Our lawns and yards are greening up before our eyes these days.

The plants are using food stored in their roots over winter to push out all that new growth. Soon, their expanding leaves will begin the process of photosynthesis, where plants convert sunlight, carbon dioxide and water into carbohydrates to use as energy.

Through photosynthesis, plants are able to make their own food. Plants thrived on this planet long before humans showed up and developed fertilizer. Keep this in mind as you make decisions about if and when to use fertilizer in your yard or garden.

Trees and shrubs do fine without any additional fertilizer from us. But when we want to push production, such as in producing lots of flowers, big vegetables or a lush lawn, some fertilizer can be helpful.

When used at the proper rates at the appropriate times, fertilizer is a useful tool. But when used in excess, too much at a time or at the wrong time, fertilizer can run off into ground water or surface water and have a negative impact on our fresh water resources.

Before you even begin thinking about adding fertilizer, take a good look at your soil and site conditions. Nothing is more important to the success of your plants than good quality soil. Adding fertilizer does not make up for lousy soil!

It also will not make up for not matching your plants to the site, such as make a sun-loving plant happy in a shady site. But it can enhance vigor and production for plants that are reasonably well sited.

What's in fertilizer?

All fertilizers have a series of three numbers somewhere on their labels, indicating the percentage of each nutrient it contains. Some familiar numbers are 33-3-3 for lawn fertilizer, 10-10-10 for vegetable garden fertilizer, and 3-4-2 for some of the organic based fertilizers. The actual numbers will vary with the source; these are just some examples.

The first number is nitrogen (N), the second is phosphorus (P) and the third is potassium (K), the three nutrients plants need the most of. So a product labeled 33-3-3 contains 33 percent N, 3 percent P and 3 percent K. Some products may have minor nutrients added as well and many of the lawn products have weed killer or insecticide added so read the label carefully to be sure you're getting what you want.

Excess N becomes a problem because it goes into solution and moves into the ground water. In severe situations it can affect drinking water quality. Excess P moves with surface water, running into rivers and lakes and promotes algae blooms there. So to help protect our fresh water resources, use these nutrients wisely. Potassium (K) is not known to cause this kind of problem.

How to choose

Chances are when you visit the fertilizer aisle in your favorite store you'll be overwhelmed by all the choices. Start by considering the needs of the plant you are feeding. Why do you want to fertilize them?

Flower and vegetable gardens need strong plants that produce lots of flowers (which may then turn into vegetables such as tomatoes or squash). Lawns produce lots of green leaves, as do the leafy vegetables and corn.

Nitrogen (N) is always the first number in the series. It promotes green tissue, so it's good for lawns and leafy vegetables. Phosphorus (P) comes second and it enhances flower and fruit production and helps roots get established. Potassium (K) is last and it is good for the overall strength and vigor of the plant.