Blend a soilless media by ratio and get its bulk density, total porosity, water-holding capacity, and air-filled porosity. Add a container size to see dry media weight and water held at capacity. Note that these products are sold in varying grades- coarse, fine, etc. This tool is for generalization, not forming research-grade mixes. For now, anyway.
About this tool: what makes a good potting mixEvery pot holds a saturated layer at its base, the perched water table, the same depth whatever the pot size, so a short wide container is wetter at the bottom than a tall narrow one. Gravel makes it worse; extra holes do nothing.
The air left after draining. Most crops want 10 to 20 percent; below 10, roots drown and it looks like drought. Shallow containers need a coarser mix.
| Component | Contributes | Watch out for |
|---|---|---|
| Sphagnum peat | Water holding, light weight, low cost | pH 3.5 to 4.5, so it needs lime. Hard to rewet once dry |
| Coco coir | Water holding, rewets easily, renewable | Can carry high sodium and potassium. Low in calcium |
| Perlite | Air space and drainage, inert, very light | Dust is a respiratory hazard, so wet it before mixing |
| Vermiculite | Water and nutrient holding (real CEC) | Compresses over time and loses air space |
| Composted bark | Air space with structure that lasts | Ties up nitrogen as it continues to break down |
| Compost | Nutrients and biology | Variable and often salty. Keep it to 10 to 20 percent |
| Coarse sand | Weight and ballast for top-heavy plants | In small amounts it fills pores and worsens drainage |
Lime peat mixes to pH 5.8 to 6.2, about 5 to 10 lb dolomitic per cubic yard. Coco needs none, but buy it buffered or feed extra calcium early.
The starter charge in a bagged mix runs out in weeks. Home-made mixes stall after a month and the medium gets blamed instead of the feed.
Wet it as you mix, since dry peat sheds water. Expect 10 to 20 percent shrinkage. One cubic yard is 27 cubic feet.