Plentiful rains have kept most area lawns green throughout the summer. Even though lawns are already green and growing, the prime time to fertilize is in the next two weeks.

Fall is a crucial time for growth for lawns, and proper fertilization helps assure good growth that will carry through into 2025. The two nutrients of need now are nitrogen and potassium. Nitrogen, the first number of the three on the bag, usually is the key nutrient in lawn fertilizers in any application. Potassium, the third number, is found in winterizer fertilizers and does play a role in improving plant hardiness.

Phosphorus, the middle number on the bag, is a concern due to potential runoff and source of water quality issues. Soil tests will determine phosphorus levels and if deficiencies exist. Wisconsin law limits phosphorus application on lawns only to either newly established lawns or lawns where a soil test has shown phosphorus deficiency. For this reason, lawn fertilizers sold in garden center outlets will have zero levels of phosphorus in them.

When choosing lawn fertilizers, the kind of nitrogen, rather than total percentage in the bag, should be your main concern. Look for controlled-release, slow-release, or slowly available nitrogen sources, as they release lesser amounts of nitrogen over an extended time, which is best for turfgrass growth. Examples found in guaranteed analysis sections on fertilizer labels include sulfur-coated urea, ureaform, coated urea, and IBDU. Organic fertilizers often list controlled release as water-insoluble nitrogen, or WIN.

Adjust the amount of fertilizer applied based on nitrogen percentage in fertilizer products. Lower percentages call for applying more product, likewise higher percentages mean less actual product is applied. One pound of actual nitrogen for every 1,000 square feet of lawn area is the suggested rate. As an example, for a fertilizer with 20 percent nitrogen, it would take five pounds of actual fertilizer to supply this one pound. Fertilizer labels often have suggested setting guides and indications on the lawn coverage the bag will provide are usually based on a one-pound rate.

Use care to ensure fertilizer is uniformly applied over the lawn. Be careful to avoid fertilizer landing on impervious surfaces, as the potential increases to then wash into storm drains, waterways, or other nontarget areas. Research has shown turfgrass is very efficient at holding nutrients in place and avoiding runoff issues, but the fertilizer needs to be applied directly to lawns.

Ready or not, fall is coming soon. Help your lawn prepare with a timely fertilizer application.