Materials

Perlite

Mineral ingredient

Perlite is volcanic rock heated until it expands into lightweight, popcorn-like granules.

Air space

High Its expanded granules leave air-filled pores that last

Weight

Low Very lightweight

How long it lasts

High Does not decompose but can fragment during handling

Names on the bag

perlite, horticultural perlite, expanded perlite

Sold in fine, medium and coarse grades. The bag may not say which.

• Iowa State calls perlite the most common air-space ingredient in potting mixes.
• Perlite tends to float to the top of a mix.
• Perlite can be dusty; the University of Florida recommends a mask when handling large quantities.

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Pumice

Mineral ingredient

Pumice is volcanic rock that is mined, crushed and sorted by size for growing mixes. It is not heated and expanded the way perlite is.

Air space

High University of Florida: excellent aeration

Water holding

Medium University of Florida: good water retention

Weight

Low Described as lightweight; no grade-matched comparison with perlite

How long it lasts

High Does not decompose, or breaks down slowly

Names on the bag

pumice, horticultural pumice

Graded by particle size after crushing. The bag may not say which grade.

• The University of Florida describes pumice as adding air and holding some water; Purdue lists it as an air-space ingredient for container mixes.
• Like perlite, pumice does not decompose, or breaks down only slowly, in a mix.
• Pumice is described as lightweight, but the sources don't compare its weight with a given grade of perlite.

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Vermiculite

Mineral ingredient

Vermiculite is a naturally layered mineral heated until it expands into shiny, flaky particles.

Air space

Medium Rated as giving good aeration, though holding water is its main job

Water holding

High Holds water and, to some degree, nutrients

Weight

Low Lightweight

Names on the bag

vermiculite, horticultural vermiculite

Sold in several grades from fine to coarse. The bag may not say which.

• The EPA's handling advice for vermiculite is to use it outdoors or in a well-ventilated area and keep it damp to avoid dust.
• The University of Florida notes that fine vermiculite can produce dust that irritates when inhaled.
• Vermiculite can compact if pressed and lose air space over time.
• Iowa State calls vermiculite expensive and, with a good-quality mix and a fertilizer program, likely unnecessary.

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Coco coir

Organic ingredient

Coco coir is fiber from the outer husk of coconuts, a co-product of the coconut industry sold as a peat-moss replacement.

Weight

Low Listed as lightweight

Names on the bag

coco coir, coconut coir, coir, coconut husk fiber, coco chips

Chunky and fine coir behave differently; the bag or brick may not say which.

• The University of Florida advises washing or rinsing coir before use because of its salt content.
• Purdue and Arkansas Extension describe coir as easier to rewet than peat, while Iowa State cautions that dried coir can resist wetting. Follow the preparation directions for your product.
• Coir comes from many countries, which makes it hard to characterize; Arkansas Extension advises checking a product's soluble salts, sodium and chloride.

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Coco peat

Organic ingredient

Coco peat is the fine fraction of coconut husk, sold loose or as compressed bricks; the University of Florida describes fine coir as holding more water than the chunky type.

Air space

Medium The University of Florida rates the fine type's aeration as good

Water holding

High High water-holding capacity

Weight

Low Listed as lightweight

Names on the bag

coco peat, coir pith, coir dust, cocopeat brick

• Coco peat is a form of coir. The University of Florida advises washing or rinsing coir before use because of its salt content.

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Peat moss

Organic ingredient

Peat moss is partially decomposed plant material, mostly sphagnum moss, taken from the lower layers of a bog.

Air space

Medium Described as draining well as well as holding water

Weight

Low Added to mixes partly to reduce weight; low bulk density

Names on the bag

sphagnum peat moss, peat moss, reed-sedge peat, black peat

• Iowa State says peat moss is still the base of most potting mixes.
• Dry peat can resist wetting. Check the preparation directions and whether a wetting agent is included.
• A mix high in peat shrinks as it dries and can pull away from the sides of the pot; water then runs down the gap instead of soaking in.

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Sphagnum moss

Organic ingredient

Sphagnum moss comes from young or living bog-surface growth and is sold as long fibers or shredded material.

Names on the bag

sphagnum moss, long-fibered sphagnum, New Zealand sphagnum

• A bag labeled “sphagnum peat moss” holds peat moss, the decayed layer from lower in the bog.
• The University of Florida notes that sphagnum is expensive compared with other mix ingredients.

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Garden soil

Bag category

Bagged garden soil is a manufactured product sold for beds and for mixing into existing soil. Its ingredients vary, and it is a different material from soil dug out of a garden.

Names on the bag

garden soil, organic garden soil, in-ground soil, flower and vegetable garden soil

• For soil dug from your yard, Washington State University advises a potting mix made for indoor pots instead; clay-rich garden soil can drain poorly in containers. Read a garden-soil bag's directions before using it in a pot.

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Potting mix

Bag category

Potting mix is a prepared growing blend sold for containers. The name overlaps with potting soil and does not define a separate formula.

Names on the bag

potting mix, potting soil, container mix, seed starting mix

• Retail mixes vary widely between brands and even between bags of one brand.

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Potting soil

Bag category

Potting soil is a prepared growing blend sold for containers. Its ingredients may overlap with products called potting mix.

Names on the bag

potting soil, potting mix, container mix, indoor potting mix

• Retail mixes vary widely between brands and even between bags of one brand.

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Raised bed soil

Bag category

Raised-bed soil is a bagged blend sold for filling raised beds. The name does not set its ingredients.

Names on the bag

raised bed soil, raised bed mix, raised bed and planter mix

• Maryland Extension's fill advice depends on whether the bed sits on existing soil or on a hard surface.

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Topsoil

Bulk material

Topsoil is mineral soil, made of sand, silt and clay particles, sometimes blended with compost. Bagged products may use the name loosely, so read the ingredients or ask the supplier.

Names on the bag

topsoil, screened topsoil, premium topsoil, lawn soil

• Arkansas Extension says soil should be a minor ingredient of modern container mixes, or left out.
• Maryland Extension says topsoil should be dark and crumbly, with an earthy smell, and advises asking a bulk supplier for soil-test results and the soil's origin.

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