Nitrogen Leaching & Run-off
Leaching occurs when irrigation or rainfall carries nitrogen, primarily in the nitrate form, downward through the soil profile. As nitrate moves below plant root systems, it continues to move downward, eventually ending up in groundwater. How much nitrogen is leached from a lawn depends on the soil type; the amount and rate of precipitation; and the nitrogen source, rate, and timing of application.
The greatest potential for leaching is in sandy soils during periods of wet weather or under excessive irrigation, and following applications of quick-release nitrogen at high rates. Leaching can be reduced by using slow-release nitrogen sources on high-sand-content soils or by using low rate applications of quick-release nitrogen sources. Leaching can also be curtailed by restricting nitrogen applications when plants are not actively growing (during midsummer and winter) and/or during extremely wet periods of the year. Since leaching of nitrogen can sometimes occur even in loam soils, be sure always to follow good fertility and irrigation practices.
When nitrogen is applied to turf, some may be carried in runoff into surface or groundwater. Runoff is water that reaches the turf-soil surface and is not absorbed into the ground or accumulated on the surface, but runs downslope. The rate of runoff is determined by the amount and rate of precipitation, slope, infiltration capacity of soil, geological features of the site, vegetation cover, and cultural practices.
Runoff is most likely to occur following sudden, heavy rainstorms on soils with poor infiltration characteristics that support little or no vegetation. The most significant runoff threat, however, is from impervious surfaces such as sidewalks, driveways, roads, and frozen soils. Some runoff from impervious surfaces is carried into storm sewers and finds its way into surface or groundwater.
Research conducted at Penn State has shown that where a dense, well- established turf exists, the amount of nitrogen removed from the site via runoff is very low—provided the site has good infiltration characteristics. The dense cover of leaves, stems, and thatch of turf slows the rate of surface flow, allowing water and nutrients to infiltrate the soil.
Tips for Getting the Most Out of Your Nitrogen Fertilizer
The following are suggestions for maximizing the efficiency of your nitrogen fertilizer program while minimizing losses to leaching, runoff, and the atmosphere.
- Soil test. Applications of phosphorus, potassium, and lime according to soil test recommendations allow more efficient use of nitrogen fertilizer by turfgrasses.
- Apply nitrogen in amounts needed by the species you are trying to maintain—more is not necessarily better.
- On turf, apply nitrogen fertilizer in multiple applications over the growing season so as to meet the needs of your turf at the appropriate time—usually mid to late spring, late summer, and late fall.
- Returning clippings to lawns can cut nitrogen fertilizer use by up to one-third.
- Don’t overwater—too much water can leach nitrogen below root systems and into groundwater.
- Use slow-release fertilizers when making infrequent, high-rate applications in areas where soils are prone to leaching.
- Keep nitrogen on the lawn and not on pavement. Shut off your spreader when moving across driveways or maintenance roads, or blow or sweep up granules from pavement. In small lawns enclosed by sidewalks and driveways, use a drop spreader or a liquid application for greater accuracy.
- Do not apply nitrogen to lawns under summer dormancy or on frozen surfaces in winter.
- Water-in urea or ammonium fertilizers, especially when applications are made in warm weather.
- Fill and empty fertilizer spreaders in an area where spills can be easily cleaned up. Use your spilled fertilizer—don’t wash it into the street or storm sewers.