Responsible fertilizer practices for lawns
Few soils have enough natural nitrogen (N) to maintain desired turfgrass quality and recuperative ability throughout the growing season. Shortages can cause slow growth, yellowing of the plants, thinning out of the turf, and increased incidence of some diseases. However, an overabundance of N can lead to excessive shoot and leaf growth, reduced root growth, low plant carbohydrate (food) reserves, increased susceptibility to environmental stresses, and some diseases. A primary consideration in using N fertilizers responsibility is to match the site conditions and the desired maintenance program with the proper N fertilizer sources.
Nitrogen fertilizer sources
Nitrogen fertilizer sources are often categorized as inorganic types or organic types. A brief description of several N sources is given in Table 1.
Once beyond the rootzone, nitrates can continue moving through the soil and may find their way into groundwater sources. Organic fertilizer products, natural or synthetic, contain carbon in their chemical structure. Nitrogen from natural organic sources becomes available only after the product breaks down through soil microbial action. These are considered slow-release N sources as N is gradually released to the soil becoming available for plant use. Soil temperature and moisture are key factors governing the microbial activity and thereby the N release. Compared to quick-release sources, slow-release N sources have a lower leaf-burn potential and can be applied at slightly higher rates without damaging the turf.
The primary synthetic organic fertilizer product is urea. It is considered a quick-release N product with a relatively high leaf burn potential. Urea has been further processed and/or combined with other materials giving these fertilizer products more or less of a slow-release characteristic. The N released from these slow-release N products depends on soil chemical and/or microbial action, has a fairly low-leaf burn potential, and can be applied at slightly higher rates than quick-release N sources.
Nitrogen fertilizer use
The amount of N required by turfgrass depends on the type of grasses present and the management practices used. High maintenance lawns often contain the more vigorous, improved Kentucky bluegrass and turf type perennial ryegrass varieties. These lawns will perform better when adequate water and fertilizer are regularly provided. Low maintenance lawns usually consist of common types of bluegrass in combination with a mixture of fine fescue grasses. These lawns grow and spread more slowly and usually receive little extra water or N fertilizer. Table 2 describes the annual application of N requirements for these lawn types and how clippings left on the lawn impact yearly N requirements.
On highly leachable soils--sands and sandy loams-- the recommended N application rates may result in excessive loss of nitrate-N due to leaching. Where soluble N sources are used on these soil types, reducing the N rates to 1/4 to 1/2 lb. N/1,000 ft2 = 1000 square feet per application may minimize potential nitrate-N leaching. If frequent, lower N rate applications are not practical, slow-release N sources may be a better choice for these soils. This practice is adaptable to late season N fertilization and may be especially true where sandy soils are in close proximity to surface water or groundwater.
Watering practices that move water beyond the root zone may increase potential nitrate-N leaching. Daily irrigation during cool, moist periods increases the potential for leaching. Irrigation practices that take into consideration the grass plant's needs during any particular climate condition are more effective. Adding enough water to compensate for that removed by plant uptake and evaporation minimizes potential N pollution problems from leaching. Sloped areas may require more frequent but smaller amounts of water per application. They are more vulnerable to runoff before water has infiltrated into the soil.
Irrigation of 1/4 to 1/2 inch of water immediately after applying a quick-release N source helps move the N into the surface soil where it can be used by the grass plant. Also, the N will be protected from runoff and possible volatilization back to the atmosphere.
Nitrogen fertilizer product knowledge and being familiar with the site may minimize or even eliminate potential adverse impacts on water quality. In addition, always follow manufacturer guidelines or consult with local extension turf specialists for appropriate application rates.
PhosphorusPhosphorus (P) is an essential nutrient contained in every living grass plant cell. The amount of P needed by the grass plant is significantly less than nitrogen or potassium. It has positive effects on turfgrass establishment, rooting, and root branching. Phosphorus is particularly important during early grass seedling growth and development stages.
While P is an important nutrient for grasses and other green plants, it is also an important nutrient for algae and weeds in our lake systems. Phosphorus is often the least abundant nutrient in freshwater lakes, which limits growth of algae and weeds. Lake enrichment with P can cause undesirable algae blooms and vigorous growth of other lake weeds, a process termed eutrophication. For this reason, much concern has been raised about the contribution of lawn and garden fertilizers to lake pollution.
Off-site movement of phosphorusPhosphates, P combined with oxygen, are removed from the soil solution and immobilized in the soil. Consequently, phosphates are not prone to leaching and pose little or no threat to groundwater resources.
Phosphorus can also be part of organic material such as manures, composts, various natural fertilizers, grass clippings, or tree leaves. In the soil solution, these organic forms of P may not be bound tightly to soil particles. Thus, they are more prone to leaching or runoff than the phosphates. Of course, sandy soils will pose a greater leaching concern than heavier loam or clay soils.
Off-site transport of P to surface waters tends to be associated with sediment erosion. Phosphorus readily bonds to the very fine soil mineral and organic matter particles. These very fine particles are easily eroded by runoff water and wind, thus potentially carrying the P bonded to them to lakes and rivers. Living plants such as trees, shrubs, and grassy areas around lakes can help stabilize the soil against wind and water erosion. Also, they act as filters to help remove these fine soil particles from the air, thus trapping both the soil particles and any associated nutrients bonded onto them.
Phosphorus management practicesPhosphorus fertilizer additions to lawn areas should be based on a reliable soil test. A soil test can usually be obtained from land grant universities such as the University of Minnesota or through private soil testing laboratories.
In established turfgrass areas, runoff potential is quite low due to dense turfgrass and its extensive fibrous root systems. Therefore, where P is applied to turfgrass areas, it should be watered into the soil where it is immobilized and generally protected from loss by runoff.
During the winter months, freezing and thawing can break down leaves, dead grass plants, and other organic debris and release soluble forms of phosphate and nitrates. These nutrients potentially can run off from frozen ground, especially slopes, during spring snowmelt and early spring rains and could be carried into surface water areas.
Grass clippings, leaf litter, and other forms of organic debris should be removed and kept off hard-surface areas where they could be carried in runoff to surface water areas. Obviously, these same materials should not be dumped on or near shoreline areas where nutrients released during decomposition can move directly into the water.
It is often advisable to add some P when establishing a lawn even though soil P levels may be adequate for turf. This ensures that P will be available near the soil surface for the young developing grass roots. Protecting newly seeded areas, especially slopes, with some type of mulch cover during establishment helps prevent runoff and erosion of soil and possible nutrients. Applying P to an established turf following core cultivation helps get P down into the soil, thereby protecting it from loss by runoff.
General fertilization practicesIn addition to the specific N and P management practices already mentioned, following are some general lawn fertilization practices that can help reduce potential water pollution.
Never directly deposit or inadvertently apply fertilizer materials into lake areas.
Fill granular fertilizer spreaders on a hard surface where any spills can be easily cleaned up. NEVER wash off fertilizer spills into the street or other hard-surface areas where they can easily enter storm sewers and ultimately surface water areas. Wash off granular fertilizer spreaders over turfed areas to prevent runoff of fertilizer from hard surfaces. Fill and clean liquid fertilizer applicators over turfed areas for similar reasons.
Close the gate on the fertilizer spreader when crossing hard-surface areas or go back and sweep up the material. Reuse it another time or put it back into the spreader.
Try to use a drop spreader, which is more precise but slower than a rotary type spreader near surface water. Next to shoreline areas, apply fertilizer around the perimeter of the property with a drop spreader to create a safety zone. The rest of the area farther away from the shoreline can be fertilized with a rotary spreader. Since the perimeter has already been done with the drop spreader, it is not necessary to hug the shore because fertilizer may get into the water. The same kinds of precautions should be taken when using liquid fertilizer.
Avoid getting fertilizer into natural drainage areas or pathways on a property. These areas may not necessarily be hard-surface areas, but they can carry fertilizer directly into the surface water before having the chance to infiltrate into the surrounding turf/soil area.
Leave a buffer zone of unmanaged grasses or possibly natural vegetation growing around the shoreline. This can help prevent soil erosion and may retain some of the nutrients that might otherwise enter the lake.
Leave grass clippings on the lawn area to decompose and recycle nutrients back to the turf area. They should not be blown or raked into street gutters or onto sidewalks and driveways where they may be carried with runoff water to surface water. Nutrients released in water through decomposition may cause undesirable algae and vegetative growth.
NEVER apply N fertilizers to water resources directly or to frozen ground.
Improper management or use of turf fertilizers may contribute to potential pollution of surface water and groundwater. However, combining appropriate landscape management practices with a modest lawn fertilizer program may further reduce surface water pollution.
Prepared by Robert J. Mugaas, Hennepin County Extension horticulturist, University of Minnesota; Michael L. Agnew, Extension horticulturist-turf; and Nick E. Christians, professor of horticulture, Iowa State University.
Originally a joint publication of the Minnesota Extension Service, University of Minnesota, and Iowa State University Extension.
This material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 92-EWQI-1-9265.