As you peruse the soil aisle in any big box hardware store, you will notice huge bags labeled Garden Soil, Raised Bed Soil, Lawn Soil, Flower and Vegetable Garden Soil, Outdoor Potting Mix, Potting Soil with Fertilizer, and many others. Why are there so many different types of soil, and does it really matter which one you use? It does indeed. University and industry researchers have spent exhaustive hours experimenting with different soil blends to develop the best planting medium for each growing situation. And, while you have some leeway in terms of in-ground soil mixes, it is vital that plants grown in containers use a planting medium specifically designed for them. 

Image

Air

The ideal soil for most plants is comprised of a ratio of 45% mineral matter, 25% water, 25% air, and 5% organic matter. In an in-ground garden setting, this can be achieved through diligent soil management, which includes minimizing chemical fertilizers and adding plenty of organic matter to the soil. 

Container soil must provide a reservoir for moisture, air, and mineral nutrients. A container limits the soil volume that roots can develop in and is relatively shallow in depth. As a result, plants grown in containers are subjected to more intense environmental stresses than those grown in-ground.

It’s crucial that plant roots receive oxygen. In addition to absorbing water and nutrients, roots also respire. They use oxygen to convert sugars produced by the leaves into energy needed for growing new roots, absorbing nutrients, transporting water and fighting disease. Lack of air leads to a decrease in nutrient uptake, cessation of growth, root rot, and eventual death of the roots and plant. 

In general, the components of planting medium specifically designed for containers must contain larger soil particles than in-ground mixes. This accommodates the requirement for essential air and water space. However, plants vary in the amount of air space they need in planting medium. Orchids, ferns and azaleas need the most amount of aeration, at greater than 20% of the volume of the planting mix. African violets, begonias and gardenias need 10% to 20%; chrysanthemums, hydrangeas and lilies need 10%; carnations, geraniums and roses 5% to 2%; and Dutch irises 2% to 0%.

Water

In the ground, water drains and is held in the soil. In containers, the water that hasn’t poured out of drainage holes remains at the bottom of the pot, potentially depriving the roots of air. The shorter the container, the more water is held, resulting in less air. In his excellent 9-Part series on container soil mixes for the University of California’s Department of Agriculture and Natural Resources (UCANR), Steven A Tjosvold states that, in containers, air and water porosity combined must be greater or equal to 50% of total planting media volume. (https://ucanr.edu/blog/nursery-and-flower-grower/article/soil-mixes-part-3-how-much-air-and-water).

Soil/Planting Media

Prior to the 1990’s, container potting medium was usually soil-based. However, horticulture professionals have since developed peat or coco coir-based soilless mixes, which provide the larger particles container plants need, as well as other beneficial components for sowing seeds and healthy vegetative growth. There are benefits to both soil and soilless mixes. Mediums containing soil will hold the nutrients associated with the minerals in the soil, and may not need to be fertilized as soon as a soilless mix. Soilless mixes comprise components with larger particles needed for container plants. Both types of planting mediums will normally contain some or all of the following ingredients:

Sphagnum Peat Moss: Good aeration and water-holding capacity. Can restrict drainage if too much is used. There is some controversy over peat not being a sustainable resource, and the strip-mining process used to harvest peat moss causes environmental damage. 

Coco Coir: Made from the outer husks of coconuts that are grown for food, oil, and other fiber products. Good aeration and water holding capacity. Neutral pH. 

Bark: Provides structural support, prevents soil compaction, and promotes excellent drainage. Use aged bark fines or composted bark rather than fresh bark. The United States Department of Agriculture (USDA) advocates pine bark over other wood species. 

Plaster or Builder Sand: Improves drainage and aeration, but does not improve water-holding capacity. The ratio of sand to other ingredients may change depending on the type of plant in the container.

Perlite: Expanded volcanic rock, perlite provides great drainage and holds air (but not water). 

Vermiculite: A mica clay that has been heated to 1,400˚F, vermiculite is often substituted for perlite. Holds water, nutrients and air. 

Nutrients

Plants need a variety of nutrients in order to survive. Some nutrients are present in the soil, and can be made available to the plants through a symbiotic process between the plant and microorganisms present under the ground. Numerous plants need additional nutrients added as fertilizer. Most container plants, especially those using a soilless mix, need added fertilizer in order to thrive. 

Plant nutritional requirements are categorized as macronutrients (needed in large quantities) and micronutrients (needed, but in lesser quantities.) The three major macronutrients are listed on all fertilizer packaging – N (nitrogen), P (potassium) and K (potash). Other macronutrients include calcium, magnesium, sulfur, carbon, hydrogen and oxygen. Plants need the following micronutrients in trace amounts: iron, manganese, zinc, copper, boron, molybdenum, chlorine, and nickel. Nutritional needs vary by plant – research into the nutrient needs for each plant type will determine which fertilizer is best.

Soil and Planting Media Formulas

Many years ago, my husband and I researched and developed a “super soil”. We bought all the components, mixed them together in a huge pile, let the pile sit out to meld for a week or two, then used the soil in our raised beds. Since that time, several excellent soils have reached the market, and we can buy a few bags for the same cost as making the super soil ourselves.

If you are interested in making your own potting mix, several US universities sport formulas on their website: 

• The University of California has four recipes on the UC Riverside website, each using a combination of plaster sand, bark, peat moss, with macro- and micronutrients. https://agops.ucr.edu/soil-mixing#uc-soil-mix-iii
• The University of Florida Extension shows several formulas, based on the type of plant. https://sfyl.ifas.ufl.edu/lawn-and-garden/homemade-potting-mix/
• Penn State Extension provides a recipe for different types of mixes. The soilless mix is comprised of 1 part compost, 1 part peat moss, 1 part vermiculite, (screen the first 3 ingredients with ¼” screen to mix), add 0.6 oz blood meal, 0.4 oz clay phosphate, and 0.4 oz green sand, per each gallon. https://extension.psu.edu/potting-media-and-plant-propagation

Soil Shrinkage

The soil or planting medium in containers tends to shrink over time. Many organic ingredients in the medium will decompose – a benefit to the health of the plant, but a problem in containers if not addressed. The total volume of the soil will be reduced, resulting in less total water-holding capacity and the need for more frequent irrigations, potentially endangering the root system. The problem is easily rectified by “topping up” the soil or planting medium in the container.  Container soils with organic components such as peat and coco coir are more stable and not prone to shrinkage.

Container gardening is an excellent method of growing edible and ornamental plants in areas with limited space. With container plants, there is no need to worry over poor soil, weeding is minimal to nonexistent, plants can be mobile, and pests are easier to control. With the proper care and attention to soil or media mixture, container plants will thrive and beautify the environment for decades. 

Resources: 

UCANR Nursery and Flower Grower Website: https://ucanr.edu/blog/nursery-and-flower-grower/article/soil-mixes-part-9-properties-common-soil-mix-components

UC Riverside Soil Mixing Website: https://agops.ucr.edu/soil-mixing#uc-soil-mix-iii

Washington State University Coco Coir v Peat Moss Informational Flyer: https://s3.wp.wsu.edu/uploads/sites/2080/2018/03/coconut-coir.pdf

USDA Pine Bark Proves Best for Potting: https://agresearchmag.ars.usda.gov/2017/may/pinebark/

Penn State Potting Medium and Plant Propagation Website: https://extension.psu.edu/potting-media-and-plant-propagation