Feeding Without Guessing

The bag says plant food, but a plant makes its own food: the sugar from photosynthesis, paid for by light, as in Light Is the Budget. What a fertiliser sells is building materials: mineral nutrients such as nitrogen, phosphorus and potassium that the plant needs to turn sugar into leaves, roots and fruit. Roots take them up dissolved in water.

Think of a builder with plenty of money but a short supply of one material. If there's no cement, more bricks won't raise the wall any faster. Growth is held back by whichever input is scarcest, whether that's a nutrient, water or light. Adding more of something that isn't short changes nothing, and adding too much can harm the roots.