There are 16 nutrients needed for plant growth and development. 13 of these nutrients are typically already in your soil. Three nutrients, however, need to be added to your soil to make sure you get the most out of your yard and garden. They are nitrogen, phosphorous, and potassium (N,P & K).
Fertilizers available at your local garden center will provide these essential elements. The key is knowing how much fertilizer to apply.
Most homeowners tend to estimate and end up putting down too much fertilizer. The excess fertilizer then leaches to the ground water and ends up in nearby ponds and creeks. Too much fertilizer can harm the natural habitat. Too many nutrients can also stress your plants, causing other growth issues.
The other side of the risk you take with estimating fertilizer is not applying enough. Not giving your lawn & garden enough nutrients risks slower or even unhealthy development.
A soil test gives you precise nutrient requirements for the soil type and plant type in your situation. Testing your soil will help you save money by purchasing only the fertilizer you need as well as providing your yard & garden the nutrients they need to thrive.
Most state cooperative extension offices (every county has one) will test your soil for free. Find your local Extension office.
There are also soil test kits available at most local garden centers ranging in price from $5 to $25.