A fertilizer label carries three numbers, and the whole skill of feeding a pot is knowing which of those three the plant in front of you is short of. Each element does a different job, they come from different materials, and the symptoms of excess and shortage look nothing alike between the three.
Nitrogen: leaf growth, and the price of pushing it
Nitrogen is the growth element. It drives shoot development, builds chlorophyll, and is what makes a plant look deep green and busy. The organic sources are the ones a gardener can actually lay hands on: well-rotted animal manures, fish waste, and the seed cakes left after oil pressing, meaning bean cake, rapeseed cake, cottonseed cake and sesame residue. On the mineral side it is urea, ammonia and the ammonium salts, which act fast, are normally used as a top-dressing, and are also the reason repeated feeding leaves a pot compacted and sour-smelling.
Too much nitrogen is easy to see and easy to cause. Tissue goes soft, the internodes stretch, the plant grows tall and pale, and both pests and disease get a foothold far more easily. Hardiness drops along with it, which is why the heavily nitrogened plant is the one that dies outside in February. The shortage looks different again: a plant that stays short, yellows from the older leaves upward, grows slowly and shows no interest at all in flowering.
Phosphorus: buds, roots and ripening
Phosphorus is the element behind flowering and fruit set. It firms up stem growth, is needed for a flower bud to form in the first place, and speeds ripening. Bone meal, rice bran, fish scales and poultry manure are the organic materials worth remembering for it; phosphate rock, superphosphate and calcium-magnesium phosphate are the mineral equivalents.
A phosphorus shortage shows later and less dramatically than a nitrogen one. Growth slows, the flowers come small, and whatever fruit sets is small and late. This is also the element people under-use when what they want is a display of bloom, reaching instead for a high-nitrogen liquid and paying for a leafier plant that flowers less.
Potassium: the resistance element
Potassium builds stem strength, root mass, and the plant’s general ability to withstand drought, cold, disease and physical abuse. It improves fruit quality as well. Plant ash is the traditional organic source and can go in as either a base or a top-dressing; potassium nitrate and monopotassium phosphate are the soluble ones, used on pot plants as a spray at a strength the foliage can survive.
Deficiency has a signature look. Dead spots appear along the leaf margin, spread until the edge scorches, and leave the centre of the leaf green and apparently healthy. Excess is rarer in an ordinary garden but it exists, and when it happens the plant dwarfs, the joints shorten and the colour yellows.
Reading the three together
The practical value of all this is that a plant tells you which element it wants, and the diagnosis is worth doing before the feeding. A leggy green plant that refuses to bloom is not underfed. A plant with scorched leaf edges does not need more of the same fertilizer it is already getting. The ratio matters more than the quantity, and a feed applied at the wrong moment in the season is a reliable way to spend money on a worse plant.
How often to apply, and what to do in the first weeks after a repot, is covered at feeding potted plants. The two families of fertilizer and how they behave in a pot are compared in organic versus inorganic fertilizer, and the leaf-by-leaf version of this diagnosis is at reading a yellow leaf.