The forest, with the help of the animals that inhabit it, fertilizes itself. Animal droppings and leaves fall to the earth and decompose to release valuable nutrients into the soil.

In most American gardens, we tend to rake up the leaves along with pet droppings and discard them. A clean yard is the result, but not necessarily a nutritious one. Not unless we add fertilizer.

A compost pile is a good way to convert biodegradable waste into a nutritious meal for your plants and shrubs. But, this may not be practical if you don't have sufficient room or if you don't have the time. If this is the case, you need to know a little bit about packaged fertilizers.

There are sixteen nutrients that your garden needs for growth, three of which are primary nitrogen, phosphorus and potassium (potash). The soil provides most of the nutrients, but not all.

As you amble down the fertilizer aisle in your local garden center, you will see that most of the containers are marked with three large numbers separated by hyphens such as "16-10-4" or "21-0-0." These numbers indicate the amount by volume of each of the primary nutrients and always in the exact same order nitrogen, phosphorous and potassium. In the first example above, the fertilizer contains sixteen-percent nitrogen (N), ten percent phosphorous (P) and four percent potassium (K).

Fertilizer can be purchased dry or in liquid form. Dry fertilizers are spread on the ground and liquids can be sprayed onto the ground or onto the leaves. Liquids sprayed directly onto the leaves of a plant will give an instant boost and the results can be seen very quickly. Unfortunately, liquids will leach through the soil more quickly than granules and therefore don't usually last very long.

It is easy to remember how the three primary nutrients can affect the plants and lawn in your garden. All you need to remember is UP, DOWN and ALL AROUND. Nitrogen up, phosphate down and potassium all around.

The first number, nitrogen (UP), promotes greening and blade and leaf development. The second number, phosphorous (DOWN), is used to develop root growth and structure. And the third number potassium (ALL AROUND), affects the all-around health, hardiness and strength of the plant.

Plants need to have food available when they are growing or blossoming. Shrubs and trees grow most vigorously from mid-winter to early spring. Adding fertilizer a month before this growth cycle normally occurs will ensure a good food supply and a healthy growth cycle.

On the other hand, roses like to be fed all spring and summer because it helps them blossom. Some types of lawns need quarterly shots of nitrogen to stay green (winter and summer in warmer climates) and azaleas and other acid lovers are fertilized once or twice a year, depending on how often you want them to blossom.

Feeding stimulates growth, so fertilizing late in the summer in colder climates can result in new growth being frost-damaged.

As opposed to late summer feeding, which can be detrimental, feeding just before the first frost actually can be beneficial. Once it has gotten cold enough, plants will stop growing above ground even when fertilized. But, feeding during this period will allow the roots to absorb plenty of nutrients for the spring growth period. Isn't nature interesting especially when we help it a little!