Early fall is a good time to fertilize the cool-season grasses such as bluegrass, fescue, and ryegrass grown in cold-winter areas of the country. Warm season grasses such as bermuda grass and St. Augustine grass do best when given their main feeding in late spring. 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. 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 zero as 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.) Phosphorus is helpful in the establishment of a new lawn the first year after sodding or seeding and does not contribute to runoff when it is applied to new lawns according to recommended rates. 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 percent 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.