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How to Make a Soilless Potting Mix

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Homemade soilless potting mix is an effective, less expensive substitute for commercial potting mixture. Soilless potting mix is far superior to regular garden soil, as soil is too heavy, becoming compacted and preventing air from circulating around the roots. As a result, plants often develop potentially fatal problems such as stem rot. Garden soil also tends to develop a hard crust that seedlings are unable to penetrate, and it often contains weed seeds. Making soilless potting mixture isn’t an exact science. Start with a few basic ingredients, and then adjust the recipe to suit your needs.

Measure 2 gallons of peat moss into a large, clean container such as a garbage can or bushel basket. Peat moss is inexpensive and is available at nearly any garden supply center or nursery. For blooming plants — especially bromeliads — substitute bark chips for half of the peat moss.

Mix in an equal amount of vermiculite or perlite. Both are natural, lightweight substances that improve soil drainage. Although both are effective additions to soilless potting mixture, vermiculite tends to become compacted when it is handled roughly. Alternatively, clean, coarse sand can replace all or part of the vermiculite or perlite, as sand also promotes excellent soil drainage. Although sand is heavy, it makes a useful potting medium for tall, top heavy plants and is also effective for succulents.

Place 4 tablespoons of ground limestone into a separate bowl or bucket. While limestone isn’t absolutely required, it is especially helpful for growing plants that remain in the same pot for extended periods of time. Add a small amount of general-purpose time-release fertilizer to the limestone at the rate suggested on the container.

Mix the limestone and fertilizer thoroughly, and blend the combined ingredients into the potting mixture.

Sift the potting mixture through a screen with 1/2-inch wire mesh. For starting seedlings, create a finer texture by sifting the mixture through 1/4-inch hardware cloth.

Moisten the potting mixture before planting.

How to Make a Soilless Potting Mix. Homemade soilless potting mix is an effective, less expensive substitute for commercial potting mixture. Soilless potting mix is far superior to regular garden soil, as soil is too heavy, becoming compacted and preventing air from circulating around the roots. As a result, plants …

Soilless Potting Mix

Seed starting instructions invariably say to use a good soilless potting mix. What kind of potting mix doesn’t have soil, and what’s wrong with soil anyway?

Starting Seeds in Soil

You certainly could use soil right from your garden to start seedlings indoors, but garden soil comes with two major disadvantages:  

  • You don’t know what’s coming in with it: Disease spores, bacteria, plant-eating insects, weed seeds, and other unwanted materials can easily hitch a ride with your garden soil. There are all kinds of natural predators and weather phenomenon outdoors that help keep these things in check. To use this soil indoors, you’d need to sterilize it first, with some sort of heat treatment.
  • Lack of drainage: Garden soil tends to be somewhat heavy and without tilling, either by you, earthworms, or other insects, it begins to compact after several waterings. This compaction is especially hard on the tender roots of young seedlings just getting established.

Soilless Mixes

A soilless mix gives you more control.   Besides being free of disease and other contaminants, you can blend ingredients for preferred drainage, water retention, and airspace. It is also lighter in weight, which you’ll appreciate when you have to move the pots outdoors.

Most soilless mixes are predominantly sphagnum peat moss. Sphagnum peat is lightweight and inexpensive. Just as importantly, it’s well-draining yet water retentive. Granted, until you get the peat thoroughly moistened, the particles can be very unpleasant to work with. Peat is also on the acidic side, and most seed starting mixes have a soil pH around 5.8, which is fine for starting most seeds.

Fun Fact

Peat takes hundreds of years to form. Alternatives, like coir, are being sought. Expect to start seeing more potting mixes that omit peat altogether.

Amendments

  • Bark: Bark is added to improve drainage and airspace within the mix. This means it will also decrease water retention slightly. Bark mixes are better for use with mature plants that need to dry between waterings than for starting seeds.
  • Coir: Coir is a coconut fiber by-product and works similarly to peat in providing good drainage while also retaining water. As mentioned above, coir is becoming a substitute for peat.
  • Perlite: Perlite is that stuff that looks like pebbly Styrofoam. It’s a volcanic mineral, although it does not affect the nutrient quality or the pH of the mix. It does add in drainage and in air and water retention, that magical balance. In fact, it is sometimes used in outdoor gardens to prevent sandy soil from leaching nutrients.
  • Vermiculite: Vermiculite is those silvery-gray flecks you see in potting soil. It’s a mica-type material 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 access them. Perlite is also good as a soil covering for seeds that need to remain consistently moist to germinate. You may see vermiculite for sale at home improvement stores, for use in insulation or plaster. This grade vermiculite is not really suitable for potting mixes since it does not absorb water easily.

Fertilizer and Trace Elements

  • Seeds don’t require fertilizer for germination, so it is somewhat wasted if you are using it for seed starting. By the time the seedlings have grown true leaves and require supplemental food, whatever was in the mix has begun to dissipate.
  • Wetting agents are becoming increasingly popular. That’s understandable if you’ve ever worked with straight peat moss. Wetting agents are polymers added to the soil to greatly improve their water-absorbing ability. Certified organic wetting agents may not be possible, perhaps because by nature, a soil wetting agent can’t be quickly bio-degradable or they’d be useless. I’ve always had good luck without a wetting agent but making sure my mix is well moistened before I put it into pots or cell packs and then, not letting it dry out. Watering containers from the bottom will help with this.
  • You may also see pH adjusting amendments such as limestone or gypsum. Mixes will vary by manufacturer and region. Occasionally a particular plant will favor certain amendments over others, but for seed starting a basic mix is generally sufficient. These will be labeled for seed starting or as a starter or germination mix.

The best way to judge a potting mix is to see how well your seedlings do. If you get good germination and the seedlings start off a healthy green, all is well. Otherwise adjust your mix, starting with the pH.

A soilless potting mix is preferable to using outdoor garden soil for several 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.

Soilless Potting Mix

4 to 6 parts sphagnum peat moss or coir
1 part perlite
1 part vermiculite

Mix with Compost

2 parts compost
2 to 4 parts sphagnum peat moss or coir
1 part perlite
1 part vermiculite

Mix with the Addition of Nutrients

Add ½ cup each per every 8 gallons of the mix:
½ cup bone meal (for added phosphorous)
½ cup dolomitic limestone (raises soil pH and provides calcium and magnesium)
½ cup blood meal, soybean meal, or dried kelp powder (for added nitrogen)

Starting seeds with a sterile potting mix is a wise idea. Here is what is in a soilless potting mix and why it is the best option.