready-made mixes. Blending your own may be cost effective if you require a large amount of potting mix—and if you have a place to store it.
Here are three potting mixes you can blend at home:
Cornell University Potting Mix:
Here is a potting mix developed by Cornell University (this formula makes enough mix for about three to four 1-gallon (4L) containers):
- 8 quarts (7.5L) vermiculite
- 8 quarts (7.5L) shredded peat moss
- 2 level tablespoons superphosphate
- 2 tablespoons limestone
- 8 tablespoons steamed bone meal or dried cow manure
Mix all the ingredients in a trash can lined with a can liner. Shake vigorously. This is an odorless mix and what you don’t use can be stored in plastic bags.
University of California Potting Mix:
Here is a potting mix developed by the University of California (this formula makes one cubic yard or 27 cubic feet (0.76 cbm) of planting mix that is equal to 202 gallons (765L)):
- 13 cubic feet (0.36 cbm) composted redwood sawdust
- 7 cubic feet (0.2 cbm) ground fir or pine bark
- 3½ cubic feet coarse sphagnum peat moss
- 3½ cubic feet horticultural grade or sharp builder’s sand
- To 1 cubic yard (0.02 cbm) of these ingredients add:
- 5 pounds (2.2 kg) dolomitic sand
- 2 pounds (0.9 kg) superphosphate fertilizer
- 1 pound (0.45 kg) calcium nitrate
Home-Made Vegetable Potting Mix:
This mix is enough for one 10-gallon (38L) container (for larger batches, increase the ingredients proportionally):
- 5 gallons (19L) fully mature compost (either commercially prepared or homemade)
- 1 gallon (4L) sharp sand
- 1 gallon (4L) vermiculite or perlite
- 1 gallon (4L) ground sphagnum peat most
Blend these ingredients and then add 1 cup (236 mL) of 5-10-10 chelated fertilizer and then blend again.
Commercially blended potting mixes come to you sterilized. Home blends that contain garden soil or homemade compost are not. You may want to sterilize your mix to destroy weed seed and nematodes (small microscopic worms) and to neutralize fungus diseases. You can sterilize your mix by spreading it out in a shallow pan and baking it at 275°F (135°C) for an hour; soak the soil before baking to avoid odor.
