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.
On the average, 90 percent of all soil tests in Oklahoma County are either adequate or high when it comes to phosphorus. Thirty percent have levels higher than 500. These high levels are rapidly contributing to pollution in our streams and rivers.
A soil test also checks the pH level of your soil. The ideal pH level for most plants is 6.5 to 6.8. If a test determines that there is too much acid in your soil, this can also mean aluminum is available to plants, which can be toxic. To help remedy this, you will probably need to add lime to help raise the pH level.
If you have a high pH alkaline soil, this may mean that some of the nutrients your plants will need to grow, like iron, may not be available. A high pH soil can usually be corrected by adding sulfur to the soil.
If you would like to have your soil tested, dig down to a depth of about six inches of dirt and take several samples (12 to 15) in the general area you want tested. Mix this together in a clean bucket or pail and bring in about two cups to our offices at 930 N. Portland in Oklahoma City. We will send the sample to the Soil Testing Laboratory at Oklahoma State University. The report you get back will tell you what you need to apply to your soil to make it the most fertile. The cost for each sample is $10 and the results of the test will be mailed in about two weeks.
No matter how much work you put into your lawn or how many expensive fertilizers you apply, a beautiful lawn must start with the right amounts of nutrients and the correct pH level. We hope you will decide to test your soil before you begin these applications.