How to read the bag. On each bag of fertilizer, there is a ratio, such as 10-5-10 or 29-6-4. These numbers tell you the ratio of nutrients in the fertilizer. The first number denotes the amount of nitrogen in the fertilizer. A ten-pound bag of 10-5-10 fertilizer would contain 10%, or one pound, of nitrogen. The second number is the phosphorus ratio. Thus, the ten-pound bag of 10-5-10 would contain 5%, or eight ounces, of phosphorus. The last number is the potassium ratio. The same bag of fertilizer would contain 10%, or one pound, of potassium. This number is also sometimes called the NPK number. The remainder of the material in the fertilizer is trace minerals and inert filler.
What does nitrogen do? Nitrogen promotes the growth of foliage. A fertilizer with a higher ratio of nitrogen would be used when you want a lot of top-growth. This is the type of fertilizer you might apply to your lawn in spring. You wouldn't want to apply a high-nitrogen fertilizer in the fall, when the roots of your grass should be strong and deep in preparation for winter. Slow-release nitrogen is preferable to quick-release nitrogen for two reasons. The first reason is that there's less chance of burning your plants with slow-release nitrogen because the grains have been specially coated to break down slowly. The second reason is that, since it breaks down slowly, it feeds the soil, not the plants.
What does phosphorus do? Phosphorus promotes root growth. A fertilizer with a higher percentage of phosphorus would be applied to a vegetable garden in the spring, when you want your plants to get established.
What does potassium do? Potassium, also known as potash, encourages root development. It also encourages disease-resistance in plants, so many vegetable garden fertilizers are high in potash.
Liquid vs. granular form. Liquid fertilizer has to be applied more often, but it has one major advantage--you will see results quickly. A lawn of tired, brown grass can be revitalized in a matter of days. Liquid fertilizers are more expensive than granular forms, but if you need nutrients for your lawn or garden ASAP (or if you're just impatient), it can be worth the expense. Liquid fertilizer is normally applied with a hand-held sprayer, which may attach to your garden hose. Granular forms stay in the soil longer than liquids, so they need to be applied much less frequently. If you use a granular fertilizer, you'll need to apply it by hand or with a spreader. The simplest spreaders look like a small plastic wheelbarrow with a dispenser on the bottom of the tray that broadcasts the fertilizer over an area quickly. After applying the fertilizer, you'll need to water it in. All slow-release fertilizers are in granular form.

Balanced vs. specialty fertilizers. If you have average garden soil, with relatively healthy plants, a balanced fertilizer--one in which the amounts of nitrogen, phosphorus and potassium are relatively equal--should work just fine. On the other hand, if you have plants which can be difficult to grow, such as orchids or bonsai, you may want a fertilizer which is higher in some nutrients than in others. Specialty fertilizers may also have additives which can adjust a soil pH. For example, if your soil is too acidic, you may want a fertilizer with limestone, which makes your soil more alkaline. If you need to make your soil more acidic, look for a fertilizer which contains sulfur. If you want a more detailed analysis of your soil, you can have a soil test performed by your local extension service.
Organic vs. synthetic fertilizers. Organic fertilizers are always preferable to synthetic fertilizers when you are applying it to plants that you will use as food. The downside is that organic fertilizers, or soil additives, to be more accurate, do not always contain all the elements and minerals that your plant will need. Therefore, you may need to apply several different ones or apply them more frequently to compensate. Some organic soil additives that can be very valuable to your plants include dried blood meal, which is high in phosphorus, animal manure, which is great for conditioning your soil and adding nitrogen, and fish emulsion, which is very gentle on plants. The best soil additive, of course, is compost, which usually contains all of the minerals that your plants need.