Potting mix for vegetables: soil, compost, rice husk, coir, pot size in litres

Soil in a pot works harder than soil in a bed: the roots have limited room, and excess water has nowhere to go. Good container soil is light, airy and quick to drain while still holding enough water. A starting mix that suits most vegetables is equal parts loam, compost and an airy material, in a pot big enough for the crop.

In the last lesson we counted sun hours and chose crops. This lesson is about the roots’ home.

Why plain garden soil does poorly in a pot

Soil in a bed has earthworms, old root channels and deeper layers that carry water away. Scoop it into a pot and that structure is gone. After a few weeks of watering it slumps and packs down, water collects at the bottom, and the roots run short of air.

US university extension services agree: container vegetables need a light, well-draining mix, and plain topsoil or fill soil is not recommended because it is heavy and drains poorly. If you blend your own, the usual recipe splits it evenly between soil, organic matter and something that keeps it open.

What each local ingredient does

Material	Job	Note
Loam	Weight, anchors roots, holds nutrients	Sift out stones, clay lumps and grass roots
Compost	Organic matter, nutrients, water holding	Only finished compost: cool, no smell
Charred rice husk	Adds air space, drainage	Raises pH slightly
Raw rice husk	Lightens and opens the mix	Mix it through; do not leave a layer on top
Coco coir	Holds water, keeps the mix crumbly	Soak compressed blocks until they swell before mixing
Composted manure	Nutrients	Never use fresh manure

A Brazilian study that blended carbonized rice hulls into a growing medium found they increased air space, reduced the water left after draining, and raised the pH of an acidic material (from 4.2 to 5.3 at the ratio tested). More on pH in the next lesson.

You can make compost from kitchen scraps; see Composting kitchen scraps at home.

The mix: start at 1:1:1, then adjust

Starting recipe, by volume (measure with the same bucket or tin):

• 1 part loam
• 1 part compost
• 1 part airy material, for example half charred rice husk and half coir

Adjust as you go. Check the soil after watering: if water soaks in slowly and puddles on top, the mix is too dense. If it is bone dry within half a day, it is too open.

Situation	Adjustment
Rainy season, rain falls on the pots often	More charred husk, less coir
Hot season, sunny balcony, pots dry fast	More coir or compost
Your garden soil is clay	Cut soil to half a part, add more airy material
Hanging pots, or a balcony with a weight limit	Less soil, more coir and compost to lighten it

Mix it lightly damp before filling the pots. Dry ingredients do not blend, the dust flies, and the first watering makes the soil sink a lot.

Drainage holes beat any recipe

However good the mix, a pot without holes will hold water at the bottom until the roots rot.

• Every pot needs holes. If you reuse a bucket or tub, drill several holes in the base, or along the side right at the bottom, no more than about 1 cm up.
• Raise pots on bricks or wooden slats so water can run out instead of sitting under the base.
• If you use a saucer, tip out the water after each watering. Standing water is also a place for mosquitoes to breed.

Pot size: how many litres each crop needs

The figures come from Texas A&M AgriLife Extension’s guide to container vegetables, converted from US gallons to litres (1 gallon ≈ 3.8 litres) and rounded.

Crop	Minimum size	Plants per pot
Lettuce (leafy greens)	About 4 litres	2
Spring onions	About 4 litres	3-5
Small herbs such as coriander	Pot 15-25 cm across	Several
Small radishes	About 4 litres	3
Beans (yard-long, green)	At least about 8 litres	About 8 cm apart
Large cabbage-family crops, e.g. broccoli	About 8 litres	1
Chillies / peppers	About 19 litres	1-2
Eggplant, tomatoes	About 19 litres	1

A bigger pot does no harm: more soil dries out more slowly. The downside is weight and moving it around.

Worked example: soil for two chilli pots

Say you have two cylindrical pots, 30 cm across and 30 cm deep.

1. Radius = 30 ÷ 2 = 15 cm
2. Volume = 3.14 × 15 × 15 × 30 = 21,195 cubic centimetres
3. 1,000 cubic centimetres = 1 litre, so about 21 litres per pot: more than 19 litres, fine for one chilli plant each
4. Two pots = 42 litres; add about 15% for settling = about 48 litres
5. At 1:1:1 that is 16 litres loam, 16 litres compost and 16 litres airy material (8 litres charred husk, 8 litres coir)

A pot that is wider at the rim than at the base holds less than the cylinder you calculate from the rim. An easier way is to fill it with water using a scoop of known size and count the scoops.

Common mistakes

• Pots too small for fruiting crops. A chilli in a 3-4 litre pot dries out twice a day in the hot season; the plant stays stunted and fruits poorly.
• No drainage hole, or a blocked one. Check the base every time you move the pot.
• Fresh manure or compost that is still hot. It damages roots and can carry germs onto vegetables you eat raw.
• Fill soil from a building site. It often contains cement and stones, packs hard, and you do not know where it came from.
• Filling to the brim. Leave 2-3 cm below the rim as a basin for water while you pour, so it does not run off.

Sources

• Vegetable Gardening in Containers (E-545) — Texas A&M AgriLife Extension
• No yard? Grow vegetables in containers — Oregon State University Extension
• Use of carbonized rice hulls as a horticultural amendment for an organic soil — Pesquisa Agropecuária Brasileira (Embrapa)
• Self-Watering Wicking Container (AEN-157) — University of Kentucky Extension