Bentgrass Response to Dormant Applied Milorganite
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Bentgrass Response to Dormant Applied Milorganite By Dr. Wayne R. Kussow Department of Soil Science University of Wisconsin-Madison The primary benefits of dormant influences release of N from dormant spring, the study was conducted on application of Milorganite in Wiscon- applied Milorganite and what are the two sites approximately 200 feet sin are hastening of the breaking of spring responses of creeping bent- apart. One site was on a uniform 5% dormancy by turfgrass and earlier grass response to these influences. slope and the other on an area with spring greenup. Experience has The study was established in the virtually no slope. During snowmelt, shown that the effectiveness of the fall of 1993 in an effort to define the water ponded to a depth of more than practice varies from season-to-season factors that regulate bentgrass 6 inches on the flat site but quickly and site-to-site, even within the con- response to a dormant Milorganite ran off the sloping site. Hence, while fines of a given golf course. One rea- application. One assumption made in it was not measured, one can assume son is varying viewpoints on what designing the study was that turf- that a moisture differential existed constitutes a dormant fertilization grass uptake of N from Milorganite between the two sites in early spring. and, therefore, the time of fertilizer must be preceded by microbial release Ask any Soil Scientist and they'll application. of organic N. This assumption leads tell you that for all practical purposes One definition of dormant fertiliza- to the hypothesis that whatever en- soil microbial activity virtually tion is application of fertilizer after hances springtime warming of turf ceases when soil temperature drops turf shoot growth has ceased. This oc- will favor microbial release of Milor- below 50 degrees. If this were entire- curs when the turf is subjected to ganite N and, therefore, bentgrass ly true, then why snowmold? This line several consecutive days whose mean response. of reasoning led to the hypothesis that daily temperatures are 50 degrees F Absorption of radiant energy from there is measurable microbial release or less. In southern Wisconsin, this the sun is what causes soil to warm of Milorganite under snow cover, lb often occurs around October 15 to 20. in spring. Sunny days contribute more gain evidence for this, Milorganite Fertilization at this time is what I call energy than do cloudy days and favor was sealed in porous polyethylene late fall fertilization. I view dormant the warming process. But how rapid- packets that were placed on the turf fertilization as fertilizer application ly soil temperature rises also depends surface at the time of dormant fertili- that does not take place until the turf- on the nature of the surface that the zation and again immediately after grass has truly gone dormant; the solar radiation strikes and the heat snowmelt. Several packets were turfgrass shoots turn brown and dry capacity of the soil. Light-colored sur- placed in the plots to allow for due to nighttime temperatures well faces reflect more of the solar radia- removal at different times and anal- below freezing and the soil surface tion than do dark surfaces and slow ysis for the amount of organic N re- temperature is below 32 degrees F. soil warming. Once the radiant ener- maining in them. This technique This often does not occur in Wiscon- gy is absorbed at the turf surface, the allowed me to verify whether or not sin before the advent of permanent rate of soil warming is determined mineralization of organic N occurred snow cover. Thus, I also include in my primarily by its moisture content. It prior to snowmelt and to track miner- definition of dormant fertilizer that takes much more energy to warm a alization until such time that clipping which is applied when the weather wet soil than a dry soil because water N content could served that function. forecast leads one to believe that per- has a very high heat capacity. Tb help explain the effects of surface manent snow cover is imminent. In the present study, I varied the drainage, turfgrass clipping height With my definitions of late fall and color of the turf surface in two ways. and color of topdressing material on dormant fertilization, the distinguish- One was as simple as mowing the spring response to dormant applied ing feature is when fertilizer N up- bentgrass at two different heights in Milorganite, plot temperatures were take occurs. With late fall fertilization the fall. The more light, tan-colored measured at a V2-inch soil depth. This N uptake occurs yet that season. In grass on the surface after snowmelt, was accomplished with the ther- dormant application of a SRN such as the greater the amount of radiant mocouples connected to dataloggers Milorganite, fertilizer N uptake most energy that is reflected. I also varied that recorded soil minimum, maxi- likely does not take place until the fol- soil surface color by topdressing in the mum and mean temperature on a lowing spring. The purpose of the fall with sand, soil or charcoal. To cre- daily basis. present study was to determine what ate a difference in soil moisture in the (Continued on Page 31) Bentgrass Response mineralization. These were 0.0967 Fig. 1. Effects of surface drainage, mg N/degree rise in temperature for lb Dormant Applied height of cut and topdressing material the dormant applied Milorganite and on the mineralization of dormant applied Milorganite. 0.0670 mg N/degree for the spring ap- Milorganite — pllied Milorganite. What this sug- (Continued from Page 29) gests is that as soil temperatures rose, the rate of mineralization of dormant Mineralization of Dormant applied Milorganite was 43% faster Applied Milorganite than for spring applied Milorganite. Among the three treatments im- posed, surface drainage had the Fig. 2. Rates of mineralization of dor- greatest and most consistent in- mant and spring applied Milorganite. fluence on the amounts of Milor- ganite N mineralized during winter and the first 41 days following snow- April 1 to April 16 melt on March 5, 6 and 7. As antici- pated, N mineralization was greatest on the site with good surface drainage. Influences of height of cut did not become apparent until about April 1, when somewhat more miner- winter under the generous snow cover alization was observed at the %-inch on the plots. It is of interest to note cutting height than at the %-inch here that due to this deep and con- height. The effects of the topdressings tinous snowcover, soil temperatures at on Milorganite N mineralization were the y2-inch depth never dropped below variable over time. As a general rule, 25 degrees. On a percentage basis, topdressing with charcoal resulted in over-winter mineralization of the dor- as much or more N mineralization as mant Milorganite amounted to 10.5 did topdressing with sand or soil. to 12.3% of the N applied. The influences of surface drainage, Rates of dormant Milorganite N Bentgrass Greenup height of cut and color of the topdress- mineralization increased noticeably Regrowth of the bentgrass shoots ing material applied on Milorganite between April 1 and April 16 (Fig. 1). commenced on or about March 28, N mineralization were found to be ad- This was a period during which daily when mean daily soil temperatures ditive. Therefore, mineralization was average soil temperatures steadily in- were in the range of 34 to 36 degrees. greatest on the site with good surface creased from about 38 to 47 degrees.. Periodic color ratings then served to drainage in plots mowed at %-inch and By April 16, the amounts of dormant characterize the effects of the study topdressed with charcoal. Conversely, Milorganite mineralized ranged from variables on greenup. As in the case N mineralization was slowest on the 0.27 to 0.33 lb. N/M. It was at this of dormant Milorganite mineraliza- site with poor surface drainage on time that there was sufficient tion, the rate of greenup was fastest plots mowed at %-inch and topdressed regrowth of the bentgrass to begin where there was good surface with sand. The magnitude of the clipping collection. drainage, the grass had been mowed differences in Milorganite N miner- Unlike earlier in the season, be- the previous fall at %-inch, and it had alization between these best- and tween April 1 and 16 there was a been topdressed with charcoal (Fig. 3). worst-case situations is indicated in linear relationship between Milor- Slowest greenup was where surface Figure 1. ganite N mineralization rates and drainage was poor, the mowing height Soil temperatures did not rise above soil temperatures. This condition was %-inch and the turf was top- freezing until March 16, which was 9 presented the opportunity to address dressed with sand. However, differ- days after completion of snowmelt. another question I hoped the study ences in greenup between these Yet, by March 12, 0.16 to 0.21 lb/M of would help answer. The question is, "best" and "worst" sets of conditions dormant applied Milorganite N had "Does the overwintering of Milor- disappeared by April 20. While they already been mineralized (Fig. 1). ganite affect its springtime minerali- persisted, the best and worst condi- This is evidence that mineralization zation rates?" This is where analysis tion differences in spring greenup did occur between the time of appli- of the packets placed in the plots right amount to about one week in terms cation of the dormant Milorganite on after completion of snowmelt on of how long it took to achieve the November 23 and the time of snow- March 7 came into the picture.