Lawn care is big business in the United States. Americans spend over $40 billion annually on watering, fertilizing and mowing lawns, according to Fox Business. Applying the right fertilizer at the right time saves time, money and prevents pollution. The ideal time to fertilize, according to Cornell University, is in the fall for most grass types, when conditions are cool and dry.
Most lawn fertilizers used in the United States are granular, although the nutrient values may vary, depending on the formula. Slow-release formulas are more expensive but are a better choice for most homeowners, according to Cornell University. They break down slowly, providing long-term nutrients. They are also less likely to run off during heavy rain.
A soil test conducted at a university county extension office or a commercial lab provides a detailed analysis of the soil composition, pH and nutrient level. Based on the analysis, gardeners can more accurately fertilize lawns according to the fertility needs of the soil. Many soils are naturally high in phosphorus. Adding more phosphorus unnecessarily causes the nutrient to build up in the soil and run off into lakes and streams during rainfall. For a healthy lawn and environment, use the least amount of fertilizer necessary and apply it at the right time, according to Cornell University.
When homeowners fertilize before a heavy rain, most of the fertilizer ends up in the gutter, where it makes its way to streams and lakes. Phosphorus fertilizer causes excess algae and weed growth, choking out fish and aquatic life. Fertilizers also leach into ground water during heavy rains. Excessive nitrogen in drinking water causes serious health problems in infants, according to the University of Minnesota Extension.
Weed-and-feed products contain toxic pesticides that may cause health problems, especially in young children. These products should be avoided altogether, according to Thurston County Washington Public Health and Social Services.
Applying fertilizer before a rain literally washes money down the drain. While most fertilizers require a "watering in" to activate them, rainfall is an unreliable and ineffective method. Too little rain won't dissolve the granules, while too much rainfall washes the fertilizer away before it can benefit lawns. Sandy or coarse soils are especially prone to leaching during heavy rains, according to the University of Minnesota.
Rotary spreaders apply fertilizer quickly but also hurl it into unwanted places, such as vegetable gardens and hard surfaces. Unless swept up, the fertilizer washes into gutters, polluting water systems during the next rainfall. Drop spreaders are slower but apply fertilizer more efficiently, according to Cornell University Extension.