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The Dirty Truth Of Common
Potting Mix Ingredients

Witch Ingredients Are In Your Potting Mix?

Potting Mix Ingredients

Sphagnum moss peat and coconut coir fiber are two of the most common base component in potting mixes. Listed below are other materials commonly added to both commercial and homemade potting soil. 

Uunderstanding their intended use within a potting mix can help you, when purchasing, to decide whether the potting mix is going to do the job intended.

While many potting mix ingredients are also used as soil amendments and common to both the open garden and potting mixes this section will deal mainly with their usefulness within a potting medium for growing vegetable seedlings.

Sphagnum Moss Peat

Sphagnum peat is a very stable organic material that holds water and air well, does not decompose quickly and drains freely. It is very acidic, pH around 4.0, and yellow to light brown in color. Sphagnum peat is the least decomposed of the general categories of peat.

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Sphagnum peat moss has for a long time been the main base material in Soilless potting mixes for seed raising. For more about sphagnum peat follow this link to Sphagnum moss peat

Coconut Coir Fiber

Coconut coir is an excellent substitute for sphagnum peat as the base potting mix ingredients in Soilless potting mixes. it is a renewable organic resource that lasts longer than sphagnum peat before breaking down. It is used to add bulk and aeration to potting mixes, can hold a large amount of water. Because of its bulk and texture, once saturated the excess water is able to drain freely from the potting mixture.

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Coconut coir is purchased in compressed blocks that only need water added to rehydrate it for use in potting mixes. For more about using coconut coir in potting mixes follow this link to coconut coir fiber


While there is nothing better for a vegetable garden than well made compost, it can cause problems in a potting mix designed for growing vegetable seedlings.

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Most importantly, compost used in potting mixes should have gone through a hot compost process to ensure all weed seeds and plant pathogens have been eliminated. For more about using compost in potting mixes follow this link to Compost potting soil

Composted Forest Products

As a potting mix ingredients there is little difference between composted forest products and composted pine bark, mostly it is used as a cheap replacement for the peat component whose purpose is to provide drainage and air spaces within a potting mix.

In the right place, some specialized potting mixes and potting mixes for older potted plants, these products may well be an acceptable substitute.

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However they are a poor addition to a potting mix that is going to be used for raising seeds and young vegetable seedlings. For more about compost forest products and composted pine bark follow this link to Composted forest products 

Seedling Fertilizer

When the nutrients provided by the seed have been exhausted, which is about the time a young seedlings first true leave appear, they require small amounts of fertilizer to ensure good strong healthy seedlings.

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This is especially true if you're using a soilless seed starting mix. For more about fertilizing seedlings follow this link to Seedling fertilizer


Perlite is the snowy white granular particles that look like small pieces of Styrofoam you will see in many potting mediums.

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For more about the different ways of using perlite as a potting mix ingredients in growing mediums follow this link to Perlite in potting soil

Potting Sand

Sand doesn't hold water well and too much can hinder drainage. Clean, washed sand has a near neutral pH with no nutrient value for plants; it is usually added as potting mix ingredients simply because it is a cheap filler. In seed starting and potting on mixes for growing vegetable seedlings it is neither needed nor recommended.

Away from vegetable seed raising mixes sand can help in potting mixes for older, taller potted plants giving them some ballast to prevent them from toppling over, it should be used sparingly and ensure it is clean sharp sand.

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Sharp sand refers to sand that has a course and gritty texture. In most cases this type of sand is made from ground quartz rocks. It is best to avoid plaster and fine sands for use in potting mixes because these will create a dense mix with poor structure and drainage.


Although soil is sometimes incorporated in both commercial and homemade potting mixes it is considered by many as an unsuitable potting mix ingredients to use in a potting, container or seed raising medium. Soil or garden loam comes with a range of problems, as well as containing bacteria, organisms and spores that cause diseases, soil can harbor a huge range of insects and weed seeds.

Even when the soil is sterilized to eliminate these problems, it is inherently dense causing a lack of porosity and the ability for good drainage. In a contained environment soil begins to compact after being watered several times, causing lack of aeration of the soil. This is especially hard on young seedlings trying to get established because, without air, roots will suffocate and rot.

Starting with a disease and problem free growing medium is the most important first step when growing in containers, pots or raising seeds and seedlings. It can be the difference between your plants thriving or just surviving.

lime Fertilizer

Most vegetables thrive when the soil has a pH in the range of somewhere between 6.0 - 7.0 It is in this range that the soil provide the broadest availability of nutrients. Agricultural, or garden lime acts as a neutralizing agent for acidic soils and is why it is commonly added to soilless potting mixes like sphagnum peat.

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While it is not needed in seed germination mixes any potting on mix using sphagnum peat or similarly acidic base material should contain lime at the rate of 1/2 tablespoon for every gallon (1 US gal-dry is equal to 4.40 liters) of sphagnum peat used.


Vermiculite is used as potting mix ingredients in a similar way as perlite, but with the advantage of having some nutrient content. However it is not as hard as perlite and if handled too roughly it will compact and lose its ability to hold both water and air.

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Vermiculite provides a good environment for seed germination. It doesn’t form a crust and allows the young seedling to freely emerge it can be used either within a potting mix or for covering seeds after sowing. For more about using vermiculite in potting mixes follow this link to Vermiculite

Wetting Agents

Wetting agents are a common ingredient in many commercial potting mixes. They are polymers added to the soil to retain and regulate moisture within a potting mix. Most wetting agents are not approved by organic certifying agencies, however a natural derivative of the yucca plant has been found to aid in the uniformity of water absorption in soils and is being added to some potting mixes. While some products from this source are "OMRI listed" not all products containing yucca extract are.

The Organic Materials Review Institute (OMRI) provides organic certifiers, growers, manufacturers, and suppliers an independent review of products intended for use in certified organic production.

For the gardener, buying any product that has this  Photo of OMRI-listed-logo logo on the label is an indication that the ingredients within the product comply with the USDA’s National Organic Program (NOP)


Vermicompost or Worm Castings,  are an excellent organic seedling fertilizer. The nutrients in worm castings are water soluble, making them immediately available for plants to use. Mix approximately 10 - 15 percent vermicompost into the seed starting mix and it will last until transplanting without having to add any other plant food.

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For more about using worm castings in potting mixes for growing seedlings follow this link to Vermicompost

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