This page has the numbers for potatoes: seed potatoes planted 4 inches deep and 12 inches apart with 30 inches between rows, two weeks before your last frost, an inch and a half of water a week, and 2 to 5 pounds per plant over a 70 to 120 day season.
One figure sets potatoes apart from everything else in the bed: they want soil at pH 5.0 to 6.5, more acid than any other crop here, because scab gets worse as pH rises. The page covers that, the earthing up that keeps tubers from greening, and why certified seed potatoes are worth the money.
Seed potatoes
Buy certified seed potatoes. They cost a little more than eating potatoes and they are tested for the virus and blight problems that, once introduced to a bed, are very hard to get rid of. Supermarket potatoes are frequently treated to stop them sprouting anyway.
Chitting — standing them in a cool, bright place for a few weeks until stubby green shoots appear — gives a modest head start. Cool and bright is the point; a warm dark cupboard produces long pale shoots that snap off when you plant.
Large seed potatoes can be cut into pieces with at least two eyes each, left a day or two for the cut faces to dry, then planted.
Planting
Plant about two weeks before your last frost date, as soon as the soil is workable. Potatoes are not frost-hardy above ground, but they take a while to emerge, and a light frost on early foliage sets them back without killing them.
Set the tubers four inches deep, twelve inches apart, with 30 inches between rows. That row spacing is not padding: the plants get large and need the room, and you need somewhere to draw soil from when earthing up.
Do not lime the bed
Potatoes prefer soil at pH 5.0 to 6.5, more acid than most vegetables want, and scab gets noticeably worse as pH climbs towards neutral. If a bed is in rotation, apply any lime in the year it grows brassicas rather than the year it grows potatoes.
On feeding: moderate nitrogen and generous potassium. Too much nitrogen gives magnificent foliage and modest tubers. Fresh manure is best avoided, as it encourages scab.
Earthing up
As the shoots reach eight inches or so, draw soil up around the stems to make a ridge, leaving the top few inches of foliage showing. Repeat once or twice more over the following weeks.
This does two jobs. It keeps developing tubers covered, which prevents them greening in the light, and it gives the plant more buried stem to form tubers along. A thick straw mulch does the same job if drawing soil is impractical.
Water
An inch and a half a week, and it matters most from flowering onwards, when the tubers are actually forming. Inconsistent watering at that stage produces knobbly, cracked potatoes and hollow centres. Mulch helps considerably.
Ease off as the foliage begins to die back, which is the plant finishing rather than a problem.
Harvest
For new potatoes, start feeling into the ridge around flowering time and take a few from each plant without disturbing the rest.
For storage, wait until the foliage has yellowed and collapsed, then leave the tubers in the ground for another two weeks so their skins toughen. Lift on a dry day, let them dry on the surface for a few hours, and store in the dark somewhere cool. Any tuber with a spade wound should be eaten rather than stored.
Common trouble
Colorado potato beetle where it occurs: hand-pick the adults and scrape off the orange egg clusters under the leaves.
Late blight turns foliage brown and collapses a crop quickly in warm humid weather. Cutting and removing the foliage at the first sign protects the tubers below.
Hollow heart and cracking are water problems, not diseases.