RESEARCH

Surface organic matterin bermudagrass greens:A primary stress?Excessive organic matter can deprive bermudagrass greens of oxygen and nutrients.

Robert N. Carrow, Ph.D.

In the preceding article, "Surface organicmatter in creeping bentgrass greens," I discussprimary and secondary problems associatedwith organic matter dynamics in the surfacesoil zone (0-2 inches [5.1 centimeters)) ofcreeping bentgrass putting greens andresearch on managing these stresses (1). Manyof the creeping bentgrass and bermudagrasscultivars used on putting greens have a strongtendency to accumulate organic matterwithin the surface zone (1,3,6). The preced-ing article also presents an overview of pastand current research related to the influenceof organic matter content on soil physicalproperties of putting greens constructedwith a high-sand-content root zone (hereafterreferred to as "high-sand greens").

HypothesisThe hypothesis I present here is that

adverse soil physical conditions triggered byexcessiveorganic matter accumulation in ultra-dwarf bermudagrass greens are usually theprimary stresses that lead to many of the prob-lems observed on these greens. Examples ofthese problems are root-rot organisms (forexample, Gaeumannomyces species,which causebermudagrass decline) (Figure 1); Curvulariaspecies; root rot; limited rooting depth and asso-ciated irrigation and fertilization challenges, ete.

The distinction between primary stressesand secondary stresses or symptoms is veryimportant for the development of successfulmanagement protocols. A primary stress is acondition that causes direct physiological orpest stress on a plant, but it also creates con-ditions favorable for the occurrence of othersecondary stresses. Symptoms or secondarystresses are often the problems that are seenby superintendents and golfers. In turf grassmanagement, secondary stresses must bedealt with when they occur, but to preventfrequent reoccurrence, the long-term main-

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tenance goal should be elimination of the pri-mary stresses or underlying causes.

Other scientists or superintendents maynot agree about which stresses are primaryand which are secondary for bermudagrassgreens, but I hope to stimulate dialogue andresearch directed to these issues. The mostconstructive dialogue takes place when analternative hypothesis is presented thatexplains the observed responses.

Organic matter dynamicsThe first problem

In the preceding article, I noted two typesof organic matter problems on creeping bent-grass greens (1,5). One problem occurredwhen direct high-temperature stress causedrapid deterioration of creeping bentgrass sur-face roots and changed the nature of theorganic matter from live roots with a struc-ture to gelatinous dead tissue. When liveroots are present, higher organic matter con-tent may be present without causing adversesoil physical responses, such as low aeration

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Excessive organic matter accumula-tion is a primary stress on bermuda-grass greens that can lead to sec-ondary stresses.

Secondary stresses include: exces-sive moisture retention, low oxygenlevels, reduced water infiltration anddecreased gas exchange.

Aggressive topdressing along withhollow-tine aeration helps preventorganic matter buildup.

Managing the primary stress canreduce secondary stresses.

or low water infiltration. After the roots havedied over a period of a few days, the gelati-nous dead tissues can rapidly seal the surface.

Because bermudagrasses have superiorhigh-temperature tolerance, high temperatureswould not be expected to kill bermudagrassroots. However, the cool-season grass used inoverseeding bermudagrass greens could sufferthis injury in late spring. In that situation, thebermudagrass green would exhibit low infil-tration and a soggy surface for a period of timeuntil the microorganisms break down the freshorganic matter. During this period oflow oxy-gen, some bermudagrass root damage couldoccur. Salt or other stresses could also causerapid death of overseeded grasses on occasionand induce temporary low-oxygen levels.

The second problemThe second organic matter problem,

excessive accumulation of organic matter, ismore likely to be serious on bermudagrassgreens and sometimes fairways and tees. Forall grass species, organic matter levels greaterthan 4-5% (by weight) favor adverse soilphysical responses. At these levels, the num-ber of larger macropores (aeration porosity,drainage porosity) rapidly starts to decline,and moisture retention increases at theexpense of aeration within the 0-2-inch sur-face zone (1,5). With fewer macropores, waterinfiltration and oxygen diffusion declineacross the zone. Thus, common themes areassociated with excessive organic matter in thesurface zone of both creeping bentgrass andbermudagrass greens with high sand con-tent(1,5):• excess moisture retention• low aeration (oxygen) within the zone• reduced water infiltration• decreased gas exchange across the zone

unless cultivation creates temporarymacropore channels for water/air flow

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Figure 1. Thinning associated with bermudagrass decline of Cynodon species turf (Reprinted from Compendium ofTurfgrass Diseases, 2nd ed., APS Press, 1992).

Primary stressesI propose that organic matter accumula-

tion and associated adverse soil physicalresponses are the primary stresses on ultra-dwarf bermudagrasses, because in mostinstances they occur before other commonlyreported stresses are observed. As with creep-ing bentgrass, subsurface layers of fines, sur-face compaction or a push-up green with toomany fines can create the same high-moistureand low-oxygen conditions caused by exces-sive organic 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 matter

accumulation seems to be associated with oneor more of the conditions discussed below.

Grass typeUltradwarfbermudagrass cultivars have a

tendency 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.I-centimeter) zone is limited enough to cre-ate anaerobic conditions, decomposition oforganic matter greatly declines because micro-bial activity slows. Thus, when organic mat-ter accumulation 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 sufficient

macropores to allow oxygen diffusion acrossthe surface zone. However, within the surfacezone, conditions are more anaerobic.Anaerobic conditions at the surface often stim-ulate grasses to produce adventitious or surfaceroots, which add to the surface organic matterbiomass. Poor internal drainage deeper withinthe soil profile would cause a waterlogged con-dition at the surface and foster organic matteraccumulation during wet weather.

Acidic conditionsAn acidic condition in the first inch (2.5

centimeters) of the soil surface can occur onacid sands in humid regions, when acidic nitro-gen fertilizers 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 bac-terial populations that are important indecomposing more-resistant organic matter .

Poor air drainage or humid weatherWhen the surface zone does not rapidly

dry after irrigation or rain, longer periods ofanaerobic conditions are favored (Figure 3).

Secondary problems or symptomsDiverse secondary problems or symptoms

arise in response to the adverse soil physical

conditions created by excessive organic mat-ter, 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 depth

within one to two years after establishmenthas been commonly observed on bermuda-grass greens 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 oxy-gen, root growth rate declines, and under. (

severe oxygen stress, roots may dIe back.Creating temporary macropores by cultiva-tion operations 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 obvi-ously influences deeper roots.

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Figure 2. Ultradwarf bermudagrass greens show evi-dence of excessive organic matter in the top 2 inches ofsoil two years after establishment.

Prolonged cloudy weather and close mow-ing may contribute to root decline by provid-ing less favorable conditions for producing theplant carbohydrates (food) necessary to main-tain current roots or generate new ones (3).

Root and crown tissue injuryAll living tissues within the surface 2

inches (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 rooting

depth and injury to root or crown tissueinhibits nutrient and water uptake. Whenexposed to low-oxygen conditions, even rela-tively healthy roots are less efficient at nutrientand water uptake. When roots become so lim-ited that frequent irrigation becomes necessary,the surface zone then remains moist most ofthe time, which promotes low oxygen and lim-ited rooting conditions. Although cationexchange capacity (CEC) tends to be adequatefor nutrient retention within 2-4 centi-moles/kilogram, most of the CEC resideswithin the surface 0-2 inches (0-5.1 centime-ters) in the organic matter. When roots areshallow, fertilization is a challenge. Duringdrier periods, frequent irrigation is required to

avoid drought stress. With a limited root sys-tem and roots that may not be very healthy,spoon-feeding all nutrients is necessary.

Bermudagrass decline and root rotRoot rot associated with Gaeumannomyces

species is probably the most recognized root-rot

104 GeMMay 2004

Photos courtesy of p, 0 'Brien and C, Hartwiger

injury, but other causes may be anthracnosebasal rot, Helminthosporium speciesand Pythiumspecies. Prolonged periods of rainy weather willtrigger root-rot activity.The combination of pro-longed cloudy and wet weather is especiallycon-ducive to root rot development: Cloudy weathercan place carbohydrate stress on roots while fos-tering a moist surface, which favors the diseaseorganism and low-oxygen stresson the root andlower crown tissues (3).A high-sand surface con-dition would allow excesswater to drain, the sur-face to 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 cre-ates conditions 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, super-intendents may wish to consider a solid tineoperation (X inch [6.4 millimeters] in diam-eter) or a HydroJect operation just beforefungicide application to allow the fungicide to

penetrate though the zone.

Other secondary problemsOther secondary problems include the

following.• Algae.• Soft, soggy surfaces. If water infiltration

declines too much, the surface may remaintoo soggy or waterlogged and prone to track-ing from vehicles.

• Greater potential for intracellular freezingdamage during wet periods of late winterand early spring may occur in the morenorthern areas ofbermudagrass adaptation.

• Excessive drying, desiccation and extra-cel-lular freezing 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 to

explain 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 exces-sive organic 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 environ-mental conditions alone may cause a diseasethat is usually associated with excessiveorganic matter, even though excessive organicmatter is not present. In such a case, the dis-ease would be considered a primary problem.

Should all organic matter be removed?As previously noted, organic matter con-

tent of 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 lit-tle organic matter accumulation still had1.5% organic matter content in the 0-2 inch (0-5.1 centimeter) surface zone, and this organicmatter was live turE Living and dead organicmatter are very difficult to separate, but a back-ground level of 1.5% is reasonable. Thesecourses also had very low « 1.0 centimoles/kilo-

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Photo courtesy of R.N. Carrow

Figure 3. Greens with poor air circulation and drainage often exhibit excessive organic matter in the surface layer.Excessive organic matter induces high moisture and lower oxygen content in the root zone, inhibiting microbial decom-position of the organic matter.

gram) CEC because little dead organic matterwas present at CEC sites and essentially no claywas found. Thus, some organic matterenhances CEC status.

Thatch or mat?Is the surface zone of high organic mat-

ter thatch or mat? Thatch is a zone of deadand living organic matter without apprecia-ble sand integrated into it. Sometimes a'thatch zone created during grow-in becomesburied and remains as a thatch layer problem.Mat is an organic zone that has considerablesand integrated within it, with sand as thedominant matrix.

From field observations, it appears that thequantity of sand topdressing sufficient to main-tain a uniform, stable surface with goodputting speed is often less than the quantity oftopdressing required to create a true mat. Thesurface zone of many greens appears to be a"near mat," but with insufficient sand to negatethe organic matter. When sand is not main-tained as the primary matrix, the result is con-siderable sand floating in the organic matter. Inthat case, the organic matter, not the sand, con-trols the aeration and moisture status.

Management practicesSurface organic matter accumulation is

managed by a combination of the followingpractices.

Dilution. Dilution of the surface organicmatter through aggressive topdressing, espe-cially in combination with hollow-tine aera-tion and venting operations, is the backboneof any successful program. A recent article (5)provides excellent options and guidelines thatare appropriate for bermudagrass and creep-ing bentgrass greens. The suggested targetrange for annual topdressing on many sites,40-50 cubic feet/l,OOO square feet (121.9-152.4 cubic meters/hectare), is based on his-torical rates that were used until a few yearsago when ultralight but more frequentapplications became the norm and resultedin less total sand applied.

Using adequate hollow-tine core aerationoperations and venting operations to maintainmacropores across the surface organic matterzone at all times is important for ensuringoxygen diffusion across the zone to reachunderlying roots. Developing and maintain-ing deeper roots allows less-frequent irrigationso that the surface zone has the opportunityto dry somewhat, which greatly improves oxy-gen status within the zone. If topdressing and

cultivation operations are insufficient to con-trol the surface organic matter zone quickly,the result will be shallow roots, which requirelight, frequent irrigation and frequent spoon-feeding. This situation fosters even moreorganic matter accumulation.

Surface pH levels. The surface pH shouldbe higher than 5.5 to allow adequate bacter-ial populations for organic matter decompo-sition. One practice that has beenrecommended for bermudagrass decline hasbeen the use of acidifying fertilizers. Oneresponse that occurs on soils with at leastsome clay is that at pH below 5.5, more sol-uble manganese is often observed. However,on sands with CEC from primarily organicmatter, this is not the case. Adequate man-ganese is important for suppressing take-allpatch (3,4). If the surface pH is below 5.5,the reason for a low pH should be deter-mined. Water acidification can cause a lowpH at the surface while also adding appre-ciable soluble sulfur. If anaerobic conditionsstart within or below the surface organic mat-ter zone, the presence of soluble sulfur willgreatly increase the chance for black layer (6).

Take-all. On sites with an existing organicmatter problem and a history of take-all, itmay be worthwhile to test not only foliar appli-cations of manganese, but also some granularapplications to achieve higher manganese lev-els within both the plant and organic zone.

Secondary stresses. If excessive organicmatter stimulates a particular secondarystress, then practices to control the secondarystress will be of immediate concern.

SummaryIn summary, the new ultradwarfbermuda-

grasses used on putting greens have a strongtendency to accumulate organic matter withinthe surface zone, and certain stresses are fre-quently observed on these grasses (decreasedrooting over time, root-rot pathogens, others).The challenge for the superintendent is toidentify the primary or underlying stresses andmanage them aggressively.

References1. Carrow, R.N. 2004. Surface organic matter in bent-

grass greens. Golf Course Management 72(5):96-101.

2. Duncan, R.R., and R.N. Carrow. 2000. Seashorepaspalum: The environmental turfgrass. Wiley,Hoboken, N.J.

3. Elliot, M. 2001. Got root rot? Don't panic. Florida TurfDigest March/April 20-23.

4. Heckman, JR., B.B. Clark and JA. Murphy. 2003.Optimizing manganese fertilization for the sup-pression of take-all patch disease on creeping bent-grass. Crop Science 43:1395-1398.

5. O'Brien, P., and C. Hartwiger. 2003. Aeration andtopdressing for the 21st century. USGA GreenSection Record 41 (2):1-7.

6. Unruh, JB., and S. Davis. 2001. Diseases and heatbesiege ultradwarf bermudagrasses. Golf CourseManagement 69(4):49-54.

Robert N, Carrow, Ph.D. (rcarrow@griffin.uga.edu), is aprofessor of turfgrass science and a research scientistat the Georgia Experiment Station, University of Georgia,Griffin. Carrow was an instructor at GCSAAeducationseminars in San Diego, and he is one of the subjectmatter experts for GCSAA'sonline WAlE.R. course.

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