Nitrogen (N), phosphorus (P) and potassium (K) are the “big three” in plant fertilizer, and the minerals that gardeners are most concerned about. Too little of any of the three and the plants will be weak-stemmed and poorly rooted. Too much and the gardener risks not only burning the roots of the plant but having the chemicals drain out of the soil and contaminate the area's water table or streams. Inexpensive test kits are available at any store that sells garden supplies. These test kits can give you a good idea of the soil condition, but for precise results the soil should be sent to your local county extension service for a professional test.
Purchase a soil test kit at the store. One with multiple testing strips is best, since the several areas of the garden should be tested over a period of time.
Collect soil samples from various areas. A difference of even a few feet may make a difference in the NPK ratios. Carefully note the location of each sample from the yard or garden.
Mix the soil sample with distilled water as per the instructions included with the kit.
Use a testing strip to measure the NPK available in the soil by dipping it in the soil sample that was mixed with the distilled water. Test kits are usually color coded for the minerals. For example, one kit uses a range of white to red for nitrogen, with little available nitrogen scoring white and substantial nitrogen presence scoring red. Pink notes an amount in the middle.
Test again several weeks after you have amended the soil with fertilizer or compost to ensure that the areas that were lacking are now at the proper levels.