Never spend money on any fertilizer or soil amendment for your lawn or garden without first consulting the results of a soil test.
These diagnostic results — available from virtually all Cooperative Extension offices across the U.S. — will tell you exactly how much N (nitrogen), P (phosphorus) or K (potassium), lime, sulphur or other nutrients to add. Too much nitrogen and phosphorus can harm oceans, lakes, rivers and drinking water. Other excess nutrients can weaken and even kill grass and other plants.
The bottom line, in other words, is to avoid guessing. That can be bad, for the environment, for your landscape and for your pocketbook.