Fertilizers are supplemental minerals and nutrients given to plants to stimulate growth and development. Before applying any fertilizer, acquiring a basic understanding of where and how the plants you have grow best is a good idea. After setting them appropriately for sun and drainage, perform a soil test to determine how to adjust your plant's environment to its individual requirements. Even if you know that azaleas need an acidic soil, you won't truly understand how to supply that unless you know how much acid is already in your soil. Test, and then feed.
Azaleas and Rhododendron
Because azaleas and rhododendrons do best in acidic soil with a pH range from 4.5 to 6.0, older plants often do well without supplemental fertilizer, relying instead upon the natural decay of organic matter, which increase acidity. In fact, some older plants may be plagued by too much organic decay elevating the acid level. Test the soil. If the soil shows an elevated pH range, mulch with pine bark, which can naturally lower acid. For newer plants, fertilize with an acid-rich azalea/rhododendron food, available at most nurseries. More specifically, add ground sulfur or ferrous sulfate, to increase soil acidity. Treat in the spring or fall and use slow-release, granular formulations.
A common misconception about perennial flowers is that once they're established in a garden, they are relatively carefree. While ever-blooming perennial gardens can indeed offer years of beauty, they still require regular maintenance such as weeding and feeding. Delphiniums, especially, require feeding. Treat your delphiniums with an all-purpose granular fertilizer upon planting and then again in the summer after they have been cut back.
How you treat your lawn depends on the type of grass and your climate zone. However, there are a variety of general all-purpose products on the market that tackle lawns with an all-in-one approach that mixes fertilizers with weed control. As a general rule, feed your lawn four times a year with a 30-10-10 mix of nitrogen, phosphorus and potassium.
Fruit-bearing trees have their own fertilizing schedules and needs. As long as you've purchased a healthy tree, chosen your site well and followed planting instructions, your apple tree will get most of what it needs from the soil. If the soil pH is above 7.8, feeding annually with nitrogen should be enough to keep your tree healthy. For a new (under 4 years) planting, fertilize monthly in the spring. Once fully established--unless soil testing indicates necessary adjustments--give your apple tree a single dose of nitrogen in the form of ammonium sulfate each spring.