Time your lawn fertilizing right to give your grass the biggest benefit and reduce the likelihood of fertilizer-laden runoff polluting the watershed. Cool-season grasses (e.g. bluegrass, fescue, perennial rye) benefit most from a main feeding in early fall, while warm season grasses (e.g. St. Augustine, bermuda, centipede, zoysia) are best given their main feeding in late spring. To make sure the fertilizer you put down helps your lawn, rather than running off to cause problems in the watershed, use one that has at least 50% of its nitrogen in organic or slow release (water-insoluble) form. Sweep up any fertilizer that lands on driveways and sidewalks as you spread it, and avoid fertilizing right before heavy rain is predicted.
Research has shown that, on established lawns growing on soils that did not test deficient in phosphorus, adding this potentially watershed-harmful nutrient in fertilizer did nothing to benefit the growth of the turf. So, for established lawns, test your soil every 3 to 5 years and, if the soil test does not show a phosphorus deficiency, use a fertilizer that does not include this nutrient. Look for one with a zero for the middle number in the analysis, for example 5-0-10 or 10-0-10. (That middle number indicates the percentage of phosphate in the fertilizer.)