Indoor Potting-Media Ingredient Reference

A source-indexed reference for the materials used in indoor container mixes. Compare what each ingredient does and follow the citation back to its extension or research source.

This table reports source observations, not a single tested population. University publications cover different grades, test methods, container heights, and production settings. A value from a nursery-substrate trial is not a guaranteed result in a home pot.

Ingredient properties and sources

10 ingredients

Download the source table (CSV)

Scope: source-reported properties of materials used in container media. Nursery and hydroponic trials are identified as such; they are not universal houseplant performance guarantees. Retrieved September 28, 2026.

Potting-media ingredients with source-reported properties and citations
IngredientMaterial and useSource-reported observationsLimit or caveat
Perlite Expanded volcanic glass
  • Purdue reports the ground volcanic ore expands about 4 to 20 times its original volume when heated; it describes the horticultural aggregate as lightweight, porous, and pH 6.5 to 7.5. Purdue Extension HO-255-WChecked 2026-09-28.
  • Arkansas Extension describes horticultural perlite as a closed-cell aggregate that does not absorb water, so it is used to increase drainage and aeration. University of Arkansas Extension FSA-6097Checked 2026-09-28.
  • UF/IFAS lists water-holding capacity as moderate in its hydroponic-media comparison table. This uses a different source/test context from the Arkansas closed-cell description. UF/IFAS EP623Checked 2026-09-28.
 Particle grade and test method matter. Lightweight granules may float during watering; dry perlite is dusty.
Pumice Porous volcanic rock
  • Purdue describes horticultural pumice as a light, porous volcanic substrate with pH about 7.0 to 7.4. Purdue Extension HO-255-WChecked 2026-09-28.
  • UF/IFAS lists pumice as having no CEC, neutral pH, moderate water-holding capacity, and good aeration in its hydroponic-media comparison. UF/IFAS EP623Checked 2026-09-28.
  • The ASHS study reports that particle-size distribution changes pumice air/water balance; coarse grades can hold less available water in some containers. ASHS HortTechnologyChecked 2026-09-28.
 Do not treat all grades as equivalent. Source and particle-size distribution affect performance.
Vermiculite Expanded mica mineral
  • Arkansas Extension describes expanded vermiculite as plate-like and able to hold water and potassium, magnesium, and calcium nutrients. University of Arkansas Extension FSA-6097Checked 2026-09-28.
  • UF/IFAS lists vermiculite as having moderate water-holding capacity and good aeration in its hydroponic-media comparison table. UF/IFAS EP623Checked 2026-09-28.
 Source and grade affect pH and retention; avoid compacting wet material.
Sphagnum peat moss Partly decomposed sphagnum peat
  • Purdue reports sphagnum peat pH around 3.0 to 4.0 and describes high water- and air-holding capacity for lighter peat grades. Purdue Extension HO-255-WChecked 2026-09-28.
  • WSU Extension notes that a peat-based mix becomes harder to wet after it dries completely and recommends moistening mix before potting. WSU ExtensionChecked 2026-09-28.
 Properties vary with decomposition and grade. These are source-specific values, not a universal finished-mix pH.
Coco coir Coconut fibre
  • In the named south Florida nursery-substrate trial, coconut fibre retained more water per unit dry weight than the tested pine bark, Florida peat, and prepared mix. UF/IFAS AE562Checked 2026-09-28.
  • UF/IFAS lists coir as having high water-holding capacity and good aeration in its hydroponic-media comparison. UF/IFAS EP623Checked 2026-09-28.
 These results describe the tested materials and setup; retail coir products vary. They do not establish one universal coir value.
Composted pine or fir bark Processed bark aggregate
  • UF/IFAS lists composted pine bark as high water-holding with good aeration and moderate-to-low CEC in its hydroponic-media comparison. UF/IFAS EP623Checked 2026-09-28.
  • Purdue and Arkansas Extension describe bark as a common structural substrate component; bark-heavy mixes are used where aeration and drainage are needed. Purdue Extension HO-255-WChecked 2026-09-28.
 Species, composting, particle size, and age change properties. The row does not claim one recipe suits all plants.
Expanded clay (LECA) Fired expanded clay aggregate
  • UF/IFAS lists expanded clay as neutral pH, good aeration, low water-holding, and reusable in its hydroponic-media comparison table. UF/IFAS EP623Checked 2026-09-28.
  • Purdue notes lightweight expanded clay aggregate among substrate aggregates used in greenhouse media. Purdue Extension HO-255-WChecked 2026-09-28.
 LECA product and system differ from soil-based potting mix; its inclusion here is for comparison, not a soil recipe recommendation.
Coarse washed sand Inorganic mineral aggregate
  • UC Davis advises using coarse, washed sand for media and avoiding beach sand or sand intended for highway de-icing; its root-media notes list bulk density around 100 to 120 lb/ft3. UC Davis ExtensionChecked 2026-09-28.
  • UF/IFAS lists sand as low water-holding with good aeration in its hydroponic-media comparison. UF/IFAS EP623Checked 2026-09-28.
 Bulk-density figure is source-reported for sand, not a bag or mix measurement.
Medium horticultural charcoal Carbonized material
  • University of Maryland lists medium charcoal as one ingredient in its published orchid-mix example (1 part peat moss, 6 parts fir bark, 1 part charcoal). University of Maryland ExtensionChecked 2026-09-28.
 The cited extension example documents recipe use, not a universal performance benefit. No general water-holding or nutrient claim is made here.
Worm castings Earthworm-processed organic matter
  • LSU AgCenter reports variable NPK values (about 1-0-0 to 5-5-3) and micronutrients; it notes the value depends on feedstock and storage. LSU AgCenterChecked 2026-09-28.
  • LSU AgCenter describes mixing castings into potting mix and storing them moist but not overly wet, away from ultraviolet light and temperature swings. LSU AgCenterChecked 2026-09-28.
 NPK varies. Castings are not a fixed-dose or replacement-fertilizer recommendation. A commonly repeated percentage was excluded because its primary CSU citation was not verified.

Source values depend on material grade and test method. If a linked source changes or a row needs correction, contact us through the contact form and name the ingredient and source.

Method and corrections

Each row keeps the cited source's context and retrieval date. We do not average values from unlike materials or tests, and blank properties are left out rather than guessed. Derived calculator outputs are kept separate from these observed source notes.

Sources were checked on September 28, 2026. Recheck a source before relying on it for a new recommendation. To flag a broken citation or factual correction, use the contact form and include the ingredient and source title.

The downloadable CSV preserves each source observation as its own row: potting-media ingredient data (CSV).