Most gardeners should use a complete fertilizer with twice as much phosphorus as nitrogen or potassium. An example would be 10-20-10 or 12-24-12. These fertilizers usually are easy to find. Some soils contain enough potassium for good plant growth and don't need more.

In general, fertilize plants when you start to see new growth - leaves, buds, roots, etc. - and when planting transplants. Don't fertilize when plants are very dry or drought-stressed. Water the root area before fertilizing.

It's a building block for growing new stems and leaves, plus it is a necessary part of chlorophyll, which makes the leaves green and helps plants photosynthesize. Phosphorus (P) is needed for developing flowers, fruits, and root systems. Potassium (K) keeps roots healthy and also aids flowers and fruits.

Water to moisten the soil and distribute the fertilizer to a 12- to 18-inch depth. Apply in early spring or autumn when roots are actively growing.