Why growing-media ratios are calculated by volume
A volcanic component is used to change pore distribution, water movement and air space within the root-zone volume. Blend ratios should therefore be calculated by volume. Weight becomes relevant for logistics only after the bulk density of the actual grade and batch is known. A generic cubic-metre-to-tonne conversion can be inaccurate because particle size, moisture, grading and packing method affect bulk density.
Core formulas
| Project type | Formula | Result |
|---|---|---|
| Rectangular bed | Length m × width m × depth m | m³ of root-zone profile |
| Open area | Area m² × treatment depth m | m³ of amended profile |
| Circular container | 3.1416 × radius² × height | m³ when dimensions are metres |
| Rated pot capacity | Pot count × litres per pot ÷ 1,000 | m³ of total mix |
| Component volume | Total mix m³ × percentage ÷ 100 | m³ of component |
Example 1: rectangular growing bed
A bed is 12 m long, 1.5 m wide and 0.35 m deep. Total volume = 12 × 1.5 × 0.35 = 6.30 m³. If the approved mix contains 30% volcanic tuff, required tuff = 6.30 × 0.30 = 1.89 m³. The remaining 4.41 m³ is allocated to other components.
Example 2: landscape area
An 800 m² area will be amended to a depth of 15 cm. Convert depth to metres: 0.15 m. Profile volume = 800 × 0.15 = 120 m³. At a 25% volcanic component, requirement = 30 m³. This is a component within the amended profile, not a replacement of all existing soil.
Example 3: pots and planters
For 250 pots rated at 30 litres, total mix = 7,500 litres = 7.5 m³. A 20% inclusion rate for 4–8 mm material requires 1.5 m³. Verify actual fill volume because containers are not always filled to their nominal rim capacity.
Example 4: top dressing
A 100 m² bed with a 5 cm surface layer requires 100 × 0.05 = 5 m³. This is a separate top-dressing calculation, not a root-zone blend ratio. Subtract uncovered areas before ordering.
Operating allowance
The allowance should reflect survey accuracy, depth variation, handling loss, blending and leveling. Keep four figures separate: net design volume, component percentage, documented allowance and final commercial quantity. This prevents duplicated allowances and makes the quotation auditable.
