Don't waste money giving plants more nutrient supplements than they can absorb.

The major elements are those listed on plant food labels as N (nitrogen), P (phosphorus), and K (potassium, available as potash or K2O). Many soils are naturally high or low in any of these, but nitrogen is the most water soluble, and therefore the most likely to leach away (and into ground-water supplies). Adding more nitrogen than plants can take up is a waste of money.

The elements plants use less of are called secondary elements. They are no less critical to healthy growth than the major elements; plants just require smaller doses of them. They are calcium (Ca), magnesium (Mg), and sulfur (S). Don't waste money by adding these to your soil on a yearly basis, because plants take them in slowly, and they don't leach away. A single application usually lasts several years.
Trace or micronutrients are those that plants require only small amounts of. They include boron (B), manganese (Mn), copper (Cu), zinc (Zn), iron (Fe), molybdenum (Mo), and chlorine (Cl). Short of a soil test, the best way to tell if you need to add any of these is by plant response.