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Fertilizer Practices


The goal of any fertilization program is to compensate for nutrient deficiencies in the soil that are needed by the grass plant to sustain healthy growth and remain competitive against disease, insect, and weed invasion. While phosphorus (P) and potassium (K) are needed for a healthy lawn, it is nitrogen (N) that is required in the largest amounts by the grass plant.

Nitrogen (N)

Few soils have enough natural N available to maintain adequate turfgrass quality and recuperative capacity throughout the growing season. Nitrogen shortages can lead to very slow growth, yellowing of the plants, thinning of the turf, weed infestation and increased incidence of some diseases. However, overfertilizing with N can lead to excessive shoot and leaf growth, reduced root growth, low plant carbohydrate (food) reserves and increased susceptibility to environmental stresses and some diseases. In fact, in lawns containing grasses adapted to lower water and nutrient inputs as described earlier, overfertilizing with nitrogen and/or overwatering results in a decline in health rather then improvement. 

Nitrogen Release RatesNitrogen contained in fertilizers may be derived from either inorganic or organic sources, and is either quickly or slowly available for the plant to use. Inorganic fertilizers, such as ammonium nitrate, ammonium sulfate and the organic fertilizer urea, are all water soluble or quick-release N sources. That is, N becomes available soon after water is applied to the turf. Their response is quite predictable and results are often visible in 5 to 7 days. However, the effects are relatively short lived. On sandy soils, high application rates of these products, combined with high irrigation or rainfall amounts, will likely result in higher N losses due to leaching. Leaching is the movement of water and nutrients beyond the turfgrass root zone.Organic fertilizer products, natural or synthetic, are those containing carbon (C) in their chemical structure. Nitrogen from synthetic natural organic sources becomes available only after the product begins to break down due to soil microbial action. These are considered slow-release N sources because it is gradually released to the soil solution and available for plant use over a longer period. Soil temperature and moisture are key factors governing the microbial activity and thereby the N release. Compared to quick-release sources, these have a lower leaf-burn potential and can be applied at slightly higher rates, less often, without damaging the turf.

The primary synthetic organic fertilizer product is urea. It is considered a quick-release N product. Urea has been further processed and/or combined with other materials resulting in products with more or less of a slow-release characteristic. Nitrogen release is dependent on soil chemical, soil temperature and/or microbial action. These slow-release fertilizers have a fairly low leaf-burn potential and can be applied at slightly higher rates and less often than quick-release N sources. See Table 1 for a summary of different N sources.


Grass clippings should be returned to the lawn whenever possible. As grass clippings easily decompose once in contact with the soil, they do not contribute to thatch build-up. Grass clippings are a valuable organic source of nutrients, especially nitrogen (N). As they decompose, these nutrients become available for use by the grass plant. In fact, yearly nitrogen applications may be reduced by 1/3 to 1/2 when grass clippings are returned to the lawn.

Table 1. Characteristics of Common Turfgrass N Sources

Classification, burn potential, leaching potential, low temperature response, and residual effect on common turfgrass N sources.

Fertilizer Source

Leaching Potential

Burn Potential

Low Temp. Response

Residual Effect

Inorganic

Ammonium nitrate

High

High

Rapid

Short

Calcium nitrate

High

High

Rapid

Short

Ammonium sulfate

High

High

Rapid

Short

Organic-Natural

Activated sewage sludge

Very Low

Very Low

Very Low

Long

Manures

Very Low

Very Low

Very Low

Long

Other natural products

Very Low

Very Low

Very Low

Long

Synthetic

Urea

Moderate

High

Rapid

Short

Urea solutions

Moderate

High

Rapid

Short

Sulfur-coated urea

Low

Low

Moderate

Moderate

Resin-coated urea

Low

Low

Moderate

Moderate

Isobutylidene diurea (IBDU)

Mod. Low

Low

Moderate

Moderate

Methylene ureas and

Ureaformaldehyde*

Low

Low

Low

Mod. Long to Long

*some products may contain urea in addition to the ureaformaldehyde component.


Recently, a number of organic products have appeared on the market as "biostimulants," "growth enhancers," "soil enhancers" and have claimed other effects to improve both the soil and lawn. This has also been an area of active research to evaluate these materials and their effects. In some instances, there appear to be some benefits while in other situations results appear to be inconsistent at best. As these materials are further tested, evaluated and used more widely, it is likely that some of them will provide consistently beneficial results to our soils and turfgrass communities. If one should choose to try some of these, it may be best to proceed cautiously and evaluate their use and effects on a limited area before expending the dollars and time to apply to large areas.

Phosphorus (P)

Phosphorus is important in stimulating early root growth and promoting early plant vigor. Phosphorus moves very little in the soil with most of it bound tightly to soil particles. Soils naturally high in phosphorus generally provide sufficient phosphorus to sustain plant health and growth for many years without supplying additional phosphorus from fertilizers.

There are three reasons for correcting phosphorus-deficient soil prior to seeding. First, since phosphorus moves very little in the soil, it is desirable to mix the phosphorus throughout the root zone. This is relatively easy prior to seeding but more difficult after establishment. Second, seedling plants with newly developing root systems are most likely to be affected by lack of phosphorus. Third, mixing phosphorus into the soil (rather than applying it to the surface) reduces the chance for phosphorus to move into lakes and streams. Fully developed turfgrass root systems can absorb phosphorus from a much larger volume of soil, reducing plant deficiencies.

A soil testing laboratory can determine the current level of phosphorus in a particular soil. This inexpensive procedure is the best way to accurately determine the phosphorus requirements of a lawn.


Potassium (K)Potassium is important in the synthesis of some plant components and in the regulation of many physiological processes. Potassium deficiencies in lawns have led to increased incidence of turfgrass diseases and reduced tolerance to environmental stress.Potassium is held on the surfaces of soil particles and moves little in most soils; however, it can gradually move out of the root zone in very sandy soils. Where soils are high in native potassium, supplemental potassium fertilization may be unnecessary; however, where soils are low in native potassium, supplemental applications are very important. Soil tests are essential to determine the potassium level of a soil and to develop a potassium fertility program.

Recently, slow-release K sources have become available. Their use is intended to provide K over an extended period of time such that the plant can utilize the material as it becomes available. Potassium can leach down through the soil beyond the turfgrass root zone although it is less prone to leaching than N. These products can be particularly useful on sandy soils with relatively high leaching potentials as only small quantities become available at any one time. This reduces the leaching potential and allows greater opportunity for plant use.


Other Essential ElementsAdequate levels of the other essential elements are usually present in Minnesota lawns. Where deficiencies occur, they are usually associated with extremely sandy, highly acidic, or highly alkaline soils. Where a deficiency is suspected, rely on a soil test and advice from county extension educators as to the traits of soils in your area. For most lawns, attention to nitrogen, phosphorus, and potassium is all that is required.Soil TestsSoil TestingSoil tests are necessary to establish the proper lawn fertility program for a specific site. Soil tests can be obtained by submitting a soil sample to a private soil fertility testing laboratory or to the University of Minnesota Soil Testing Laboratory. Informational sheets and materials can be obtained through your county extension office.The proper ratio of nitrogen, phosphorus, and potassium to apply to a lawn can only be determined by soil testing. Since nitrogen can move rapidly out of the root zone with percolating water, soil tests for available nitrogen are not very meaningful and are generally not performed. Phosphorus and potassium move little in the soil and consequently soil tests will be valid for several years. The most important soil test is the first one to establish the base levels of P and K in the soil. Subsequent sampling may not be necessary for many years if phosphorus and potassium levels are adequate.The label on lawn fertilizer bags list the percentages of the three primary nutrients as a series of three numbers called the fertilizer analysis. As an example, a common lawn fertilizer analysis is 23-3-6. In this case the fertilizer contains 23 percent nitrogen, phosphorus equivalent to 3 percent P2O5, and potassium equivalent to 6 percent K2O.One fertilizer analysis is not best for all soils, since native soil phosphorus and potassium levels determine needs. If a soil is already high in phosphorus and potassium, a fertilizer with an analysis of 34-0-0 or 46-0-0 would be sufficient, while a soil with low phosphorus or potassium would require additions of fertilizer containing phosphorus or potassium.Timing

Late summer and fall are the principal times of year to fertilize lawns (Table 2). This is contrary to the traditional springtime application of most lawn fertilizer. Some advantages of fall rather than early spring applications are shown in Table 3. Excessive early spring applications of nitrogen cause a surge of top growth in plants which makes the lawn look nice in the spring but can severely deplete the plants' energy reserves going into summer. Consequently, when summer stress periods occur, plants are weaker and less able to survive. By applying fertilizer in late October or early November (when top growth is minimal but soil temperatures are still warm enough for root absorption of nitrogen), plants resume growth and green-up early the following spring. The rapid rate of shoot growth associated with excessive early spring nitrogen applications is not as apparent with fall application.

Table 2. Levels of Home Lawn Care

Lawn Care Level

Amount of Watering

Range of Mowing Heights

# of Fertilizer Applications

Timing of Fertilizer Applications

Very Low Maintenance:

(see grass types under low maintenance)

none

3+

0

-------

Low Maintenance:

Suitable grasses are commonly Kentucky bluegrass varieties & fine-leaved fescues (e.g., creeping red, chewings and hard fescue).

little to none

2 1/2"-3

1

September

Medium Maintenance:

Suitable grasses are most Kentucky bluegrasses & fine-leaved fescues

some

2"-3"

2

Mid- to late August early September, mid October

High Maintenance:

Suitable grasses are the improved bluegrass varieties & turf-type perennial ryegrasses

regularly

1"-3"

3 or 4

Mid-May to mid-June, mid-August, mid-September, and mid-October


Earlier times should be used in northern parts of the state, while later times can be used in the southern areas.

NOTE: If clippings are left on the lawn, one fertilizer application can be eliminated from the high maintenance program.

Table 3. Benefits from Applying Lawn Fertilizers in Fall Rather than Early Spring

  • Lengthened period of green in the fall. Earlier green-up in the spring without stimulating excessive shoot growth. Carbohydrate reserves (a measure of the energy stored in the plant and available for growth) remain higher during the spring and summer period.
  • Reduced incidence of summer diseases.

Lawn Fertilization and Water QualityImproper management and use of turf fertilizers can contribute to pollution of both surface waters and groundwaters. Avoiding over-application or misapplication of these materials, and basing lawn nutrient requirements on a reliable soil test, are the first steps in responsibly using fertilizers to protect water resources. Combining appropriate landscape management practices with a modest turf fertilization program may further reduce potential water pollution. Following are some additional general fertilization practices that can reduce water pollution from fertilizers.1. Never directly deposit or inadvertently apply fertilizers into lakes.

2. Fill fertilizer spreaders on a hard surface where spills can be cleaned up easily. NEVER wash fertilizer spills into the street or other hard-surface areas where fertilizer can enter storm sewers and ultimately surface water.

Fertilizer

3. Close the gate on the fertilizer spreader when crossing hard-surface areas or go back and sweep up the material for reuse.

Extra Material

4. Never apply fertilizer to frozen ground.5. Clippings not left on lawn, leaves and other plant debris should be removed as soon as possible from street gutters, sidewalks, and driveways. This plant material can be composted, used in the garden as a mulch, or disposed of through appropriate community services.

6. When mowing lawns, do not direct clippings into the street or lake.

Curb

7. Drop spreaders are more precise but slower than rotary spreaders. Near shoreline areas, apply fertilizer near the lake with a drop spreader to create a buffer zone. Then you can fertilize the area away from the shoreline with a rotary spreader. Take the same precautions when using liquid applications.

Fertilizers

8. Avoid getting fertilizer into natural drainage areas or pathways on a property. These may not necessarily be hard-surface areas. Fertilizer can be carried directly into surface water before it has a chance to infiltrate into the surrounding turf/soil area.

9. For shoreline areas, try to leave a buffer zone of unmanaged grasses, or possibly natural vegetation, around the shoreline. This natural area helps prevent erosion from adjacent shore land, and may retain nutrients that would otherwise go into the lake.

Natural Vegetation

Protecting surface water and groundwater is not something to be taken lightly. Neglecting lawn areas for fear of introducing nutrients and pesticides into water supplies is not a way to protect these resources either. Properly maintaining turfed areas with appropriate but modest use of fertilizers and pesticides will do more to protect water resources than to hurt them.

 

 
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