In order to accurately test a soil sample for fertility, the first thing that you will have to do is find soil that is representative of the entire area. Also, don't test the soil near a fence. The sample shouldn't be deeper than 6". Scoop up some soil samples. You can use a trowel. Take samples from several areas. Set it aside for a few days, in order to let it dry. You can buy a testing kit, or you can mail the soil in for testing. If you mail them in, you'll get your results in approximately 2 weeks. They will send you a chart. This will tell you the deficiency, and it will give you an idea of the steps you need to take to improve its fertility.