Without potassium, the edges of a plant's leaves will begin to turn brown. Here's an easy memory aid. These three elements are known as macronutrients, along with calcium, sulfur and magnesium. Plants with little calcium develop young leaves with hooked ends. If they're lacking in magnesium, the leaves turn yellow with green veins.
Nothing can beat a soil test for accuracy.
You can pick up a kit at your local Co-Operative Extension Service. It will explain how to properly dig the sample and how to package it for mailing. In four to six weeks, you will receive a very accurate analysis of your soil. That's because most Piedmont clay soil has a pH between 5.0 and 6.0, while a pH of about 6.5 is more ideal for general growing conditions. Fertilizers work best at this higher pH level.