Blend a soilless mix by ratio and see its bulk density, total porosity, water-holding capacity and air-filled porosity. Add a container size to see the dry weight and the water it holds at capacity. Peat, perlite and the rest come in coarse and fine grades, so treat these numbers as a close guide, not a lab result.
Pick what you're growing and your pot size. It gives a potting mix recipe and how much of each thing to buy.
About this tool: what makes a good potting mixRelated Tools
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Open the calculatorHow many teaspoons or grams of fertilizer to add, from its N-P-K and your container size.
Open the calculatorEnter your water test results to see whether the water suits irrigation and how to treat it.
Open the analyzerEvery 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. NC State puts the range for container mixes at 10 to 30 percent, measured in a standard 3 in test cell; below 10, roots drown and it looks like drought. Shallow containers need a coarser mix. Component figures are NC State porometer readings where published (sources).
| 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. Fine grades hold more water; coarse grades drain more than this tool shows |
| 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 5.5 to 6.0, about 8 to 10 lb dolomitic per cubic yard for peat mixes and 5 to 6 for bark mixes. 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.