If you think back to high school chemistry class, you may remember that N stands for nitrogen, P for phosphorus, and K for potassium. The three numbers commonly found on fertilizer bags refer to the relative amounts of these three nutrients, in this order. So a 20-5-10 fertilizer contains 20 percent nitrogen, 5 percent phosphate (phosphorus), and 10 percent potash (potassium). A complete fertilizer contains all three of these nutrients.
Each nutrient plays a specific role in overall plant health:
Most lawn fertilizers are relatively high in nitrogen; for example, the N-P-K analysis might be 30-3-3. After all, you want to promote that leafy growth! Phosphorus and potassium are also important, but they're needed in smaller quantities.
What would happen if you applied this same high-nitrogen lawn fertilizer to your tomato plants? You'd likely get lots of foliage but few tomatoes, since the plants would use the bonanza of nitrogen to grow lots of leaves. Fertilizers formulated for vegetable and flower gardens contain a more balanced ratio of the nutrients, such as 10-10-10 or 12-4-8.
Like humans, plants need a wide range of nutrients for proper growth. Nitrogen, phosphorus, and potassium are the big three, but plants also need many other nutrients, albeit in much smaller amounts. These are called micronutrients, and they include copper, boron, manganese, and iron. Although most soils contain adequate amounts of many micronutrients, some fertilizers include small amounts of these elements just in case the soil is deficient.
The best way to determine what your lawn needs is to test the soil. You can purchase a soil test kit or contact your local Cooperative Extension office for soil-testing information. The Extension office will provide you with not only the test results, but also recommendations for the type and amount of soil amendments needed.
An important -- and often overlooked -- aspect of soil fertility is soil pH, which is a measurement of the acidity or alkalinity of the soil. The pH scale runs from 0 to 14, with 7 being neutral. When the pH is below 7, the soil is said to be acidic; above 7, it is alkaline. Most lawn grasses prefer a soil pH of about 6.5 to 6.8, a slightly acidic soil. Because soil pH affects how well plants can use the nutrients in the soil, it's important to adjust your soil's pH if it's too high or too low. Otherwise, plants may not be able to use the nutrients you apply. Soils in regions with high rainfall tend to be acidic, while those in arid regions tend to be alkaline. You can raise your soil's pH by adding lime (powdered limestone) and lower it by adding sulfur. In general, you should test soil every few years and apply lime or sulfur as necessary based on the test results, rather than simply adding them yearly.
Your lawn is made up of individual grass plants. Like most garden plants, grasses perform best in healthy soil that is loose, rich in organic matter, and moist but well drained. If grass is struggling, look at the landscape as a whole before applying fertilizer. Is the soil compacted from foot or vehicle traffic? Core aerating removes plugs of soil, allowing water and nutrients to soak in. Are there areas where water puddles or runs off in a gully? Re-grading the landscape or installing drainage can help. Is the area shaded? Consider alternatives, such as shade-loving ground covers. Does your yard consist of heavy, poorly draining clay soil or sandy soil that dries out quickly? The addition of organic matter benefits both, improving drainage in clay soils and water-holding capacity in sandy soils. For new lawns, add organic matter, such as compost, at planting time. For existing lawns, a thin application of compost yearly will add organic matter.
You may have seen recommendations such as, "Apply 1 pound of actual nitrogen per 1,000 square feet of lawn." To determine how much of a particular formulation of fertilizer you need, divide 100 by the first number on the label. For example, suppose you've got a 30-3-3 fertilizer; 100 divided by 30 equals 3.3, so you'll need about 3-1/3 lbs. of fertilizer for every 1,000 square feet of lawn. An acre is 43,560 square feet. If you have a quarter-acre lot, you have about 10,000 square feet (subtracting some for non-lawn areas). So, for 10,000 square feet of lawn you'll want to spread about 33 lbs. of fertilizer evenly over the lawn to apply 1 pound of actual nitrogen per 1,000 square feet. Of course, many fertilizer bags will provide the recommended application rate, but it's helpful to understand how the calculations are done.
Grass clippings contain large amounts of nitrogen -- nitrogen that gets returned to the soil if clippings are allowed to remain and decompose on the lawn. By collecting and removing clippings, you're throwing away free fertilizer! Mow frequently, and if grass gets long and mowing yields clumps of clippings, you may need to rake these up and add them to your compost pile.
Always follow the recommendations on the fertilizer bag and remember: more is not necessarily better. Different types of grasses (fescues, bluegrass, zoysia, bermudagrass, etc.) prefer different fertilizing schedules and techniques so it's helpful to know what type of lawn you have.