Sources and research notes
Every claim used on a comparison page, with its references and any qualification, then the publications themselves. How we compare explains the research process.
Claims and qualifications
Open a statement to see its references and any recorded qualification. These are research records, not results from testing products or growing plants.
Publications behind each comparison
- Coco peat vs Coco coir: ISU [1], UF/IFAS [2], Rutgers [5], ISU [7], Arkansas [9], Purdue [10], Kentucky [11], Barrett [19].
- Garden soil vs Potting soil: ISU [1], UF/IFAS [2], Rutgers [5], UMD [6], Arkansas [9], Purdue [10], Kentucky [11], WSU [12], UMass [13], Wiberg [16], Bilderback [17].
- Peat moss vs Coco coir: ISU [1], UF/IFAS [2], Rutgers [5], ISU [7], Arkansas [9], Purdue [10], Kentucky [11], WSU [12], Barrett [19].
- Perlite vs Pumice: ISU [1], UF/IFAS [2], Rutgers [5], UMD [6], Arkansas [9], Purdue [10], Kentucky [11], Bilderback [17].
- Perlite vs Vermiculite: ISU [1], UF/IFAS [2], EPA [3], Rutgers [5], UMD [6], Arkansas [9], Purdue [10], Kentucky [11], WSU [12], UMass [13].
- Potting soil vs Potting mix: ISU [1], Rutgers [5], UMD [6], Arkansas [9], Purdue [10], WSU [12], UMass [13], Wiberg [16], Bilderback [17].
- Potting soil vs Raised bed soil: ISU [1], Rutgers [5], UMD [6], Arkansas [9], Purdue [10], WSU [12], Wiberg [16].
- Potting soil vs Topsoil: ISU [1], Rutgers [5], UMD [6], Arkansas [9], Purdue [10], WSU [12], Wiberg [16].
- Raised bed soil vs Garden soil: ISU [1], Rutgers [5], UMD [6], GA DoA [8], Purdue [10], WSU [12], UMass [13], CA Regs [15].
- Sphagnum moss vs Peat moss: ISU [1], UF/IFAS [2], Rutgers [5], ISU [7], Arkansas [9], Purdue [10], Kentucky [11], UF/IFAS [18].
- Topsoil vs Garden soil: ISU [1], Rutgers [5], UMD [6], Arkansas [9], Purdue [10], WSU [12].
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Materials and their properties
Perlite is volcanic rock that has been heated until it expands into lightweight, popcorn-like granules.
References: ISU [1]; UF/IFAS [2].
Perlite is made from siliceous volcanic rock, often from deposits in North America and the Mediterranean, that is heated and expanded.
References: UF/IFAS [2].
Perlite's job in a mix is air space and drainage: its expanded granules leave air-filled pores that last, because perlite does not break down.
References: ISU [1]; UF/IFAS [2].
Perlite does not decompose, though handling can break it into smaller pieces.
References: UF/IFAS [2].
Perlite is very lightweight.
References: UF/IFAS [2].
Perlite can be dusty; the University of Florida recommends a mask when handling large quantities.
Handling advice only. Not a health assurance in either direction.
References: UF/IFAS [2]; Arkansas [9].
Vermiculite is a naturally layered mineral that has been heated so it expands into shiny, flaky particles.
References: UF/IFAS [2]; ISU [1].
Vermiculite starts as a layered silica mineral and is expanded with heat.
References: UF/IFAS [2].
Vermiculite's job in a mix is holding water and, to some degree, nutrients.
References: ISU [1]; UF/IFAS [2].
Vermiculite is lightweight.
References: UF/IFAS [2].
Iowa State calls vermiculite expensive and, with a good-quality mix and a fertilizer program, likely unnecessary.
References: ISU [1].
The EPA's handling advice for vermiculite is to use it outdoors or in a well-ventilated area, keep it damp to avoid creating dust, and avoid carrying dust into the home on clothing.
The EPA page gives handling advice. It says nothing about the purity of any particular product, and neither do we.
References: EPA [3].
The University of Florida notes that fine vermiculite can produce dust that irritates when inhaled.
Pumice is crushed volcanic rock used as a growing-medium component.
References: UF/IFAS [2].
The University of Florida describes pumice as adding air and holding some water, in a guide to hydroponic media. Purdue lists pumice as an air-space ingredient for container mixes.
References: UF/IFAS [2]; Purdue [10].
The University of Florida and Purdue describe pumice as lightweight, but neither compares its weight with a particular grade of perlite.
References: UF/IFAS [2]; Purdue [10].
Peat is partially decomposed plant material, including mosses, grasses and sedges, removed from bogs.
References: UF/IFAS [2]; ISU [7].
Peat moss has traditionally been the base for most potting mixes, and it still is.
References: ISU [1]; Kentucky [11].
The University of Florida describes peat as holding water well and draining well.
References: UF/IFAS [2].
Iowa State warns that dried peat and coir can resist wetting. Purdue and Arkansas Extension describe coir as easier to rewet than peat, so no single rewetting rule fits every coir product.
References: ISU [1]; Purdue [10]; Arkansas [9].
The University of Florida describes peat as acidic.
References: UF/IFAS [2].
Sphagnum peat moss is the dead, partially decomposed material that accumulates in the lower levels of a peat bog; sphagnum moss is the living moss that grows on top of it.
Long-fiber sphagnum moss is used widely in the floral trade for wreaths and to line hanging baskets.
References: ISU [7].
Coco coir is coconut fiber from the outer husk of coconuts, a co-product of the coconut industry sold as a peat-moss replacement.
References: UF/IFAS [2]; ISU [1].
The University of Florida describes two kinds of coir: a chunky type with better drainage and high aeration, and a fine type with high water-holding capacity and good aeration.
References: UF/IFAS [2].
The University of Florida advises washing or rinsing coir before use because of its salt content.
References: UF/IFAS [2]; Arkansas [9].
The University of Florida describes coir as near neutral to slightly acidic.
References: UF/IFAS [2].
Rutgers warns that bag names such as Potting Soil, Container Mix, Garden Soil, Topsoil and Humus are confusing, and says to read the label to learn what is in the bag.
References: Rutgers [5].
Maryland Extension says bags sold as “organic soil” typically contain no topsoil; they combine wood waste, bark fines, compost, peat moss and other organic materials.
References: UMD [6].
Rutgers says many bagged soil products are entirely organic. Typical ingredients include forest products (“arbor fines”), wood bark, sphagnum or reed-sedge peat, coconut husk fiber (coir) and composts of various kinds and quality.
References: Rutgers [5].
Rutgers describes manufactured topsoil as a commercially blended mineral soil, usually with added compost. It says these products have no legal specifications and recommends asking suppliers for soil-test information.
References: Rutgers [5].
Rutgers notes that soil varies greatly across a landscape and with depth.
References: Rutgers [5].
Compost provides a low dose of nutrition, varies by source and may contain weed seeds.
References: ISU [1].
Bark added to potting media is composted and processed to half an inch or less; larger bark is unsuitable for seed starting.
References: ISU [1].
The University of Florida describes vermiculite as holding water well while still giving good aeration.
References: UF/IFAS [2].
Iowa State calls perlite the most common air-space ingredient in potting mixes.
References: ISU [1].
Arkansas Extension says perlite granules are closed cells that do not absorb or hold water. The University of Florida describes perlite used as a growing medium as holding water well. Neither gives a measurement for water inside a single granule.
References: Arkansas [9]; UF/IFAS [2].
Purdue notes that perlite tends to float to the top of a mix.
References: Purdue [10].
Mineral ingredients such as perlite, pumice, coarse sand and calcined clay do not decompose, or break down only slowly, in a mix. Blending them with organic ingredients helps keep large air spaces open over time.
References: Bilderback [17]; Kentucky [11].
Kentucky's growing-media guide says vermiculite can compact over time and lose air space, and Arkansas Extension advises against pressing or compacting it.
References: Kentucky [11]; Arkansas [9].
When vermiculite ore is heated, its plates move apart into an open, accordion-like structure. It is then graded extra coarse, coarse or fine, and Purdue describes it as holding a lot of water, with good aeration and low weight.
References: Purdue [10].
Horticultural pumice is mined, crushed and graded by particle size; Purdue describes it as a lightweight, porous rock.
References: Purdue [10].
Peat is light: Arkansas Extension says it is added to a mix to hold more water or to cut weight, Purdue gives it a low bulk density, and Washington State University says a peat-based potting mix weighs less than a soil-based mix.
References: Arkansas [9]; Purdue [10]; WSU [12].
A mix high in peat shrinks as it dries and can pull away from the sides of the pot. Water then runs down that gap instead of soaking in.
References: Purdue [10].
Arkansas Extension classifies horticultural peat as moss peat (peat moss, the least decomposed type, mostly from sphagnum), reed-sedge peat and peat humus.
References: Arkansas [9].
Arkansas Extension distinguishes sphagnum moss, the young or living part of the plant, from peat. It lists sphagnum moss for shipping plants, starting new plants and lining hanging baskets.
References: Arkansas [9]; ISU [7].
Purdue calls coir readily wettable, and Arkansas Extension says it rewets more easily than peat. Iowa State cautions that dried coir can still resist wetting. Follow the preparation directions for your product.
References: Purdue [10]; Arkansas [9]; ISU [1].
Coir can decompose and lose volume. Arkansas Extension says it may resist breakdown more than peat, and Kentucky lists decomposition and shrinkage among its drawbacks.
References: Arkansas [9]; Kentucky [11].
Coir comes from many countries, which makes it hard to characterize. Arkansas Extension advises checking a coir product's soluble salts, sodium and chloride.
References: Arkansas [9].
Coir pith, coir dust and coco peat are names for fine coconut-husk material. Barrett and colleagues list the names together, and the University of Florida separates fine coir from chunky forms.
References: Purdue [10]; UF/IFAS [2]; Barrett [19].
Kentucky's guide lists coir as lightweight.
References: Kentucky [11].
Washington State University advises using a potting mix made for indoor pots instead of soil from the garden, and warns that garden soils with a lot of clay can drain poorly in containers.
Arkansas Extension says soil should be a minor ingredient of modern container mixes, or left out. Where growers add it, the reasons are local supply or weight, and the concerns are weed seeds and leftover chemicals.
References: Arkansas [9].
Nurseries add sand and soil to outdoor container mixes to add weight and keep pots from tipping over, not to improve growth.
References: Arkansas [9].
A 2006 study of retail potting media found differences in physical and chemical properties among brands, and differences in seed germination among bags of one brand.
References: Wiberg [16].
The University of Florida says coarse sphagnum holds water well and leaves air spaces, which is why it is used for air layering, a way of rooting plants. Shredded sphagnum is a good medium for starting seeds.
References: UF/IFAS [18].
The University of Florida gives one drawback for sphagnum: it is expensive compared with other mix ingredients.
References: UF/IFAS [18].
Iowa State distinguishes finer, more decomposed peat sold as a soil amendment from the horticultural peat used in potting mixes. Processing peat into smaller particles increases the water it holds and reduces its air spaces.
References: ISU [1].
Maryland Extension describes topsoil as mineral soil, made of clay, silt and sand particles.
References: UMD [6].
Maryland Extension says topsoil should be dark and crumbly, with an earthy smell.
References: UMD [6].
Iowa State calls perlite the most common air-space ingredient in potting mixes, while Arkansas Extension lists pumice among amendments that growers there do not use extensively.
References: ISU [1]; Arkansas [9].
Bilderback and colleagues list perlite and pumice among the coarse aggregates that can be used in potting mixes.
References: Bilderback [17].
Arkansas Extension says the finer grades of vermiculite are used for seed germination or to top-dress seed flats, and Iowa State notes that some growers cover sown seeds with fine vermiculite.
References: Arkansas [9]; ISU [1].
Arkansas Extension says perlite is used most in greenhouse growing media and nursery propagation. Purdue notes that plug mixes for seedlings use a finer grade of perlite than mixes for larger containers.
References: Arkansas [9]; Purdue [10].
Purdue, Arkansas Extension and Washington State University each describe coir as an alternative or substitute for peat moss in a mix.
References: Purdue [10]; Arkansas [9]; WSU [12].
Rutgers says bagged “soil” products rarely contain actual mineral soil.
References: Rutgers [5].
Comparing the terms
Perlite adds air space and drainage to a mix; vermiculite mainly holds water and some nutrients. Perlite is volcanic rock puffed up by heat into light, popcorn-like granules. Vermiculite is a layered mineral heated into shiny flakes, and it adds some air too.
References: ISU [1]; UF/IFAS [2].
Just after watering, water coats the popcorn-shaped perlite and the stacked vermiculite flakes and fills many of the gaps between them.
References: ISU [1]; UF/IFAS [2]; Purdue [10].
Once the pot drains, air moves back into the larger gaps while water clings to the particles. Perlite is there for the air; vermiculite for the water.
References: ISU [1]; UF/IFAS [2].
As the mix dries, fewer water marks remain in either pot and air fills more of the gaps.
References: ISU [1]; UF/IFAS [2]; Purdue [10].
Potting soil is sold for pots; bagged garden soil is sold for beds and mixing into the ground. For indoor pots, Washington State University advises a potting mix instead of yard soil; clay-rich garden soil can drain poorly in containers.
References: Rutgers [5]; UMD [6]; WSU [12]; ISU [1].
Water coats the mixed particles in both bags and fills the gaps between them.
References: Rutgers [5]; UMD [6]; WSU [12]; ISU [1]; Purdue [10].
After draining, air moves back into the larger gaps. Packing the mix down makes those gaps smaller.
References: UMass [13]; ISU [1].
As the mix dries, fewer water marks remain and more of the gaps hold air.
References: Rutgers [5]; UMD [6]; WSU [12]; ISU [1]; Purdue [10].
Potting soil and potting mix are overlapping names for the same kind of product: a bagged blend made for pots. Either can contain peat, coir, bark or compost. Despite the name, Rutgers says bagged “soil” products rarely contain mineral soil; compare ingredient lists.
References: Rutgers [5]; WSU [12]; ISU [1].
Both bags are drawn with the same mixed particles, since both names cover the same kind of blend. Water coats them and fills the gaps.
References: Rutgers [5]; WSU [12]; ISU [1]; Purdue [10].
After draining, air returns to the larger gaps. Ingredients, particle sizes and how firmly the pot is filled set how much.
References: Rutgers [5]; WSU [12]; ISU [1]; Purdue [10].
As the mix dries, fewer water marks remain. A peat-based mix can shrink as it dries.
References: Purdue [10].
Perlite and pumice are both volcanic rock added to container mixes for air space, and neither decomposes the way organic ingredients do. Perlite is heated until it puffs into very light granules. Pumice is mined, crushed and sorted by size, and is used less widely.
Water coats the popcorn-shaped perlite and the pitted pumice pieces and fills the gaps between them.
References: UF/IFAS [2]; Purdue [10]; Bilderback [17]; ISU [1].
After draining, air returns to the gaps between the rock pieces. Keeping those gaps open is the job both are added for.
References: ISU [1]; UF/IFAS [2]; Purdue [10].
As the mix dries, fewer water marks remain on either rock.
References: UF/IFAS [2]; Purdue [10]; Bilderback [17]; ISU [1].
Sphagnum moss grows on top of a bog; peat moss is the partly decayed layer below. Sphagnum is sold in long strands or shredded, for lining baskets and starting plants. Peat moss is the usual base of potting mix. “Sphagnum peat moss” is peat moss.
References: ISU [7]; Arkansas [9]; UF/IFAS [18]; ISU [1]; Kentucky [11].
Water coats the long sphagnum strands and the fine peat fibers and fills the gaps between them.
References: ISU [7]; Arkansas [9]; UF/IFAS [18]; ISU [1]; Purdue [10].
After draining, water clings along the strands and among the peat, and air returns to the open gaps.
References: UF/IFAS [18]; UF/IFAS [2].
As they dry, fewer water marks remain in either pot.
References: ISU [7]; Arkansas [9]; UF/IFAS [18]; ISU [1]; Purdue [10].
Peat moss is partly decayed bog plants; coco coir is coconut-husk fiber sold as a peat substitute. Peat is acidic and can be hard to rewet once dry. Coir is closer to neutral; the University of Florida advises rinsing it before use because of salt.
References: UF/IFAS [2]; ISU [7]; ISU [1]; Purdue [10]; Arkansas [9].
Water coats the fibrous peat and the crumbly coir and fills the small gaps between them.
References: ISU [1]; UF/IFAS [2]; Arkansas [9]; Purdue [10].
After draining, both keep water around their fine particles, with air in the larger gaps.
References: UF/IFAS [2].
As the mix dries, fewer water marks remain and air takes over more of the gaps.
References: ISU [1]; UF/IFAS [2]; Arkansas [9]; Purdue [10].
Potting soil is a light blend made for pots; topsoil is mineral soil (sand, silt and clay) used to fill beds. Manufactured topsoil is usually blended with compost. Ask a bulk supplier where it came from and for any soil-test results.
References: Arkansas [9]; Purdue [10]; WSU [12]; UMD [6]; Rutgers [5].
Potting soil is drawn as mixed pieces and topsoil as crumbs. Water coats both and fills the gaps.
References: Rutgers [5]; UMD [6]; Arkansas [9]; ISU [1]; Purdue [10].
Soil's structure changes when it is dug and put in a pot. A potting blend is made for containers.
References: Rutgers [5]; UMD [6]; Arkansas [9]; ISU [1]; Purdue [10].
As both dry, fewer water marks remain. A peat-rich potting mix can be hard to rewet once dry.
References: ISU [1]; Purdue [10]; Arkansas [9].
Topsoil is mineral soil used as fill; bagged garden soil is often an organic blend for mixing into beds. Maryland Extension says bags sold as “organic soil” typically contain no topsoil. Topsoil usually comes in bulk; ask the supplier for its origin and soil tests.
References: UMD [6]; Rutgers [5].
Topsoil is drawn as crumbs and garden soil as mixed pieces. Water coats both and fills the gaps.
References: Rutgers [5]; UMD [6]; ISU [1]; Purdue [10].
After draining, air returns to the spaces between crumbs and pieces. Topsoil's structure depends on where it came from.
References: Rutgers [5].
As both dry, fewer water marks remain.
References: Rutgers [5]; UMD [6]; ISU [1]; Purdue [10].
Potting soil is sold for pots; raised-bed soil is sold for filling raised beds. Both are usually blends of peat, coir, bark or compost, and neither name fixes the recipe. For a bed, what's underneath matters: open ground or a hard surface.
References: Rutgers [5]; UMD [6].
Both bags are drawn as mixed pieces. Water coats them and fills the gaps.
References: Rutgers [5]; UMD [6]; ISU [1]; Purdue [10].
After draining, air moves back into the larger gaps between the pieces.
References: Rutgers [5]; UMD [6]; ISU [1]; Purdue [10].
As the fill dries, fewer water marks remain in both pots.
References: Rutgers [5]; UMD [6]; ISU [1]; Purdue [10].
Coco peat is coco coir: the fine, dusty part of coconut husk, also sold as cocopeat, coir pith or coir dust. A bag labeled “coir” may hold fine material, fibers, chunky chips or a blend. Fine coir holds more water; chunky coir holds more air.
References: Purdue [10]; UF/IFAS [2]; Barrett [19]; ISU [1].
Water coats the small coco peat crumbs and the larger coir pieces and fills the gaps between them.
References: Purdue [10]; UF/IFAS [2]; Arkansas [9]; ISU [1].
After draining, air returns to the gaps, which are larger between chunky coir pieces than between fine coco peat.
References: UF/IFAS [2].
As the coir dries, fewer water marks remain in both pots.
References: Purdue [10]; UF/IFAS [2]; Arkansas [9]; ISU [1].
Raised-bed soil is sold to fill a bed; garden soil is sold for beds or for mixing into the soil you have. Both are bagged blends, usually of bark, peat, coir or compost, and the names overlap. Read each bag's stated use.
References: Rutgers [5]; UMD [6]; CA Regs [15].
Water soaks the mixed pieces in both bags and fills the spaces between them.
References: Rutgers [5]; UMD [6]; ISU [1]; Purdue [10].
After draining, air returns to the larger spaces. Packing the fill down squeezes them smaller.
References: UMass [13]; ISU [1].
As the fill dries, fewer water marks remain in either pot.
References: Rutgers [5]; UMD [6]; ISU [1]; Purdue [10].
Purdue lists container mixes that contain both perlite and vermiculite.
References: Purdue [10]; ISU [1].
They do different jobs: Arkansas Extension says perlite is added to improve drainage, while vermiculite holds large amounts of water and nutrients. Purdue describes seed-starting mixes made with either fine perlite or vermiculite, and Washington State University describes houseplant mixes with either one.
References: Arkansas [9]; Purdue [10]; WSU [12].
Pots, beds and in-ground use
Both are common potting-mix ingredients, and Purdue lists mixes that contain both. Iowa State calls perlite the most common air-space ingredient.
References: Purdue [10]; ISU [1].
Maryland Extension describes filling raised beds with compost and a soilless mix, or with topsoil and compost. A soilless mix may already include perlite or vermiculite.
References: UMD [6]; Purdue [10]; ISU [1].
Rutgers describes improving soil that is low in organic matter with organic materials. It gives no general rule for mixing perlite or vermiculite into the ground.
References: Rutgers [5].
Potting soil is made for pots. A garden-soil bag is made for beds, so read its directions before filling a pot with it.
References: WSU [12]; Rutgers [5].
Garden-soil bags are sold for beds, and Maryland Extension lists compost with a soilless growing mix as one raised-bed fill.
References: Rutgers [5]; UMD [6].
Garden soil is sold for mixing in. Rutgers says bagged organic growing media, potting products included, can improve soil that is low in organic matter.
References: Rutgers [5].
Washington State University treats the two names alike: it advises a potting mix or potting soil made for indoor plants.
References: WSU [12]; Rutgers [5].
Maryland Extension lists compost with a soilless growing mix as one way to fill a raised bed.
References: UMD [6].
Rutgers says bagged organic growing media can improve garden soil that is low in organic matter.
References: Rutgers [5].
Purdue lists both as air-space ingredients for container mixes; Iowa State calls perlite the most common one.
References: Purdue [10]; ISU [1].
Maryland Extension describes filling raised beds with compost and a soilless mix, or with topsoil and compost, not with perlite or pumice alone.
References: UMD [6].
For garden soil low in organic matter, Rutgers describes adding organic materials. It gives no general rule for adding perlite or pumice.
References: Rutgers [5].
Peat moss goes into the mix itself. Shredded sphagnum is used to start seeds, and coarse sphagnum for air layering.
References: ISU [1]; Kentucky [11]; UF/IFAS [18].
Rutgers lists sphagnum peat among the usual ingredients of bagged soil products for beds.
References: Rutgers [5].
Iowa State describes a finer, more decomposed peat that is sold as a soil amendment.
References: ISU [1].
Peat moss is still the base of most potting mixes, and coir is sold as a replacement for it.
References: ISU [1]; Kentucky [11]; UF/IFAS [2].
Rutgers lists sphagnum or reed-sedge peat and coconut husk fiber among the usual ingredients of bagged soil products.
References: Rutgers [5].
Rutgers says bagged organic materials can improve garden soil that is low in organic matter.
References: Rutgers [5].
Arkansas Extension says soil should be a minor ingredient of a container mix, or left out. Where nurseries add it, the reason is weight or local supply.
References: Arkansas [9].
Maryland Extension describes two raised-bed fills: topsoil with compost, or compost with a soilless mix.
References: UMD [6].
Topsoil can be brought in as fill. Rutgers says bagged organic potting products can instead improve existing soil that is low in organic matter.
References: Rutgers [5].
Arkansas Extension says soil should be a minor ingredient of a container mix, or left out. Read a garden-soil bag's directions before using it in a pot.
References: Arkansas [9]; WSU [12]; Rutgers [5].
Maryland Extension's raised-bed fills include topsoil amended with compost. A garden-soil bag can supply that organic matter.
References: UMD [6]; Rutgers [5].
Topsoil can be brought in as fill; bagged garden soil is sold for mixing into existing soil.
References: Rutgers [5]; UMD [6].
Potting soil is made for pots. Read a raised-bed bag's directions before using it in one.
References: Rutgers [5]; WSU [12].
Raised-bed soil is sold for beds. Maryland Extension also lists compost with a soilless growing mix as one bed fill.
References: UMD [6].
Rutgers says bagged organic blends like these can improve garden soil that is low in organic matter.
References: Rutgers [5].
Fine coir can serve as the base of a mix; chunky coir gives a coarser, airier structure.
References: UF/IFAS [2].
Coir turns up in bagged bed products: Rutgers lists coconut husk fiber among their usual ingredients.
References: Rutgers [5].
Coir is organic material, and Rutgers says organic growing media can improve garden soil that is low in organic matter.
References: Rutgers [5].
Neither is sold mainly for pots. Read the bag's directions before filling a container with either.
References: Rutgers [5].
Both are sold for use in beds. Maryland Extension's two fills are compost with a soilless mix, or topsoil with compost.
References: UMD [6].
Garden soil is sold for mixing into existing soil. Rutgers says bagged organic blends can improve soil that is low in organic matter.
References: Rutgers [5].
Rutgers notes that raised beds are often filled with bagged products labeled potting soil, garden soil or topsoil, many of them entirely organic. It reports that beds filled only with organic growing media often grow poorer crops after several years.
References: Rutgers [5].
Reading labels and using this guide
Growing-media label rules depend on the state and product category. Georgia registers potting soils and their label information; California specifies label fields for packaged soil amendments.
References: GA DoA [8]; CA Regs [15].
Rutgers recommends a lab test to check the chemistry of organic growing media. Its saturated-media-extract test differs from the test used for mineral soil.
References: Rutgers [5].
The same mix holds far less air when it is packed down. Iowa State reports 15 percent air space when the pot was filled and brushed level, 9 percent after tapping it twice and 4 percent when the mix was pressed down.
References: ISU [1]; UMass [13].
In the same mix, a taller pot leaves more of the mix above the wet layer at the bottom. Mixes with smaller pores raise that wet layer.
References: ISU [1]; Purdue [10].
Rutgers describes an “ideal” container medium as one that drains rapidly after irrigation and holds 10 percent or more air space by volume after draining, measured at a medium depth of 14 to 15 cm.
References: Rutgers [4].
For container media, Rutgers says most particles in an amendment should fall between 0.5 and 4 mm, with less than 20 percent below 0.5 mm.
References: Rutgers [4].
Organic ingredients break down in a pot over time. Larger pieces become fine particles that hold more water and leave less room for air, so the mix changes as it ages.
References: Bilderback [17]; Purdue [10].
After a pot drains, the mix at the bottom stays wetter than the mix above it. Iowa State explains that smaller pores raise this wet layer, and a taller pot leaves more mix above it.
References: ISU [1]; Purdue [10]; UMass [13].
Pressing a mix down can sharply cut its air space; UMass Extension calls avoiding compaction the major concern.
References: UMass [13]; ISU [1].
Purdue notes that a mix shrinks after you fill the pot, so its level drops.
References: Purdue [10].
Air space, water-holding capacity, total pore space and dry bulk density are physical properties of a growing medium. Purdue describes these properties; ASTM D8593-24 provides a laboratory method for measuring them.
References: ASTM [14]; Purdue [10].
California requires packaged soil amendments to list ingredients in decreasing amounts, along with the product's purpose and directions. This is a state-specific rule, not a description of every soil product sold in the US.
References: CA Regs [15].
For packaged soil amendments, California requires the label to state the product's purpose and directions for use. Check those fields on your bag, and ask the manufacturer when its intended use is unclear.
References: CA Regs [15].
Purdue says wetting agents help water spread through dry ingredients such as peat, and starter fertilizers supply nutrients at the start. Check whether a finished mix already contains either.
References: Purdue [10].
Tell the lab whether your sample is mineral soil, potting mix or compost. Rutgers uses a different chemical test for organic growing media than for mineral soil, so one test does not suit every material.
References: Rutgers [5].
Iowa State notes that some potting-media products list traits such as drainage, particle size and water retention. It advises checking ingredients and considering the container and how the mix will be handled.
References: ISU [1].
Maryland Extension separates raised beds on existing soil from beds on hard surfaces, and its fill advice depends on the surface below and the condition of the existing soil.
References: UMD [6].
Soil dug from a garden and a bag sold as garden soil are different materials. Washington State University's warning about indoor pots concerns soil from the garden, including clay-rich soil; Rutgers says to read a bagged product's label for its ingredients.
References: WSU [12]; Rutgers [5].
Rutgers says bagged organic growing media can improve mineral soil that is low in organic matter. That is a soil-improvement use; it does not make every bag a replacement for your existing soil.
References: Rutgers [5].
Maryland Extension advises asking a bulk-topsoil supplier for soil-test results, the soil's origin, and information about mixing and storage. It also recommends looking at the material before purchase or delivery.
References: UMD [6].
Maryland Extension describes filling raised beds with compost and a soilless growing mix, or with topsoil amended with compost.
References: UMD [6].
Rutgers describes buying mineral topsoil from a landscape supply vendor, usually from stockpiles delivered by truck, and says topsoil suppliers should be able to provide soil test reports.
References: Rutgers [5].
Where a bed sits on open ground, Maryland Extension advises loosening compacted soil below with a garden fork, without turning it over, to improve drainage and aeration. Rutgers recommends improving the soil already there where possible, and blending any imported soil with it.
References: UMD [6]; Rutgers [5].
For a bed on a hard surface such as a driveway, Maryland Extension advises filling it with compost and a soilless growing mix, adding topsoil only to deep beds, and says such beds dry out quickly.
References: UMD [6].
Rutgers says organic fill loses its structure and volume as it breaks down, so a bed may need regular additions to keep its depth. Mineral soil tends to keep its volume after it first settles.
References: Rutgers [5].
Publications
- [1]
Iowa State University Extension and Outreach
Potting Media: Components and Handling, Randall Vos (March 10, 2025).
Recorded excerpt
Perlite is the most common and is a rock-like material that has been 'puffed up' kind…
- [2]
University of Florida IFAS Extension
ENH1359/EP623: Common Media Used in Hydroponics, Morgan G. Pinkerton, Brianna Whitman, Hannah Eason, Celina Gómez.
Recorded excerpt
Perlite is made from siliceous volcanic rock, often from deposits in North America and the Mediterranean.…
- [3]
United States Environmental Protection Agency
I use/used vermiculite to enhance my potting soil. Should I be concerned? (Last updated January 29, 2026).
Recorded excerpt
Use vermiculite outdoors or in a well-ventilated area.
- [4]
Rutgers New Jersey Agricultural Experiment Station
FS812: Fundamentals of Container Media Management: Part I, Raul Cabrera, Jim Johnson (November 2014).
Recorded excerpt
An ideal medium should drain rapidly following irrigation and contain 10% (by volume) or more air…
- [5]
Rutgers New Jersey Agricultural Experiment Station
FS1328: Soil for Raised Beds, Stephanie Murphy (September 2025).
Recorded excerpt
Product names can be confusing: Potting Soil, Container Mix, Garden Soil, Topsoil, Humus; check the label…
- [6]
University of Maryland Extension
Soil to Fill Raised Beds (Updated November 6, 2024).
Recorded excerpt
The bags of 'organic soil' you may see for sale typically don't actually contain topsoil (i.e.…
- [7]
Iowa State University Extension and Outreach
Is There A Problem With Sphagnum Peat Moss? (July 15, 1994).
Recorded excerpt
the dead, partially decomposed material that accumulates in the lower levels of the peat bog
- [8]
Georgia Department of Agriculture
Horticultural Growing Media (HGM).
Recorded excerpt
We register and approve potting soils and all label information, including the base material, percentage of…
- [9]
University of Arkansas System Division of Agriculture, Cooperative Extension Service
FSA6097: Growing Media for Container Production in a Greenhouse or Nursery, Part I – Components and Mixes, James A. Robbins (Revised 2018 (FSA6097-PD-8-2018RV)).
Recorded excerpt
Perlite differs from vermiculite in that the finished product is a "closed cell" that does not…
- [10]
Purdue University Extension
HO-255-W: Evaluating Container Substrates and Their Components, W. Garrett Owen, Roberto G. Lopez.
Recorded excerpt
Container capacity depends on container size, so the taller the container, the more drainage, and the…
- [11]
University of Kentucky Center for Crop Diversification
CCD-TA-02: Selecting Growing Media for Soilless Systems, Casey Byrd, Arundathi Sharma, Qinglu Ying (2025).
Recorded excerpt
vermiculite has high water retention and cation exchange capacity but can compact over time, reducing aeration.
- [12]
Washington State University Extension
The Pacific Northwest Gardener's Handbook, Chapter 14: Houseplants, Toni Gwin (adapted from the Texas Master Gardener Handbook).
Recorded excerpt
A peat-based potting mix will weigh less than a soil-based mix; however, it is more difficult…
- [13]
UMass Extension Greenhouse Crops and Floriculture Program
Greenhouse BMP Manual: Effects of Growing Media on Water and Nutrient Management.
Recorded excerpt
In all containers, there will be a certain amount of saturated medium at the bottom of…
- [14]
ASTM International
Recorded excerpt
the total porosity, container capacity, and air space are three properties that are determined on a…
- [15]
California Code of Regulations, Title 3 (via Cornell Legal Information Institute)
3 CCR § 2303: Labeling Requirements (fertilizing materials, including packaged soil amendments) (Amendment filed July 1, 2024; operative October 1, 2024).
Recorded excerpt
A list of ingredients in decreasing amounts present (for packaged soil amendments and organic input material…
- [16]
HortTechnology (American Society for Horticultural Science), record via FAO AGRIS
Variability in plant growth in retail potting media, A. Wiberg, R. Koenig, T. Cerny-Koenig (2006, HortTechnology 16(1): 7–12).
Recorded excerpt
There is extensive variability in physical and chemical properties among brands of retail potting media.
- [17]
HortTechnology (American Society for Horticultural Science), PDF hosted by USDA ARS
Healthy Substrates Need Physicals Too!, Ted E. Bilderback, Stuart L. Warren, James S. Owen Jr., Joseph P. Albano (2005, HortTechnology 15(4): 747–751).
Recorded excerpt
Mineral aggregates such as perlite, pumice, coarse sand, and calcined clays do not decompose, or breakdown…
- [18]
University of Florida IFAS, with University of Kentucky and Texas A&M (Plant Propagation course site)
Recorded excerpt
Coarse sphagnum is used most often in air layering because it has good water holding capacity…
- [19]
Scientia Horticulturae (Elsevier), open-access paper hosted by North Carolina State University
Achieving environmentally sustainable growing media for soilless plant cultivation systems – A review, G. E. Barrett, P. D. Alexander, J. S. Robinson, N. C. Bragg (2016, Scientia Horticulturae 212: 220–234).
Recorded location: Section 3.2, Coir; journal page 224 (PDF page 5)
Recorded excerpt
Coir (also known as coir pith, coir meal, coir dust and coco peat)