Simple Feeding Calculator

Two of the three numbers are not what they say

They are percentages by weight, so 10 lb of 10-10-10 holds 1 lb of each. But P is phosphate (actual phosphorus is 44% of it) and K is potash (actual potassium is 83%). A historical convention from reporting nutrients as oxides. It matters the moment you compare a label against a soil test.

Spoons are approximate, and usually that is fine

A spoon measures volume, a rate is a weight. Fine for houseplants; weigh it for a target ppm.

Underfeeding is recoverable, overfeeding often is not

Salts pull water out of roots: scorched edges, wilting in damp compost, white crust. For containers, weak and often.

Salts accumulate in pots

A pot is a closed system, so salts build up even when every dose was correct. Leach monthly: plain water until it runs freely out, and discard it.

Common errors

• Never feed a dry plant. Water first.
• Do not feed a sick plant back to health. Diagnose first.
• High nitrogen builds leaves, which is wrong for fruiting.
• Ease off in winter. Low light cannot use a summer schedule.