Nitrogen Know-How

The use of chemical fertilizers on lawns is becoming increasingly controversial, even creating tension among neighbors in many residential areas. The potential for nitrogen leaching from applied fertilizers to pollute drinking water and harm the environment is frequently reported by mass media. While many studies have shown turf to be an effective nitrogen filter, homeowners are still concerned about the use of fertilizers on their lawns and some even are exploring alternatives to traditional turf lawns. For example, The Florida Yards and Neighborhoods Program, a University of Florida Extension initiative, began in the 1990s promoting reduced turf use in favor of trees, shrubs and other plants that grow naturally in Florida. Conflicting reports about nitrogen leaching have left many homeowners and landscape professionals wondering what the appropriate landscape model should be for minimizing nitrogen pollution.

LEACHING FACTORS

When applying fertilizer nitrogen to plants, your goal is to incorporate nitrogen into the plants that need it for growth or into the soil where the nitrogen can then remain available to the plants over time. Unfortunately, losses of applied fertilizer nitrogen to the atmosphere or through runoff and leaching generally occur as unintended consequences of fertilization. Nitrogen leaching is the result when mobile forms of nitrogen in soil drainage water are transported below the vegetation roots and ultimately find their way to groundwater or neighboring water bodies such as lakes, rivers and ponds. Environmental conditions favorable for nitrogen leaching include coarse-textured soils, abundant precipitation, saturated soils and warm temperatures. Vegetation demand and ability to acquire nitrogen over a given ground area also can strongly influence nitrogen leaching. As a result, the type of vegetation you choose to plant in lawns is an important consideration for reducing nitrogen pollution via leaching.

COMPARING LANDSCAPE MODELS

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You should base the development of appropriate landscape models in part on scientific research. Recently, the University of Florida conducted a study comparing nitrogen pollution from newly established replicate residential landscapes. The main objective of the study was to compare fertilizer nitrogen leaching between a St. Augustinegrass landscape and an alternative landscape covered with eucalyptus mulch and a diverse collection of 12 different ornamental species including trees, shrubs and ground covers (e.g., Everglades palm, wax myrtle, firebush and trailing lantana). St. Augustinegrass is a warm-season moderate- to high-fertility turfgrass and is the most common lawn turf used in Florida. Many of the ornamental species were native or common to Florida. The hypothesis behind using native landscape materials is that they would require less applied fertilizer relative to turf because they grow naturally in an area. In addition, a multispecies design can possess a greater range of rooting depths, which could possibly scavenge nitrogen more effectively.

Researchers established four plots (60 square yards) of each of the two landscape types used in the study were on a 10 percent slope of sandy soil approximately 30 inches deep. They used a conventional fertilization program on both landscapes as they applied 1 pound of Nitrogen/1,000 square feet to the turf in bimonthly applications and to the ornamentals every four months. The nitrogen sources were predominantly urea (58 percent) and sulfur-coated urea (38 percent) as part of a blended (N-P2O5-K2O) granular fertilizer that was applied evenly across the landscapes. Researchers supplied supplemental irrigation when needed, frequently following planting and then when visual signs of plant water stress appeared, such as leaf blade folding. They collected surface runoff in gutters installed at the bottom of the slopes. Because polyethylene sheets isolated each of the landscapes, the water that percolated through the soil via a slotted drainage pipe placed below the soil collected. Soil drainage water was quantified and frequently collected to measure the concentration of inorganic nitrogen, providing us with a means to estimate total inorganic nitrogen lost in runoff and drainage from each plot. Data collection began approximately one month after planting both landscapes and continued for 12 months.

Rainfall was seasonally abundant and often intense as 81 inches of rainfall were received during the 12-month study period in addition to 37 inches of applied irrigation on the turf and 34 inches on the ornamentals. We observed no significant runoff from either landscape, despite abundant precipitation. In contrast, we measured a substantial amount of soil drainage, especially following heavy rainfall events. In total, 82 inches of soil drainage was measured from the St. Augustinegrass and 88 inches from the ornamentals. Water use was initially greater by the turf, but no difference in water use was seen at the end of the study when the ornamental species had become better established and increased in size.

LEACHING FROM ST. AUGUSTINEGRASS

Less than 2 percent of the fertilizer nitrogen applied to the St. Augustinegrass was lost through leaching. While the percent of nitrogen lost was relatively low, the amount of nitrogen lost through leaching was related to timing of fertilization and abundance of rainfall.

LEACHING FROM ORNAMENTALS

More than 30 percent of the fertilizer nitrogen applied to the ornamental landscape was lost through leaching. Nitrogen losses were fairly consistent over time and were strongly related to fertilization and rainfall events.

ST. AUGUSTINEGRASS MORE EFFICIENT

The turf was established from sod, completely covering the plot and allowing it to quickly form dense roots into the soil. The demand for nitrogen by the vegetation and the relatively dense and extensive root system, along with well-known fertility requirements, led to low overall nitrogen leaching on the St. Augustinegrass even with the coarse-textured soil and abundant rainfall. In fact, the turf was surprisingly effective at nitrogen uptake given the rainfall received and appreciable soluble nitrogen in the fertilizer, suggesting that a healthy turfgrass landscape can be both aesthetic and provide environmental services.

The ornamental landscape, on the other hand, provided a sparse vegetation canopy and a relatively low rooting density as it was designed to allow for growth and expansion of the species over time. Because some characteristics of nitrogen deficiency such as yellowing leaves and stunted growth were seen on several of the ornamental species, it seems as if there was plant demand for nitrogen, but that the landscape vegetation with its sparse rooting was unable to sequester the nitrogen before it was leached from the soil. As a result, it is likely that a more complex fertilization protocol is needed, especially during establishment, to increase plant nitrogen uptake and vigor while reducing the observed nitrogen leaching. Such a complex protocol might include targeted fertilization of individual landscape plants with a better understanding of individual nitrogen requirements, grouping landscape plants by nitrogen requirements and fertilizing at lower rates with a greater frequency.

MINIMIZING LEACHING

The answer to reducing nitrogen pollution from residential land use does not seem to be as simple as just changing the landscape materials. The data presented here, corroborated by numerous other studies, demonstrate that conventional turfgrass lawns are quite effective at using applied nitrogen and the use of turf in residential lawns is not mutually exclusive with respect to the goal of reducing nitrogen pollution. The concern over environmental pollution, however, is only one factor homeowners consider when choosing landscape materials for their lawns. Other considerations such as aesthetic value, cost, functional uses and ability to attract wildlife are also important. Regardless of landscape vegetation, the study emphasized the need to follow management practices that help to reduce nitrogen leaching.

Determine your customers' expectations with respect to vegetation performance and then fertilize accordingly considering soil type, landscape species and irrigation practices. Fertilization may not be required at all if you are satisfied with the appearance of the plants. When fertilization is justified, keep in mind that no single fertilizer program is right for all situations, and thus apply nitrogen accordingly at rates as low as is compatible for your vegetation use and conditions. Use slow-release nitrogen sources wherever possible. They are less likely to leach, although they can still leach under some conditions, including abundant precipitation. Avoid fertilizing if heavy rainfall is expected. Healthy plants have a greater demand for nitrogen and are better at using applied nitrogen, so always control diseases and pests to keep plants healthy. Finally, use irrigation judiciously to minimize the percolation of water beyond the rootzone of the plants.

John Erickson, Ph.D. candidate, works in plant physiology in the Department of Forestry Ecology and Management at the University of Wisconsin (Madison, Wis.).

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