This quartz soil is sold in 250ml - 500ml and 1Litre portions

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The purpose of a drainage layer in a terrarium is to prevent water from accumulating at the bottom of the enclosure, helping to create a well-balanced and healthier environment for the plants and animals inside. Here are the key reasons for including a drainage layer in a terrarium:
Preventing Waterlogging: A drainage layer allows excess water to drain away from the soil and plant roots. This helps prevent waterlogging, which can lead to root rot and other issues associated with overwatering.
Improving Soil Aeration: Adequate drainage promotes soil aeration, ensuring that the roots receive sufficient oxygen. Plants require oxygen at the root level for various physiological processes, and excessive water can displace air in the soil, leading to oxygen deprivation.
Reducing the Risk of Mold and Fungus: Excess moisture in the soil can create a favorable environment for mold and fungus growth. A drainage layer helps maintain appropriate moisture levels, reducing the risk of these issues.
Creating a Gradient of Moisture Levels: A well-designed terrarium often includes different layers with varying moisture levels. The drainage layer at the bottom establishes a gradient, with higher moisture levels near the substrate and lower levels near the surface. This allows plants and organisms to choose the moisture level that suits their needs.
Preventing Anaerobic Conditions: Without proper drainage, the soil at the bottom of the terrarium can become compacted, leading to anaerobic conditions (lack of oxygen). Anaerobic conditions can produce harmful byproducts and negatively impact plant and microbial health.
Components of a Drainage Layer: Typically, a drainage layer consists of materials with good permeability to water. Common components include:
Pebbles or Gravel: These provide a solid base for the drainage layer and allow water to flow freely.
Activated Carbon: As mentioned in a previous response, activated carbon helps filter impurities and prevent odors.
Mesh or Geotextile Fabric: Placed between the soil and the drainage layer, it prevents the soil from filtering down into the drainage layer while still allowing water to pass through.
When setting up a terrarium, the drainage layer is usually the first layer at the bottom, followed by the activated carbon and mesh, and then the substrate. Proper drainage is crucial for maintaining a healthy and balanced terrarium ecosystem.