Where does all that fertilizer go? For most homeowners, it goes on their lawns. But, that picture-perfect green lawn sometimes comes at the expense of our surface and ground waters that can absorb nitrate from the nitrogen that's meant for our lawns.
A national EPA study has found nitrate, a form of nitrogen fertilizer, in groundwater. Although lawn fertilization is only one possible source, chronic exposure to high nitrate levels has been shown to have adverse health effects, particularly among pregnant women.
Nitrate runoff in surface water alters the population balance of algae and other microflora in rivers causing disruptions of living organisms up and down the food chain.
How can you fertilize the lawn to grow healthy grass while reducing the potential for contamination of groundwater and river ecosystems?
The foremost turf fertilization tip involves the amount applied. Don't overload your lawn's system with more nitrogen than the turf can use atone time! Recommended amounts are given on a yearly basis but should be spread over several fertilizations of no more than one pound of nitrogen per 1,000 square feet each, the standard rate given on fertilizer products. Using more fertilizer than is recommended increases the potential for soil microbes to convert nitrogen into the easily dissolved nitrate form. This form can quickly bypass grass roots and move into water supplies.
The recommended amount for bluegrass, the most popular lawn in Colorado, is 2 to 4 pounds of nitrogen annually per 1,000 square feet. This means 2 to 4 fertilizer applications timed as follows: April, late May, late August, early October. Apply only in late May and early October with two applications and add late August with three. The fourth application is only necessary for people desirous of growing extremely high quality turf requiring lots of mowing.
Skipping the April application is just one of the many advantages to what is now regarded as the most important lawn fertilization application time of the year - fall. If you leave grass clippings on the lawn when you mow, you can apply less fertilizer than if you collected clippings.
A particular caution must be given if you grow a lawn on sandy soil. Sands don't hold fertilizer nutrients in the same way as clay soils do. This means you should use nitrogen fertilizers that are slowly available such as the sulfur-coated ureas, IBDU, and natural organic fertilizers (in warmer months). These types of fertilizers will reduce the potential for fertilizer movement from the turf into surface or groundwater. The same fertilizer recommendations also apply to those with turf over or near wells.
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010