Seed staring is its primary use. Most instructions for Seed starting say to use a good Soilless Potting Mix. What kind of potting mix doesn’t have soil and what’s wrong with soil anyway?
Define Soil, Dirt, Topsoil and Soilless Potting Mix
Soil is a combination of minerals, air, water, animals and other living matter along with their wastes or decaying bodies. Over time soil becomes compacted and begins to erode. When soil particles are excavated or erode, they are no longer supported by their living environment; consequently, the soil becomes dirt. In other words, dirt is soil that is out of place, like dust on the floor or mud on your shoes. Because dirt is disassociated from its ecosystem, it lacks the nutrients needed to promote plant growth and therefore dirt is nothing more than dead soil.
Another way soil becomes dirt is by stripping the minerals and nutrients from the soil by over farming and the use of chemical fertilizers and pesticides. These actions kill the soil’s biological ecosystem transforming the soil into lifeless dirt.
Topsoil is the natural soil layer that has formed at the surface of the Earth’s crust from rock decomposition, wind and water. Topsoil consist of sand or clay (ground-up rocks) mixed with organic materials such as leaves, grasses and other decaying organic matter. While topsoil is nutritious, it doesn’t always have the ideal properties for growing healthy plants. And it can hold excess moisture during rainy periods or be brick hard during droughts, affecting root development and growth.
Soilless Potting Mix is a sterile, soilless potting medium that holds water and stays fluffy so that roots can grow easily, and excess water can drain away quickly. Being sterile, it doesn’t contain pesky weed seeds, insects or diseases. Regular garden soil is too fine for containers and packs down too much, impeding root growth and drainage. Quality Soilless Potting Mixes are generally made up of differing blends of ground conifer bark called fines, commercial compost, vermicompost (worm castings), ground peat moss or coir fiber, perlite or vermiculite and sometimes a slow release fertilizer.
Starting Seeds in Topsoil from Your Garden
You could use topsoil from your yard, forest floor or garden to start seedlings indoors, but topsoil comes with 2 major disadvantages:
First, your topsoil may contain gardening cataclysmic surprises. Surprises like disease spores, bacteria, plant-eating insects, weed seeds, creepy crawly things and other unwanted gardening delights. There are all kinds of natural predators and weather events outdoors that help keep these things in check. However, to use your topsoil indoors, you’d need to sterilize it first, with some sort of heat treatment.
Second, Lack of Drainage – topsoil tends to be somewhat heavy and without tilling, either by you, earthworms, or other insects, it begins to compact after several watering’s. This compaction is especially hard on the tender roots of young seedlings just getting established.
Solution to Topsoil – Soilless Potting Mix
Successful container gardening requires a Soilless Potting Mix that meets the plant’s needs. The Soilless Potting Mix must create a stable reservoir for moisture and nutrients and remain loose enough to allow for root and water movement and the exchange of gases in the root zone. A Soilless Potting Mix must also have a pH (a measure of the alkalinity or acidity of a medium) that can support adequate nutrient uptake, and it must be free of soil-borne diseases, weed seeds, and toxins. Finally, a Soilless Potting Mix must provide adequate anchorage and support for the roots while still being heavy enough to provide enough ballast to prevent plants from tipping over. A well-blended Soilless Potting Mix can easily satisfy all these requirements and do so without the inherent problems and variability frequently encountered when using topsoil in containers.
Soilless Potting Mix
Most soilless mixes contain a base of Sphagnum Peat Moss or Coir a coconut fiber by-product. Sphagnum/Coir is lightweight, fluffy and inexpensive. They drain well yet they are water retentive. Sphagnum Peat Moss is on the acidic side with a pH of 3.3 to 4 while Coir pH between 5.2 and 6.8.
Amendments Added to Peat or Coir
Bark: Bark can be added to improve drainage and airspace.
Perlite: Perlite looks like packing Styrofoam. It’s a volcanic mineral. It does not affect the nutrient quality or the pH of the mix. It does create drainage; air spaces and increases water retention.
Vermiculite: Vermiculite is a silvery-gray fleck type of material. It’s a mica-type substance that is heated up and expanded, to increase its water holding capacity. The particles soak up water and nutrients and hold them in the mix until the plants are ready to use them. Vermiculite is also good a soil covering for seeds that need to remain consistently moist to germinate.
The best way to judge a potting mix is to see how well your seedling do. If you get good germination and the seedlings start off a healthy green, all is well.
A soilless potting mix is preferable to using outdoor garden soil or topsoil for many reasons, but if you need a large quantity of mix or have a need for a special blend, it is often easier to simply create your own potting mix.
Basic Soilless Potting Mix
1-part Sphagnum Peat Moss or Coir
Basic Mix with Vermicompost
2-4 parts Sphagnum Peat Moss or Coir
Basic Mix with the Addition of Nutrients
Add ½ cup each per every 8 gallons of mix:
½ cup Bone Meal (Phosphorous)
½ cup Dolomitic Limestone (Raises soil pH and provides calcium and magnesium)
½ cup Blood Meal or Soybean Meal or Dried Kelp Powder (Nitrogen)