Surface organic matter in bermudagrass greens: A primary ... - GCSAA

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RESEARCHPrimary stressesI propose that organic matter accumulationand associated adverse soil physicalresponses are the primary stresses on ultradwarfbermudagrasses, because in mostinstances they occur before other commonlyreported stresses are observed. As with creepingbentgrass, subsurface layers of fines, surfacecompaction or a push-up green with toomany fines can create the same high-moistureand low-oxygen conditions caused by excessiveorganic matter. However, on high-sandgreen mixes, these conditions are most oftencaused by too much organic matter. Push-upbermudagrass greens frequently containmore clay and silt than high-sand greens, butvigorous topdressing often creates a sand cap,which can exhibit surface organic matteraccumulation just as a high-sand green does.Conditions favoring organic matterOn bermudagrass greens, organic matteraccumulation seems to be associated with oneor more of the conditions discussed below.Grass typeUltradwarf bermudagrass cultivars have atendency to accumulate organic matterwithin the surface zone (Figure 2). The densesurface and very close mowing heights usedon these grasses also make incorporation oftopdressing difficult. Seashore paspalum ongreens is less prone to developing excessivesurface organic matter than the ultradwarfbermudagrasses if it is maintained accordingto paspalum requirements and not as abermudagrass (2). For seashore paspalum,reducing nitrogen rates and irrigating on adeeper and less frequent basis are effective inpreventing excessive organic matter.Anaerobic conditionsWhenever oxygen within the 0-2-inch (0-5.1-centimeter) zone is limited enough to createanaerobic conditions, decomposition oforganic matter greatly declines because microbialactivity slows. Thus, when organic matteraccumulation is sufficient to limit aerationwithin this zone, reduced microbial activitywill probably cause even more accumulation.Prolonged wet weather or daily irrigation isespecially conducive to enhancing organicmatter buildup because the moist conditionsmaintain a consistently anaerobic status.On a high-sand green, soil oxygen may beadequate for deeper roots below the zone ifcultivation operations have created sufficientFigure 1. Thinning associated with bermudagrass decline of Cynodon species turf (Reprinted from Compendium ofTurfgrass Diseases, 2nd ed., APS Press, 1992).macropores to allow oxygen diffusion acrossthe surface zone. However, within the surfacezone, conditions are more anaerobic.Anaerobic conditions at the surface often stimulategrasses to produce adventitious or surfaceroots, which add to the surface organic matterbiomass. Poor internal drainage deeper withinthe soil profile would cause a waterlogged conditionat the surface and foster organic matteraccumulation during wet weather.Acidic conditionsAn acidic condition in the first inch (2.5centimeters) of the soil surface can occur onacid sands in humid regions, when acidic nitrogenfertilizers are used or when routine wateracidification is practiced. When soil is sampledat 3 to 4 inches (7.6-10.2 centimeters), low pHat the surface may not be apparent. Soil pHbelow 5.5 greatly restricts the growth of bacterialpopulations that are important indecomposing more-resistant organic matter.Poor air drainage or humid weatherWhen the surface zone does not rapidlydry after irrigation or rain, longer periods ofanaerobic conditions are favored (Figure 3).Secondary problems or symptomsDiverse secondary problems or symptomsarise in response to the adverse soil physicalconditions created by excessive organic matter,and several can occur at one time. Oftena green fails because of multiple primary andsecondary problems rather than a singlecause. Common secondary problems includethe following (3,6).Limited rooting depthA tendency for reduced rooting depthwithin one to two years after establishmenthas been commonly observed on bermudagrassgreens with high sand content, eventhough bermudagrasses have the geneticcapability to be deep rooted. Limited rootdepth may be a result of one or more factors.Organic matter can limit oxygen diffusionacross the surface zone into the deeper rootmedia, especially in wet weather. Rootsrequire oxygen 24 hours a day, and oxygendemand by the plant and microorganisms canbe high in summer. Without adequate oxygen,root growth rate declines, and undersevere oxygen stress, roots may die back.Creating temporary macropores by cultivationoperations across this zone can helpmaintain better oxygen diffusion. Integratingsufficient topdressing sand into the zone canenhance water and air flow.Limited oxygen within the zone stressesroots and may cause root pruning. This obviouslyinfluences deeper roots.May 2004GCM 103
RESEARCHFigure 2. Ultradwarf bermudagrass greens show evidenceof excessive organic matter in the top 2 inches ofsoil two years after establishment.Prolonged cloudy weather and close mowingmay contribute to root decline by providingless favorable conditions for producing theplant carbohydrates (food) necessary to maintaincurrent roots or generate new ones (3).Root and crown tissue injuryAll living tissues within the surface 2inches (5.1 centimeters) must have adequateoxygen to maintain viability. Prolongedexposure to low oxygen can cause root andlower crown injury, making these tissues lessviable and more susceptible to root pathogensand drought stress if they occur. Thus, lack ofoxygen and not just root-rot organisms maycause root rot or lower crown rot.Inhibition of nutrient and water uptakeThe combination of restricted rootingdepth and injury to root or crown tissueinhibits nutrient and water uptake. Whenexposed to low-oxygen conditions, even relativelyhealthy roots are less efficient at nutrientand water uptake. When roots become so limitedthat frequent irrigation becomes necessary,the surface zone then remains moist most ofthe time, which promotes low oxygen and limitedrooting conditions. Although cationexchange capacity (CEC) tends to be adequatefor nutrient retention within 2-4 centimoles/kilogram,most of the CEC resideswithin the surface 0-2 inches (0-5.1 centimeters)in the organic matter. When roots areshallow, fertilization is a challenge. Duringdrier periods, frequent irrigation is required toavoid drought stress. With a limited root systemand roots that may not be very healthy,spoon-feeding all nutrients is necessary.Bermudagrass decline and root rotRoot rot associated with Gaeumannomycesspecies is probably the most recognized root-rot104 GCM May 2004Photos courtesy of P. O’Brien and C. Hartwigerinjury, but other causes may be anthracnosebasal rot, Helminthosporium species and Pythiumspecies. Prolonged periods of rainy weather willtrigger root-rot activity. The combination of prolongedcloudy and wet weather is especially conduciveto root rot development: Cloudy weathercan place carbohydrate stress on roots while fosteringa moist surface, which favors the diseaseorganism and low-oxygen stress on the root andlower crown tissues (3). A high-sand surface conditionwould allow excess water to drain, the surfaceto dry, and oxygen within the surface to beadequate.The important issue is whether thesuperintendent approaches the problembelieving the primary problem is a rootpathogen or excessive organic matter that createsconditions conducive to root rot. Whenroot-rot organisms are active, it is importantto focus on treating the disease, but, in thelonger term, controlling the organic mattermay be the way to reduce the frequency ofthe problem and fungicide treatments.In my experience, the organic matter zoneis very effective in screening the fungicidefrom the organisms below the surface zone orin the lower region of the zone. Thus, superintendentsmay wish to consider a solid tineoperation ( 1 ⁄4 inch [6.4 millimeters] in diameter)or a HydroJect operation just beforefungicide application to allow the fungicide topenetrate though the zone.Other secondary problemsOther secondary problems include thefollowing.• Algae.• Soft, soggy surfaces. If water infiltrationdeclines too much, the surface may remaintoo soggy or waterlogged and prone to trackingfrom vehicles.• Greater potential for intracellular freezingdamage during wet periods of late winterand early spring may occur in the morenorthern areas of bermudagrass adaptation.• Excessive drying, desiccation and extra-cellularfreezing of tissues in the upper surface ofthe high-organic-matter zone may increase inregions with cold winters without snow cover.• Light frequent irrigation with saline watercan lead to salt accumulation in the organicmatter zone and root stress.ManagementThe primary reason for this article was toexplain the adverse effects of excessive organicmatter in the surface zone in the context ofproblems observed on bermudagrass greens. Itis important for superintendents to determinewhether a problem is a secondary problem orsymptom, or whether it is the result of excessiveorganic matter in the surface.Proper identification of the primary orunderlying problems allows superintendentsto focus on preventing future problems withless emphasis on crisis management. It isimportant to note, however, that environmentalconditions alone may cause a diseasethat is usually associated with excessiveorganic matter, even though excessive organicmatter is not present. In such a case, the diseasewould be considered a primary problem.Should all organic matter be removed?As previously noted, organic matter contentof more than 4-5% (by weight) favorslow aeration and low water infiltration. Doesthis mean that all organic matter should beremoved from putting greens? No!Along the Gulf coast from Florida toLouisiana, golf courses that exhibited very littleorganic matter accumulation still had1.5% organic matter content in the 0-2 inch (0-5.1 centimeter) surface zone, and this organicmatter was live turf. Living and dead organicmatter are very difficult to separate, but a backgroundlevel of 1.5% is reasonable. Thesecourses also had very low (
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Surface organic matter in bermudagrass greens: A primary ... - GCSAA

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