Fertiliser is not food in the way a meal is food. A plant makes its own sugars from light. Fertiliser supplies mineral nutrients, mainly nitrogen (N), phosphorus (P) and potassium (K), that the plant needs to build new leaves, roots and flowers. Potting mix holds only a limited store, and some of it washes out each time you water.
The three numbers on a label, such as 15-13-20, tell you the share of those three nutrients by weight, always in the order N-P-K. For most houseplants, extension services suggest a balanced product where the three numbers are not wildly different.
The key idea is that a plant only uses nutrients while it is actively growing. New leaves, new roots or flower buds are your signal. A plant sitting still in a dim corner, or resting through a cool, dark winter, has little use for extra feed, and the unused minerals simply build up in the pot.
So fertiliser helps a plant that already has enough light and water. It cannot fix a plant that is struggling for those.
Key points
- Fertiliser supplies minerals, not energy; light provides the energy.
- The label numbers are N-P-K percentages by weight.
- Feed only when the plant is visibly growing.
Try this
Find any fertiliser you own and read the three numbers on the label. Write down which is nitrogen, which is phosphorus and which is potassium.
Check yourself
Read more in the reference
Sources
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