It's a good idea for homeowners who are interested in fertilizing their lawns to begin the entire process with a soil test. A soil test can save residents money when they find out they don't need to spend extra for fertilizers they don't need. Soil tests also help protect our rivers and streams, since too many of us continue to buy fertilizers that contain phosphorus, even though most lawns already have too much. This excess phosphorus is carried off during rains and often ends up polluting our rivers and streams.
At the Oklahoma County OSU Extension Center, our soil test is a calibrated test, meaning it is a dip-stick for your soil. In addition to checking basic pH levels, this test checks for nitrogen (N), phosphorus (P) and potassium (K). For phosphorus, the full mark is 65. If you are still applying a fertilizer that contains phosphorus to a soil that is above a 65 index, it is the same as checking the oil in your vehicle and adding two quarts of oil even though your car is showing full.
The calibrated full mark for potassium is 250, but as a rule, our Oklahoma soils are naturally high in potassium. But nitrogen often tells a different story.
Think of nitrogen as “gas” for the engine that will grow your yard. If you were going to Amarillo, you would use your dipstick to check the P and K (phosphorus and potassium) levels. P and K would probably be fine (just like your oil usually is), but you would still have to fill up the gas tank a couple of times. Since we like to keep nitrogen at the 40 to 60 lbs. per acre mark during the growing season, some fast growing grasses such as Bermuda may need a nitrogen application monthly.
When you go to buy a fertilizer, you will find that it has three numbers, such as 46-0-0, or 10-20-10, on the package. These stand for the N P K (nitrogen, phosphorus and potassium) levels. N is always the first number, P the second and K the third.
A soil test can help homeowners choose the right fertilizer, since you will know if you need a fertilizer with just nitrogen (46-0-0) or one with complete nutrients (10-20-10). Since you usually pay a higher price for fertilizers with all three nutrients, you can see why buying a fertilizer with just nitrogen can save you money.