Late Fall Fertilization of Kentucky Bluegrass

not result in higher color ratings in the spring and resulted in inefMany turfgrass managers apply a portion of the total yearly N to cool-season turfgrasses in the late fall (November). The purpose of this field study was to compare fertilization programs with and without N applications in November using both slow-release and soluble N sources. Turfs of two different cultivars of Kentucky bluegrass (Poa pratensis L. cv. Baron and cv. Newport) growing on a Flanagan silt loam (fine, montmorillonitic, mesic Aquic Argiudoll) received 10 fertilization programs utilizing urea, isobutylidene diurea (IBDU), or sulfur-coated urea (SCU). Urea was applied four times per year with either a spring application or a late-fall application combined with applications in early June, mid-July, and early September (171-196 kg N ha1 yr '). For IBDU and seu, application dates and N rates (kg haI) consisted of June 98 + September 98, June 98 + November 98, and June 49 + September 49 + November 74. The turfs were rated for color for 3 yr, and clipping weights were determined weekly for the final 2 yr of the study. Results were generally similar for both cultivars, except fewer significant differences in spring color ratings were found on Newport. An application of urea in November, without a subsequent spring fertilization, resulted in higher turf color ratings in the early spring but lower turf color ratings in May and June, compared to turf receiving a spring fertilization. Results indicate that a late-fall application of urea may not eliminate the need for spring fertilization but may allow a reduction in the amount of N applied in spring. Turfs fertilized with seu in November received higher color ratings in the spring than did turf fertilized with seu in September. With IBDU, the June + September program resulted in the highest number of ratings with acceptable turf color. November IBDU applications did 466 ficient use of the N applied. Additional Index Words: Isobutylidene diurea, Sulfur-coated urea, Urea, 'Baron', 'Newport', Nitrogen, Turfgrass, Poa pratens;s L. T HE PRACTICE of applying a portion of the total yearly N to cool-season turfgrasses in late fall (November) has gained acceptance by turfgrass managers in Illinois and surrounding states. The major benefits of a late-fall N fertilization are improved (darker green) turfgrass color in early spring and more extensive root growth. Koski and Street (1985), using a rhizotron in Ohio, have reported that root growth is more extensive because the improved spring color resulting from late-fall fertilization reduces the need for an early spring N application. Spring N fertilization can reduce the root growth of cool-season turfgrasses by promoting shoot growth. Research on the use of late-fall N has been conducted in states with maritime (Ledeboer and Skogley, Dep. of Horticulture, Univ. of Illinois, Urbana, IL 61801. Contribution from the Dep. of Horticulture, Univ. of Illinois, Urbana. This study was part of Project no. 65-267 of the Agric. Exp. Stn., Univ. of Illinois, Urbana. Received \8 May \987. ·Corresponding author. Published in Agron. J. 80:466-47\ (\988). 467 WEHNER ET AL.: FERTILIZATION PROGRAMS FOR KENTUCKY BLUEGRASS 1973; Wilkinson and Duff 1972) or transitional clipH, soil temperature, soil moisture, and particle size mates (Hanson and Juska, 1961; Powell et aI., 1967a, (Hamamoto, 1966). Dissolution of the fertilizer par1967b) where it was possible to maintain green turfticle is followed by hydrolysis of the molecule to regrass color almost year-round. Initial hesitancy to use lease urea and isobutyraldehyde. Hughes (1976) inlate-fall N applications in the Midwest was based on dicated that, in alkaline soils, delayed plant response fear of reduced winter hardiness (Beard, 1973; Carroll to IBDU fertilization can occur and that particle size and Welton, 1939). However, field observations have greatly influences the release rate ofN. Lunt and Clark indicated that winter injury does not usually increase (1969), working with temperatures of 10 and 2re, with the use of late-fall N fertilization. reported that soil temperature did not have a major The development of new sources of N and the effect on the conversion ofIBDU-N to soluble forms. growth of the professional lawn-care industry have Allen et ai. (1971), working with several experimengenerated the need for more information on the practal seu sources, found that N release from SeD was tice of late-fall N fertilization. affected by coating weight, placement of the fertilizer Sulfur-coated urea (SeU) and isobutylidene diurea particle (surface vs. incorporated into soil), and tem(IBDU) are two popular slow-release N sources used perature. Dissolution of the granules layered in the for turfgrass fertilization. Evaluations of these fertilsoil was relatively fast at 20 or 30°C but relatively izers on Kentucky bluegrass (Wilkinson, 1977; Wadslow at lOoe. dington et aI., 1977; Hummel and Waddington, 1984; The lawn-care industry relies on the use of urea for Moberg et aI., 1970) have not involved a late-fall apturfgrass fertilization because of its low cost, solubility plication. Yolk and Horn (1975) made large (200 or for tank mixing, and relatively low salt index com300 kg N ha I) single applications of IBDU and an pared to other quick-release N sources. Information experimental application ofSeU (9% dissolution rate) is needed by the lawn-care industry to determine the to bermudagrass [Cynodon dactylon (L.) Pers.] in effectiveness oflate-fall fertilization in altering the need Florida. They reported that the IBDU did not show for spring applications ofN. Spangenberg et a1. (1986) the cold weather depression of latent-N release exhibevaluated fertilization programs similar to those used ited by seu. However, these results must be viewed by the lawn-care companies. Their treatments, howwith the knowledge that the currently available comever, did not involve an N application in November. mercial seu products for turfgrass have dissolution The purpose of this research was to evaluate fertilrates of 30 to 35%. Moberg et a1. (1970) found better ization programs with or without a late-fall applicacool-weather response with IBDU than with ureafortion of IBDU, seu, and urea in comparison to promaldehyde fertilizers. grams with a September application of N. Hummel and Waddington (1984), in evaluating the use of seu with different dissolution rates, reported MATERIALS AND METHODS that the most uniform results were obtained with two This study was initiated on 7 Sept. 1982 and concluded or three split applications of 245 kg N ha I from feron 1 Nov. 1985. Adjacent pure stands of 'Newport' and tilizers with dissolution rates of 25 and 30%. Wilk'Baron' Kentucky bluegrass growing on a Flanagan silt loam inson (1977) evaluated multiple rates and timing of were used for this research. Plots were established for each application of IBDo. He reported that two applicacultivar, with an individual plot size of 0.9 by 3.1 m and tions of IBDU were required for the most uniform three replications in a randomized complete-block design. turf-quality response. The cultivars were established in June 1982. An application The rate of release of N from IBDU is affected by of 49 kg N hafrom 10-4.4-8.3 (N-P-K) was made at es' Table 1. Application timings and N rates for fertilization programs and number of ratings when treatment resulted in acceptable (rating 2: 7.0) color. Color was rated on a scale of 1 to 9 with 1 = yellow turfgrass color and 9 = dark green color. Color was rated on 64 dates over 3 yr.