Container and bed soil: how much you actually need, and what to feed

Two questions get skipped and both cost real money and a season.

How much soil, exactly

Gallons → cubic feet: multiply by 0.1337. Cubic feet → bags: divide by the bag size, usually 1.5 ft³.

Setup	Volume	Bags of 1.5 ft³ mix
One 5-gal pepper pot	0.67 ft³	0.5
Seven 5-gal pots	4.7 ft³	4
One 10-gal tomato bag	1.34 ft³	1
Four 10-gal tomato bags	5.3 ft³	4
One 4×8×1 ft raised bed	32 ft³	21

That last row is the one that surprises people at checkout. Filling a raised bed entirely with bagged potting mix costs several hundred dollars. Almost everyone blends: bulk topsoil and compost for the lower two-thirds, quality mix in the top third where the roots actually are.

The Plan Builder does this arithmetic for whatever you select.

Minimum container sizes

These are working minimums, not comfortable ones.

Crop	Minimum	Better
Basil	3 gal	5 gal
Cilantro	3 gal	5 gal
Pepper (any)	5 gal	7 gal
Tomatillo	10 gal	15 gal
Tomato (determinate)	10 gal	15 gal
Tomato (indeterminate)	10 gal	20 gal
Carrot	5 gal, 10″+ deep	deeper
Onion, garlic	ground or bed	—

Undersize the pot and you do not get a smaller plant. You get a plant that dries out twice a day, runs out of nutrition in July, and drops fruit.

Why containers dry out so fast

A 5-gallon fabric pot in full July sun can need water daily, sometimes twice. Fabric drains and breathes brilliantly — which is exactly why it also loses water fast.

That matters beyond convenience, because moisture swings cause:

• Blossom end rot — calcium moves in the transpiration stream, so an interrupted stream means calcium never reaches the fruit tip.
• Split fruit — dry, then heavy water, and the interior swells faster than the skin grows.
• Flower drop — a plant under water stress aborts flowers first.

The fix is not a product. It is mulch on the surface and drip on a timer. Every container-growing problem on this list is downstream of watering by memory.

Feeding: the ratio matters more than the brand

The single most common feeding mistake is a high-nitrogen general-purpose fertiliser on a fruiting crop. Nitrogen builds leaves. You get an enormous, dark green, magnificent tomato plant with four tomatoes on it.

Want: low N, higher K. A 3-4-6 with calcium is a good general sauce-garden ratio.

Stage	Feed
Transplant → first flower	Charged mix does the work; nothing needed for ~30 days
First flower → first fruit	Low-N granular, monthly
Fruit set → end of season	Same, monthly. Peppers at about ⅔ the tomato rate

Blossom end rot, properly explained

Because this is the number-one container failure and the internet mostly gets it wrong.

BER is not a soil calcium deficiency in almost any real garden. Calcium is abundant in most soils and in every commercial potting mix. Calcium is also immobile in the plant — it moves only in the transpiration stream, dissolved in water travelling from root to leaf and fruit.

Interrupt that stream and the fastest-growing tissue — the blossom end of a young fruit — is starved first, and its cell walls collapse. That is the black leathery patch.

Therefore:

• Cause: inconsistent soil moisture. Full stop.
• Actual fix: mulch, and consistent watering, ideally automated.
• Foliar calcium: genuinely useful, but it protects the next fruit set. Fruit already marked will not recover.
• Adding lime or gypsum: does nothing unless a soil test shows a real deficiency, which is rare.

The evidence is in the pattern: the first flush is often lost, later fruit on the same plant is clean once the weather settles. Soil calcium did not change. Watering did.

In-ground: test before you amend

Container growers can skip soil testing; bagged mix is already in range. In-ground growers are guessing without it.

The number that matters most is pH. Outside roughly 6.0–6.8, nutrients get locked out chemically regardless of how much fertiliser you apply. Adding more fertiliser to soil at pH 5.2 is pouring money into a locked box. Lime raises pH, elemental sulphur lowers it, and both take months — so test in autumn and amend for next spring.