Garden Soil Mix Ratio Calculator

How This Tool Works

This calculator divides your total volume into equal thirds for peat moss, compost, and perlite, creating a balanced soil mix perfect for most plants. The 1/3:1/3:1/3 ratio is a time-tested formula that provides excellent drainage, moisture retention, and nutrient content. Peat moss holds moisture and provides structure, compost adds nutrients and beneficial microorganisms, and perlite ensures proper aeration and drainage.

This standard mix works well for most houseplants, container gardens, and raised beds. However, you can adjust the ratios based on your specific plant needs - plants that need more drainage (like succulents) benefit from more perlite, while moisture-loving plants (like ferns) may need more peat moss. The calculator provides a solid starting point that you can customize based on your plants' requirements.

Formula

The mix is calculated using:

Each Component = Total Volume ÷ 3

Examples

• 30L total: 10L peat, 10L compost, 10L perlite (standard mix)
• 15L total: 5L peat, 5L compost, 5L perlite (small container)
• 60L total: 20L peat, 20L compost, 20L perlite (large raised bed)
• 10L total: 3.3L peat, 3.3L compost, 3.3L perlite (single large pot)

Understanding Soil Mix Components

• Peat Moss: Holds moisture, provides structure, slightly acidic (pH 3.5-4.5). Good for most plants but consider environmental impact - alternatives include coco coir
• Compost: Adds nutrients (N-P-K), beneficial microorganisms, improves soil structure. Use well-aged compost to avoid burning plants
• Perlite: Lightweight volcanic glass that improves drainage and aeration. Prevents soil compaction and allows roots to breathe
• Alternative Mixes: For succulents, use 1/3 potting soil, 1/3 perlite, 1/3 sand. For moisture-loving plants, increase peat moss to 40-50%
• pH Considerations: Most plants prefer slightly acidic to neutral soil (pH 6.0-7.0). Peat moss is acidic, so balance with compost

Mixing & Using Your Soil Mix

• Mix Thoroughly: Combine all components in a large container or wheelbarrow, mixing until uniform
• Moisten Before Use: Lightly moisten peat moss before mixing - it repels water when completely dry
• Store Properly: Keep unused mix in a covered container to prevent it from drying out or getting contaminated
• Refresh Annually: Replace or refresh potting soil annually as nutrients deplete and soil compacts
• Drainage Layer: Don't add gravel or rocks at bottom - this actually reduces drainage. Use well-draining mix throughout
• Container Size: Use appropriate container size - too large holds excess moisture, too small dries out quickly
• Test Drainage: After mixing, test that water drains freely - if it pools, add more perlite

FAQ

• What is a good soil mix ratio?
  A common and effective ratio is 1/3 peat moss, 1/3 compost, and 1/3 perlite. This provides excellent drainage, good moisture retention, and essential nutrients. The balanced mix works well for most houseplants, vegetables, and container gardens. However, you can adjust ratios based on your plants' specific needs - succulents need more drainage (more perlite), while ferns prefer more moisture retention (more peat moss).
• Can I use different ratios?
  Yes, absolutely! Adjust ratios based on your plants' needs: For succulents and cacti, use 1/3 potting soil, 1/3 perlite, 1/3 sand. For moisture-loving plants (ferns, calatheas), use 40-50% peat moss, 30% compost, 20-30% perlite. For plants needing excellent drainage, increase perlite to 40-50%. For seed starting, use finer mix with more peat moss. The 1/3:1/3:1/3 ratio is a great starting point that works for most plants.
• Can I substitute coco coir for peat moss?
  Yes, coco coir is an excellent, more sustainable alternative to peat moss. Use it in the same 1/3 ratio. Coco coir has similar moisture retention properties, is pH neutral (unlike acidic peat), and is renewable. It may require more frequent watering as it dries out slightly faster than peat. Mix coco coir with water before using, as it comes compressed and needs to expand.
• What type of compost should I use?
  Use well-aged, finished compost that's dark, crumbly, and smells earthy (not sour or ammonia-like). Homemade compost is excellent if properly aged (6-12 months). Commercial compost works well too. Avoid fresh compost as it can burn plants and may contain pathogens. Vermicompost (worm castings) is excellent and nutrient-rich. You can also use a combination of compost types. Ensure compost is free of weed seeds and pathogens.
• How much soil mix do I need for my container?
  For containers, fill to about 2-3cm below the rim to allow for watering. Calculate volume: For round containers, use π × radius² × height. For rectangular containers, use length × width × height. Add 10-20% extra for settling. A 30cm diameter, 25cm deep pot needs approximately 15-18L of mix. It's better to mix a bit extra than to run short mid-planting.
• Do I need to add fertilizer to the soil mix?
  Compost provides some nutrients, but you may need additional fertilizer depending on your plants. For most houseplants, the compost in the mix provides nutrients for 2-3 months. After that, supplement with liquid fertilizer or slow-release granules. Heavy feeders (tomatoes, flowering plants) may need fertilizer from the start. Always follow fertilizer instructions and avoid over-fertilizing, which can burn roots.