Gardening | Columnist Examines Soilless Seed Starting Mix

What is a soilless mix? Why use a soilless mix to start seeds?

A good indoor seed starting media is sterileófree of weed seeds, insects, fungus, mold, bacteria and viruses. It is water retentive and light enough to provide aeration and good drainage. It provides optimal physical properties that promote seed germination and seedling growth.

Soilless mixes contain a variety of organic and inorganic materials. Gardeners often make their own mix because it is less expensive than buying bags of commercially prepared media, or, they like their own recipe better than what comes pre-made.

Sphagnum peat moss and coconut coir are interchangeable base media for a soilless mix. They are both sterile and lightweight. They provide adequate aeration and hold water well.

However, there are a few differences between the two.

Peat moss is acidic. The pH is 3.5ó4.5, which is fine for starting most seeds and good for acid-loving plants. Peat holds water well but can be a challenge to moisten. It is considered a finite resource because peat bogs take thousands of years to form, and our rate of harvest is greater than the rate of accumulation.

Coir, a by-product of the coconut processing industry, is the fiber between the coconut fruit and husk. It has a natural pH of 5.5ó6.8. It retains more water than peat yet drains freely. It is more expensive than peat moss. Coir is available in compressed bricks that quickly rehydrate to original volume when soaked in water. Coir is a renewable organic resource and lasts longer than sphagnum peat before breaking down.

PittMoss, although not yet available to the consumer, is a new alternative to watch for in the future. It is composed of proprietary additives and recycled paper. Cheaper than coir and closer in cost to peat moss, it has no effect on the environment.

Perlite is an amorphous volcanic glass that greatly expands when adequately heated. It adds drainage and holds air. It does not retain water so it needs to be mixed with something that does, like peat or coir. It is sterile, lightweight and it floats.

Vermiculite is a mined mineral that has a mica-like appearance. It expands under high heat and exfoliates into worm-like pieces. It is sterile, lightweight and helps retain water, but does not provide aeration as well as perlite.

Perlite and vermiculite may be used interchangeably or together in a sterile mix.

Coarse sand is an option that adds drainage and aeration, but does not hold water. It adds weight to the mix. It may replace all of part of the vermiculite and perlite in a mix.

Worm castings are a nutrient rich fertilizer suitable for inclusion in a seed starting mix. Worm castings are considered sterile because their production eliminates bad bacteria, while only the good microorganisms remain. You can use sterilized compost but the beneficial microorganisms have been killed in the sterilization process.

Seeds donít need fertilizer to germinate, but require nutrients after they have developed their first set or two of true leaves. Emerging seedlings use stored food for their energy to start growth, but that energy source does not last long.

Perlite, vermiculite and sand contain no plant nutrients. Neither does peat moss. Consequently, if your mix does not contain slow release fertilizer, seedlings need liquid fertilizer diluted to half strength almost immediately after the true leaves appear.

Recipes for soilless mix often include ground limestone because it adds calcium and magnesium. That increases the pH of the mixture (think peat moss with its low pH).

Coir is potassium rich and contains several micro nutrients. Use fertilizer low in potassium to adjust for the potassium.

Examples of recipes for simple soilless seed starting mixes:

Equal parts (by volume) coir or peat, vermiculite or perlite and worm castings

2 parts peat or coir, 1 part perlite, 1 part vermiculite, optional: teaspoon lime per gallon of peat moss

Read the contents of a pre-made soilless mix you use it. Note whether it contains fertilizer or not. You might also buy peat or coir seed starting pellets. Use both the mix and the pellets to find which is better.

This story was originally published February 13, 2016 at 12:00 AM with the headline "Gardening | Columnist Examines Soilless Seed Starting Mix."