Many homeowners, when faced with the task of fertilizing their lawn, go to the garden store and buy whatever popular brand is displayed--no questions asked. Is it 10-10-10? Or 10-6-4? Or 21-10-6? Who knows? Nevertheless, if you want to get the very best out of your fertilizer dollar, you ought to take the time to understand the meaning of the numbers that are printed on fertilizer sacks.
All complete fertilizers have such numbers. A complete fertilizer is one that contains nitrogen, phosphorus and potash, and those three ingredients are represented by those numbers. For example, a 10-6-4 fertilizer contains 10% nitrogen, 6% phosphorus and 4% potash. If your bag of 10-6-4 weighs 50 pounds, you have five pounds of nitrogen. So if your lawn expert tells you to apply nitrogen at the rate of five pounds per 1,000 square feet, you will need two bags to cover your 2,000-square-foot lawn.
Also, some grasses--Bermuda and zoysia for example--require more nitrogen than others. For them, you might consider using 46-0-0, or urea, which is all nitrogen. But be careful; if you don't water it in--thoroughly--immediately after application, it will burn the grass leaves to a dead brown.
Agronomists know that at least 17 elements are necessary for healthy plant growth: boron, calcium, carbon, chlorine, cobalt, copper, hydrogen, iron, manganese, magnesium, molybdenum, nitrogen, phosphorus, potassium, oxygen, sulfur and zinc. But of all those, the three most likely to be lacking in your lawn or garden--or in a farmer's field--are nitrogen, phosphorus and potassium, the last of which is represented in your fertilizer by potash.