Soil tests should be conducted every three years. Your soil tests will let you know if your lawn needs fertilizer and, if so, how much and where. Nitrogen is the key plant nutrient for building a thick, green lawn. Applied at the right time and in the right amount, fertilizers will supply the nitrogen your soil needs. If you apply fertilizer at the wrong time or in the wrong amount, you may make conditions worse, and insect and disease problems can increase.
Excess fertilizer is likely to wash away before the grass takes it up.
Fertilizer in runoff contributes to unwanted plant growth in nearby streams or lakes. Especially in sandy soils, or soils that are shallow to bedrock, nitrogen and other chemicals can seep downward and enter groundwater used for drinking. If you hire a lawn-care service, make sure the workers test your soil before applying fertilizer. Sweep excess fertilizer off walks and back onto the lawn before it is washed away by rain. Non-chemical fertilizers, such as compost and fish meal, and other soil amendments should be applied based on the needs of your lawn.