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Nitrogen

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Recognize that no soil test currently exists that can reliably inform nitrogen (N) applications to turf in the Northeast.

  • There are no generic N fertilization recommendations that can be applied to all situations.
  • N rates must be determined based on variables such as expected quality of the turf, use of the turf, characteristics of the growing environment, grass species and varieties present, and available fertilizer and nutrient containing materials.
  • N should be applied at a frequency and rate that will promote vigorous growth without causing surge growth, overstimulation or loss of N from the turf system.
  • Nitrogen from all sources in the management plan should be factored into total N applied. Remember that materials including organic amendments, organic fertilizers, composts and compost derivatives, topdressings and recycled clippings can all contribute N to the turf system.

NITROGEN SOURCE

In choosing a nitrogen source, carefully evaluate the readily-available nitrogen content and the slowly-available nitrogen content, as well as the specific nutrient release characteristics.

  • Water-soluble nitrogen (WSN) is readily available to the plant.
  • Slowly-available nitrogen or slow release nitrogen (SRN) sources include water-insoluble nitrogen (WIN) and various engineered slow release nitrogen technologies referred to as controlled release nitrogen (CRN).
  • Manufactured turf fertilizers are often formulated with a mixture of WSN and SRN.
  • The percentages of WSN and SRN in a fertilizer product will affect the N release rate, price, and other factors.
  • Characteristics of WSN and SRN sources may be considered either advantageous or disadvantageous depending on the specific management situation.
  • Turf managers need to be especially cognizant of the release characteristics for nutrients from any material and how release rate is influenced by factors such as temperature and moisture.

Water-soluble nitrogen (WSN)

Table 10 lists the most common WSN sources used by turf managers. Of these fertilizers listed, urea is the most commonly used source of N in most complete fertilizers. Calcium ammonium nitrate, ammonium sulfate, and potassium nitrate have a higher salt index and are more likely to burn the turf than urea. Mono- and diammonium phosphate are used in fertilizers when phosphorus input is also desired.

Fertilizer Analysis (N-P-K) Salt Indexa CaCO3b  Equiv. Lbs. Needed to Supply 1 Lb. N 
Table 10. Characteristics of some water soluble turfgrass fertilizers.
Urea 46-0-0 1.7 71 2.2
Ammonium sulfate 21-0-0 3.3 110 4.8
Calcium ammonium nitrate 20-0-0 3.2 -4c 5.0
Potassium nitrate 13-0-44 5.3 -23 7.7
Monoammonium phosphate 11-4-0 2.7 58 9.1
Diammonium phosphate 21-53-0 1.7 75 4.8
  1. Relative burn potential compared to sodium nitrate. (>2.5 = high, 2.5 -1.0 = moderate, <1.0 = low)
  2. Calcium carbonate equivalent (CCE): lbs. of CaCO3 (limestone) needed to neutralize the acidity of 100 lbs. of applied fertilizer.
  3. A negative CCE increases pH; equivalent to applying 4 lbs. CaCO3 for every 100 lbs. of calcium ammonium nitrate fertilizer, or 23 lbs. CaCO3 for every 100 lbs. potassium nitrate

Water-insoluble nitrogen (WIN)

Typical slow release N sources classified as WIN include: ureaformaldehyde products (UF), isobutylidene diurea (IBDU), and products derived from natural organic materials such as seed meals, feather meal, activated sewage sludge, seaweed, and other plant and animal residues.

Ureaformaldehyde (UF) fertilizers (38%N) depend upon microbial activity to release N from complex mixtures of short, intermediate and long chain organic carbon polymers. Thus, factors which favor microbial activity will also favor N release. These conditions are: soil temperatures higher than 55°F, adequate moisture, adequate aeration, and pH between 6.0 and 7.0. UF fertilizers are less effective in late fall and early spring because of unfavorable temperatures (cold soils) for N release.

Methyleneurea (MU) fertilizers are similar to UF but are composed of shorter length carbon chains. MU fertilizers are less sensitive to cold temperatures compared to UF products.

Isobutylidene diurea (IBDU, 31% N) is a material which releases N as a result of very slow solubility in water. The physical process is essentially similar to dissolving of sugar or some other soluble product only at a much reduced rate. Finer particle size products are available for use on low cut areas or where a more rapid response is desired. Because moisture is necessary for release, IBDU is not a good choice for non-irrigated turf areas. Conditions that are not favorable for moisture retention such as excessive thatch will be less favorable for N release from IBDU. In addition, IBDU will not release as effectively on alkaline soil with pH above 7.7. Because release is not affected by temperature, IBDU is a good choice for early spring when adequate natural rainfall is usually plentiful. IBDU is not commonly found as the nitrogen component in most complete fertilizers. 

Natural organic fertilizers vary in composition depending upon what source of nitrogen is used. N release from natural organic fertilizers is much like that of UF fertilizer. Release depends upon microbial activity and is temperature dependent (i.e. needs warm soils). Therefore, developing a fertility program utilizing natural N sources can pose a unique challenge as N from natural organic sources will be more available during periods of warmer temperatures (when less fertility is generally needed), and less available during periods of cooler temperatures favorable for turfgrass growth. See Table 12 for information on some natural organic fertilizer materials. 

Slow release nitrogen (SRN) technologies 

In addition to WIN sources, coated technologies are available including sulfur coated urea (SCU), polymer (plastic) coated urea (PCU) and double coated technologies (Polymer-S), which combine both sulfur and polymer coatings in the same N source. These coated technologies are SRN sources that have similar strengths and weaknesses to those exhibited by WIN fertilizers. N release from SCU can be less consistent and less efficient compared to Polymer-S and PCU sources.

Sulfur Coated Urea (SCU, 32-36% N) and plastic coated urea (PCU) products release N slowly because the urea pellet (prill) is covered with a coating of sulfur, plastic or both. Thus, N leaks through the pores at a slow rate compared to uncoated urea. Prills which have an incomplete or cracked coating will behave like WSN. Thinly coated prills will release N more rapidly than thickly coated prills. Adequate moisture and warm soil temperatures (warmer than 55°F) are factors favoring release of N from SCU and other coated urea fertilizer products.

Table 11 lists the most common synthetic WIN and SRN sources used by turf managers.

Fertilizer Analysis (N-P-K) Salt Indexa CaCO3b Equiv. Lbs. Needed to Supply 1 Lb. N 
Table 11. Characteristics of some common slow release turfgrass fertilizers.
Ureaformaldehyde and Methyleneurea 38-0-0 0.3 68 2.6
Isobutylidene diurea 31-0-0 0.2 57 3.2
Sulfur coated urea 32-0-0 0.7 varies 3.1
  1. Relative burn potential compared to sodium nitrate. (>2.5 = high, 2.5 -1.0 = moderate, <1.0 = low).
  2. Lbs. of CaCO3 (limestone) needed to neutralize the acidity of 100 lbs. of applied fertilizer.
  Total N (%)a C:N ratio Fraction of organic N made available first seasonb P2O5 (%) K2O (%)
Table 12. Typical nutrient value and C:N ratio of several common organic and mineral soil amendments and nutrient containing materials.
Plant residues          
Alfalfa meal 3-4 18 0.3-0.5 0.5 2.5
Cottonseed meal 6 5 0.6-0.8 2 2
Seaweed 0 - - 0 1
Soybean meal 7 5 0.6-0.8 2 2
Corn gluten meal c 9 4 0.6-0.8 0 0
Animal products          
Dried blood 12 3 0.7-0.9 1 0.5
Bone meal (steamed) 3 4 0.5-0.7 15 0
Feather meal 13 4 0.7-0.9 0 0
Fish emulsion 4 3 0.7-0.9 2 0
Fish meal 9-12 4 0.7-0.9 7 0
Poultry litterd 3-4 15 0.4-0.6 3 3
Compost (mature)d          
Manure 1.5-2 20-25 0.1-0.15 2 1
Yard waste 0.5-1 20-25 0.1-0.2 1 1
Mineral materials          
Potassium sulfate 0 - - 0 50
Sul-Po-Mag 0 - - 0 21
Wood ash 0 - - 1 10
Colloidal rock phosphatee 0 - - 25f 0
Rock phosphatee 0 - - 20-32f 0
Granite duste 0 - - 0 3-5f
Greensande 0 - - 1f 4-9f
  1. Nutrient concentration of organic materials is inherently variable. Estimated values are provided for reference only. It is best to have materials tested in order to determine appropriate application rates.
  2. To estimate the quantity of total N expected to become plant available in the first season following application, multiply by the appropriate coefficient.
  3. Corn gluten meal inhibits germination of some small seed plants and has been promoted as a natural pre-emergent herbicide. Avoid using where turfgrass has been recently seeded or where overseeding is imminent.
  4. Compost and poultry litter also contain varying quantities of NH4, which is immediately plant available; however, NH4 is subject to volatilization losses if material is not immediately incorporated.
  5. Relative nutrient availability of nutrients from rock powders varies with origin of material, soil pH, and depends largely on fineness of grind.
  6. These values represent total K2O and P2O5. These materials are extremely insoluble therefore available K2O and P2O5 from these materials will be much lower.

Use slowly-available SRN along with readily-available WSN for fertilizer applications, especially on sandy soils prone to leaching.

  • WSN and SRN sources each contribute both positive and negative properties to a fertilizer product. A balance of both WSN (fast) and SRN (slow) types are preferred for most turf applications.
  • As a general rule, during periods of peak shoot growth typical of spring and fall it is desirable to have not less than 25 percent of the total N in the fertilizer derived from some SRN source. The fertilizer should also contain a sufficient amount of WSN to support active growth especially during favorable shoot and root growth periods of spring and fall.
  • If fertilization is necessary during periods of minimal growth typical of summer stress, it is advisable to have not less than 50 percent of the total N (though not less than 75% is preferable) in the fertilizer derived from an SRN source (WIN or CRN).
Table 13. Readily-available (WSN) sources versus slowly-available SRN sources.

Characteristics of readily-available (WSN) N sources:

Characteristics of slowly-available (SRN) N sources:

  • provide a rapid increase in both color and growth rate
  • release of N is relatively independent of temperature, so can be used throughout most of the growing season with acceptable response
  • relatively rapid plant response rate
  • relatively short period of plant response (residual of 4 to 6 weeks at normal rates)
  • potential for surge shoot growth
  • can be applied in either granular or liquid form
  • high foliar burn potential (salt index) when applied at excessive rates or during periods of high temperature
  • greater potential for loss via leaching or volatility (gaseous losses)
  • generally less expensive per unit N when compared to many SRN sources
  • release N slowly over a longer period of time than readily- available N sources
  • some sources are temperature dependent and do not release N in cold soils (< 55° F)
  • in moist, warm summers nutrient release may be more rapid
  • low potential for foliar burn (salt index)
  • do not result in flushes of rapid growth (surge growth)
  • provide a longer residual plant response
  • potential carryover of N into the following growing season(s)
  • lower potential for gaseous loss and loss via leaching
  • relatively slow color response
  • generally cost more per unit of N when compared to WSN sources

Know how to determine the SRN content of a fertilizer material.

  • One of the most critical, but commonly overlooked, considerations for turf fertilizers is the percentage of N in the material that is has slow release characteristics. 
  • Unfortunately, the SRN percentage of a material is not often readily apparent from the product label.  Follow these steps to calculate the percent SRN in a fertilizer from label information:
     
    1. Find the guaranteed analysis for total nitrogen (Example: 24%)

    2. Find the percentage of water insoluble nitrogen (Example: 2.1%)

    3. Find the percentages of any additional sources of slow release nitrogen listed, such as sulfur coated N, polymer coated N, etc.  Careful reading is required as listings of these vary.  Some may be listed directly, while others may be listed in a footnote as a component of another source. (Example: 5.8%)

    4. Add together all water insoluble and slow release sources, then divide by the guaranteed analysis for total nitrogen to determine the slow release N content of the material:

In this case it is 2.1% + 5.8% = 7.9%

7.9% / 24% = 32.9%

32.9% of the nitrogen in the example fertilizer is SRN.

  • Note that the SRN content of many naturally derived fertilizer materials (eg. composts) typically approaches 100%.

When using natural organic sources of N, take care not to over apply phosphorus.

  • Soil testing is the first step before applying any P containing fertilizers or P containing materials.
  • Since almost all natural organic nitrogen sources contain P, care should be taken that excess P is not applied in an attempt to supply adequate N.
  • Organic N sources exhibit a low N content by weight of fertilizer. The total fertilizer amount needed to meet the turf system N requirement increases with decreasing N content of the N source. This in turn may lead to over-application of P in a turf system, especially when soil test P is sufficient for turf growth.
  • Repeated applications of organic N sources that contain P can continue to overload the turf system with unnecessary P over time.
  • Where organic fertility programs are being implemented and soil test results show Above Optimum levels of P, the addition of P containing fertilizer, amendment, topdressing or other materials should be avoided.

See Table 12 for information on natural organic fertility sources.

NITROGEN RATE

Exercise particular care in determining an appropriate rate of fertilizer N.

  • As discussed previously, turfgrasses are highly responsive to fertilizer N. N is also a nutrient that can impair ground and surface water in the event of undesired migration out of the turf system.
  • The term fertilization rate can refer either to the rate of an individual application or to the amount of a fertilizer nutrient applied on an annual basis, taking into account all sources of N.
  • The N fertilization rate depends upon many factors such as: N source to be applied, time of the year, fertility requirement of the species and cultivars present, specific management goals (for example, successful overseeding or repairs), and expectations for quality and performance.
  • Turf that is intensively used (e.g. sports fields, golf courses) may need special considerations in terms of appropriate N rates. The N rate may need regular adjustments to provide for adequate growth and recovery at specific times of the year.
  • Any practice that promotes rooting activity especially into deeper portions of the soil profile will increase acquisition of both water and nutrients by the turf system. To that end, keeping N to its lowest possible level needed to maintain optimum turf function will promote greater rooting relative to shoot growth (high root-to-shoot ratio) and increase nutrient and water use efficiency.

Current fertilizer guidelines may call for as much as 4 lbs. N per 1000 sq. feet per season for the turfgrass species present on a site. Guidelines for N input to turf are exactly that: guidelines. If turf of acceptable quality can be maintained at a rate lower than 4 lbs. N per 1000 sq. feet per season, then reducing the rate is justified and helps to reduce labor, fertilizer cost, and the potential for excess nutrients in the system.

Lower N rates may be possible where:

  • fertile loam soils are present
  • traffic is not intensive
  • higher height of cut is practiced
  • grass clippings are returned
  • turf is under shade
  • turf is not irrigated
  • turf is older and well-established
  Kentucky bluegrass  Perennial ryegrass  Tall fescue Fine fescues
Table 14. Typical nitrogen fertilizer rate ranges for common cool-season lawn grasses.
Fertility level  med - high med - high med - high low - med
Lbs. N per 1000 sq. ft. per season  2 - 4 2 - 4 2 - 4

0 - 2

NITROGEN FREQUENCY

Figure 11. Key stress preconditioning periods for cool season grasses. Determine the frequency of fertilizer applications based on the nitrogen characteristics of the fertilizer material.

  • Fertilization programs for lawns may vary from 1 to 4 or more applications per season, depending on several factors, including the nutrient release characteristics of the material used.
  • Fertilization programs utilizing only WSN are not suggested for lawns.
  • Multiple, frequent applications of very small rates (0.1 to 0.2 lbs. of N per 1000 sq. feet per application of WSN sources, referred to as spoon-feeding) are sometimes utilized on heavily used, high value turf (e.g. sports fields and golf course tees and greens) where rapid and complete uptake of fertilizer nutrients is important for controlled plant growth and recovery, and where the presence of well drained soils requires strategies to reduce leaching potential. Such labor intensive spoon-feeding programs are seldom appropriate for lawn turf.
  • The proportion of SRN in a fertilizer should be increased during the pre-stress period approaching the summer (i.e. June) or during the summer to protect against foliar burn and surge growth. Similarly, for intensively used turf such as sports turf, more SRN in a fertilizer is needed in mid-fall during the pre-stress period preceding low temperatures when plants are acclimating (conditioning) to cold stress. It is extremely important to keep WSN as low as possible during these pre-stress periods.

NITROGEN TIMING

Time fertilizer applications so that maximum nitrogen availability corresponds with periods of active turfgrass growth.

  • An N application during late spring (approximately Memorial Day) is widely practiced in order to enhance quality going into summer and to encourage growth before the high temperatures and moisture stress of summer occur. It is important that such an application does not stimulate the turf into growth during the stressful summer months (especially if irrigation is not available). Fertilizer applied at this time should contain a high percentage of SRN (minimum of 50% or more).
  • Figure 12. Periods of peak nitrogen use for cool season grasses. Fertilization during July and August should be approached cautiously in order to avoid excess growth during periods of high temperature and moisture stress. If fertilization during the summer is necessary, it is recommended to have not less than 50 percent of the total N as SRN, though not less than 75% of total N as SRN is preferable.
  • Application of fertilizer materials should be avoided on non-irrigated turf in summer and during times of high temperature stress or moisture stress. More flexibility is possible on irrigated sites.
  • The late August/early September (approximately Labor Day) fertilization period is the most important for cool season grasses. Recovery from summer stress injury as well as increased shoot growth from tillering and rhizome production are enhanced by sufficient N availability throughout the fall.
  • Some sophisticated management programs employ what is referred to as a late season (or late fall) fertilizer application. The proper approach for late season fertilization is to apply after the last mowing after shoot growth has stopped but before the turf has lost green color.
    • The correct timing of late season fertilization, which can vary considerably from year to year depending on prevailing conditions, is extremely critical in order to realize positive benefits. Incorrect timing can stimulate turf into undesirable growth immediately prior to the onset of low temperature stress, and/or increase the potential for nutrients to move off-site due to leaching or runoff.
    • Because of the delicate nature of late season fertilization, it is more appropriate for specific management objectives on intensively-used, high value turf areas (e.g. sports fields and golf courses). Late season fertilization is normally not warranted for less intensively managed sites.
    • The success of late season fertilization is dependent on proper late summer-early fall fertilization.
    • Late season fertilization is best practiced by an experienced turf manager.
    • Late season fertilization should be avoided in areas that are or may be environmentally sensitive.
  • Some programs also employ an early spring application of N. This application is used primarily to enhance quality and early spring growth at the time when pre-emergence weed control materials are being applied. This application is also used to stimulate growth and enhance recovery of high-use turf (e.g. baseball and soccer fields and parks).
  • Do not apply fertilizer or other nutrient containing materials to drought dormant, cold dormant, inactive or otherwise brown turf. Do not apply fertilizer to frozen ground.
Time of year Number of N applications
Table 15. Suggested options for timing, rate, and % SRNa for N applicationsb based on number of applications per year (lbs. N/M = lbs. of nitrogen per 1000 square feet).
1x/yr 2x/yr 3x/yr 4x/yrc 4x/yrc
Spring (after ~50% green- up)   50-100% SRN
1.0 - 1.5 lbs. N/M
25-50% SRN
0.75 - 1.0 lbs. N/M
25-50% SRN
0.75 - 1.0 lbs. N/M
 
Late spring/ early summer     50-75% SRN
0.75 - 1.0 lbs. N/M
25-50% SRN
0.75 - 1.0 lbs. N/M
25-50% SRN
0.75 - 1.0 lbs. N/M
Summer (irrigated turf only)       50-75% SRN
0.75 - 1.0 lbs. N/M
50-75% SRN
0.75 - 1.0 lbs. N/M
Late summer/ early fall (~Labor Day) 75-100% SRN
1.0-2.0 lbs. N/M
50-100% SRN
1.0 - 1.5 lbs. N/M
25-50% SRN
1.0 - 1.5 lbs. N/M
25-50% SRN
0.75 - 1.0 lbs. N/M
25-50% SRN
0.75 - 1.0 lbs. N/M
Late fall (late season)d         25-50% SRN
0.75 - 1.0 lbs. N/M
TOTAL ANNUAL N 1.0 - 2.0 lbs. N/M 2.0 - 3.0 lbs. N/M 2.5 - 3.5 lbs. N/M 3.0 - 4.0 lbs. N/M 3.0 - 4.0 lbs. N/M
  1. Ranges for slow-release nitrogen (% SRN) content are approximate guidelines. Specific SRN percentages may vary from commercially available products by as much as 5% (plus or minus). Use higher SRN content when available, and especially on sandy root zones or during stress and pre-stress periods.
  2. Specific N rates may vary based on several factors including turfgrasses present, management, and turf use. For predominately fine fescue turf or shaded sites use lower listed N rates.
  3. Programs utilizing 4 or more N applications per year are best suited for intensively used, high-value turf.
  4. Final application made after last mowing while grass is still green. As noted in the text, not necessary for most lawns and not appropriate for environmentally sensitive sites.