Potting-Mix Calculator

Why mix your own potting mix?

Bagged potting mix is convenient, but mixing your own costs less per pot, lets you control the ingredients, and avoids the peat-heavy, fine-textured mixes that stay soggy in the pots of plants that hate wet feet. A homemade mix is also easy to adjust: add more perlite for plants that need sharp drainage, more coir for plants that like steady moisture, and more compost for heavy feeders. The trade-off is that you have to buy and store the ingredients separately, and you need to mix them properly — a poorly mixed batch can leave one pot full of perlite and the next full of peat. Mix in a large tub or wheelbarrow, measure by volume (a scoop, a jug or a bucket — the unit does not matter as long as it is consistent), and moisten the peat or coir first so it blends instead of clumping. Always wear a mask when handling dry peat, coir and perlite: the dust is unpleasant to breathe.

What each ingredient does

Peat moss and coco coir are the moisture-holding base. Peat holds water well and is slightly acidic, which suits most plants, but it is slow to re-wet once it dries out and its harvest raises sustainability concerns, so coir — a by-product of coconut processing — is a popular alternative. Perlite is expanded volcanic glass: light, sterile and full of air pockets, it keeps the mix open and draining. Vermiculite holds more water than perlite and suits seed-starting mixes. Compost adds nutrients and microbial life, but it must be finished and sieved — unfinished compost can rob nitrogen and may contain weed seeds. Coarse grit, pumice or fine bark add weight and drainage for succulents and cacti, which are the plants most often killed by a mix that holds too much water. Garden soil does not belong in potting mix: it compacts in containers, drains badly and can carry pests and disease.

Which recipe for which plant?

The general 2:1:1 mix — two parts peat or coir, one part perlite, one part compost — suits most houseplants, herbs and container vegetables. It holds moisture without staying soggy, and the compost gives young plants a gentle feed for the first few weeks. The cactus and succulent mix swaps much of the moisture-holding material for grit and pumice, because these plants store water in their leaves and stems and rot quickly in wet soil. The seed-starting mix is deliberately low in nutrients and very fine in texture, so seeds sit in close contact with damp material and germinate evenly; seedlings move into the general mix once they have their first true leaves. Tropical plants that like constant moisture, such as ferns and calatheas, do well with extra coir and a little less perlite, while orchids and aroids often prefer a chunky mix of bark, perlite and a little coir instead of a soil-like blend at all.

How much to make, and how to store it

Make only what you need for the job in hand — mixed potting mix does not improve with storage, and damp mix in a sealed bag can go sour. Dry ingredients keep well in their own bags in a dry shed: coir bricks, perlite and grit last almost indefinitely. Mixed dry ingredients keep for months in a lidded tub. Once you have potted up, the plant’s next question is usually water and pot size: use the watering calculator to work out how much to water, and the repotting calculator when the plant outgrows its pot. For how much soil a raised bed needs, see the raised-bed soil calculator and the soil volume calculator, and browse the houseplant care guides for the plant you are potting.