Is vermiculite necessary in container mixes?
Written by
Julia Anderson
Reviewed by
Prof. Charles Hartman, Ph.D.Vermiculite fulfills several functions in soil for container gardening. When wet, this mineral amendment expands, retaining a significant amount of moisture and even releasing essential minerals, such as magnesium and potassium. However, vermiculite tends to compress more easily than other alternatives. It should be used judiciously, based on the plant's needs.
Ideal Uses
- Seed starting mixes: Holds consistent moisture for germination
- Moisture-loving plants: Ferns and calatheas benefit greatly
- Hanging baskets: Reduces watering frequency in hot weather
- Sandy soils: Improves water retention dramatically
When to Avoid
- Cacti/succulents: Creates overly moist conditions
- Humid climates: Promotes fungal growth issues
- Heavy soils: Worsens compaction problems
- Self-watering pots: Causes waterlogging at root zone
Discover the chemical mechanism of vermiculite. This hydrated magnesium aluminum silicate expands in size as it is heated. Its layered structure allows water molecules to be trapped efficiently in the layers of the mineral. This will enable plants to access water from a reservoir between waterings. The mineral content also allows the plants to have a slow growth rate.
Vermiculite (properly combined) with other amendments. Mix substrate with perlite to balance moisture and aeration within the medium. Mix vermiculite with coarse sand when amending with sand to improve drainage. Limit amending to a maximum of 20% of the total soil volume. Mix with compost to create nutrient-rich blends. Before using the entire container of amended substrate, apply the amended mixture on a small scale to test effectiveness.
When vermiculite creates challenges, choose alternatives. Perlite provides much better aeration for succulent-type plants. Rice hulls sustainably offer similar moisture retention properties. Calcined clay enhances drainage and aeration in more humid climates. Pumice permits constant air movement and aeration without unnecessary soil compaction. Decisions should ultimately be based on local conditions and circumstances.
Keep a close eye on plants that use vermiculite mixes. Until you get a feel for them, check soil moisture daily. You will likely find that you use much less water than with perlite mixes. You will also want to pay attention to signs of compaction, such as a slower drainage period. As with perlite mixes, refresh the soil annually to maintain its effectiveness. Your plants will tell you everything you need to know.
Read the full article: Choosing the Best Container Gardening Soil
