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Choosing the best lawn fertilizer for your grass depends on your soil. In general, all grass types require nitrogen, phosphorus, and potassium. A soil test will tell you if your soil is lacking other minerals. You can also choose between liquid and granulated fertilizer, or organic and synthetic fertilizer.
All lawn fertilizers contain nitrogen, phosphorus, and potassium in varying ratios. These are called NPK fertilizers. Fertilizers are labeled with three numbers, such as 30-10-10, to show customers these ratios. This means the lawn fertilizer contains 30 percent nitrogen, and 10 percent each of phosphorus and potassium. The remaining 50 percent might be other nutrients, or other inert ingredients like fillers and stabilizers. Some lawn fertilizers are called “weed and feed,” which means that the product also contains an herbicide.
All grass requires a lot of nitrogen in order to grow well, which is why lawn fertilizer almost always contains a higher percentage of nitrogen. A soil test will tell you if your soil lacks sufficient phosphorus and potassium. If your soil's phosphorus and potassium are very low, choose a fertilizer with a higher ratio of these elements, but make sure that nitrogen makes up the largest percentage of the mixture. You can add other nutrients to the fertilizer like iron or magnesium depending on the results of the soil test.
Liquid fertilizer is fast-release fertilizer. The nitrogen in these fertilizers has a greening effect on the lawn, which you will usually notice in a day or two. A sprayer is required to ensure even application of liquid lawn fertilizer. Granulated fertilizer can be slow-release and fast-release. Slow-release fertilizer takes longer to work and the results aren't immediately noticeable, but it feeds the grass for longer and doesn't need to be applied as often. You can apply granulated fertilizer with a spread if you your lawn is large. A smaller lawn may just require a shovel to toss the grains around, and a push broom to sweep them in.
Concerns about the large amount of chemicals domestic lawns leach into the soil, storm drains, and groundwater are leading many people to choose organic lawn fertilizers. Synthetic fertilizers generally feed the grass, but many of the nutrients are washed away before the grass can use them. Organic fertilizers work much more slowly, but they feed the worms and microbiotic life in the soil. Worms and microbes make the nutrients available to grass over a longer period and keep the soil healthy.