The first step is to gather a soil sample. That is a fairly simple process. Before you add anything to your soil, take several scoops of soil from several places in the area you want tested. The larger the area the more samples you need. Start with six or so from a 1000 square foot garden. Don't take the top layer of soil, but soil from the bottom of a three or four inch deep hole. Remove sticks, weeds, grass blades and roots. Place a small amount of soil from each hole in a bag. Let the soil dry and take about two cups of the dried soil to your local University Extension Office. Tell them what you want to grow from the sample's space, such as vegetables, lawn or flowers. For a small fee you will get a soil report in the mail.
The test tells you what your soil needs for good plant growth, whether it is too acid or alkaline, how much organic matter the soil contains. The report also gives recommend amounts of minerals to add to the soil. It will not tell you how many bags of 10-6-4 fertilizer to spread on your new dug bed or what to put on your lawn.
The report will tell you if your soil needs lime or sulfur. Lime and sulfur affect the soil's acidity or alkalinity, which affects the plant roots' ability to absorb other necessary minerals from the soil. Lime and sulfur need time to work in the soil and can only be added in the fall or the spring.
If your vegetable or flower garden needs Nitrogen, wait until spring to apply bagged fertilizer. Nitrogen is water soluble, so anytime the soil is wet, Nitrogen dissolves away from where the plants need it. Some fertilizer bags have a slow-release type of Nitrogen, which slows this leaching process.
Now you have to figure out what to add and how much. Fertilizer bags have amounts of NPK written on them like 10-10-10, 10-6-4 or 20-0-0. The first number always represents Nitrogen, the second, Phosphorus, and the third Potassium. These numbers represent the percentage of each mineral in the bag. A 40-pound bag of 10-5-5 is 10 percent Nitrogen, or 4 pounds of Nitrogen; 5 percent of both Phosphorus and Potassium or 2 lbs each. The remaining 80 percent contains filler materials to help the fertilizer spread easily and if it is a complete fertilizer, other small amounts of minerals for plant needs.
If your soil report tells you to apply 4 pounds of Nitrogen to 1000 square feet, you will know one 40-pound bag of 10-5-5 will be the right amount for an area 50' x 20' or 10' x 100'. Depending on your report and the NPK numbers on your fertilizer bag, some basic math will help you figure out how much fertilizer to apply. When you know what to add to your soil and how much, find the product that best fills those needs.