Why is sand preferred over other media like gravel in iAVs?

This is covered in Chapter 20 in the iAVs Handbook

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Sand is the ideal medium for iAVs due to its superior filtration properties, enhanced oxygenation, and ability to promote a thriving microbial community. Unlike gravel or expanded clay pebbles, which have limited surface area and can lead to anaerobic zones, sand:

• Historical Significance and Proven Effectiveness: Sand has been used as a natural water filtration medium for thousands of years. Sand filters are still the preferred choice for water purification in modern facilities
• Accessibility and Practicality: Sand is widely available, reusable, requires low maintenance, and provides economic and environmental advantages.
• Optimal Growing Environment: Sand provides strong physical support for plant roots, encourages a healthy microbiological ecosystem, and balances drainage with water retention. Sand ensures plants receive adequate water and nutrients when combined with intermittent irrigation.
• Mechanical and Biological Filtration: Sand offers exceptional mechanical and biological filtration due to its specific surface area (SSA). Sand has an SSA of 7,000–10,000 m²/m³, which is about 200 times greater than typical gravel. Sand effectively removes suspended solids and supports diverse microbial communities.
• Supporting Soil Ecology: Sand is ideal for iAVs as it temporarily retains nutrients while allowing direct microbial interaction with plant root exudates, fostering a diverse soil ecology.
• Enhancing Aeration and Oxygenation: Sand beds maintain around 20-21% oxygen by volume in their air-filled pore spaces, far higher than the dissolved oxygen content in water.
• Optimizing Hydraulic Properties: Sand’s porosity provides an ideal balance of water retention for plant growth, efficient drainage to prevent waterlogging, adequate aeration for root health, and even nutrient distribution across the growing medium
• Traps solid waste near the surface, facilitating breakdown and preventing clogging.
• Promotes oxygen exposure throughout the bed, accelerating decomposition and supporting aerobic microbes crucial for nitrification.
• Increasing Plant Nutrient Uptake: The large surface area of the biofilm enhances micro-contact with dissolved nutrients, leading to improved nutrient uptake, greater plant vigor, higher yields, and more stable growing conditions.
• Provides a vast surface area for beneficial bacteria to colonize, maximizing nutrient cycling efficiency.
• Retains moisture while allowing excess water to drain, creating an ideal balance for plant growth.