The Hidden Costs of Organic Fertilisers in Turfgrass

The turf industry is under more and more pressure to use organic fertilisers and products.

Sustainability, regulatory trends, and consumer demand have driven this shift. However, for golf course supers and sports turf managers who manage sand-based rootzones, this move toward organic N sources has consequences that rarely feature in any marketing material.

The research tells a more complicated story than “organic is better.”

This article examines four related problems that can arise when you use organic fertilisers on intensively managed turfgrass:

• Increased disease development.
• Softer playing surfaces.
• Increased Poa annua levels, and
• Changes in dynamics between soil and water.

Organic Matter Build up: The Root of the Problem.

Sand-based greens and sports surfaces are designed to drain well. Their ability to drain depends on maintaining the macropore space in the upper rootzone.

Extensive research demonstrates that when you have high organic matter (OM) levels in the top 75 mm of sand based greens it reduces macroporosity, oxygen diffusion, and water infiltration whilst increasing water retention (Murphy et al., 1993¹Murphy, J.A., Rieke, P.E. & Erickson, A.E. (1993). Core cultivation effects on soil physical properties. Agronomy Journal, 85, 1–5.; Neylan, 1994²Neylan, J. (1994). Sand profiles for turf. Australian Turfgrass Management, 1(5), 24–26..

In fact, the USGA Green Section has consistently stated that excessive OM is a primary cause of soft, disease-prone greens³Moeller, A. & Lowe, T. (2016). Managing organic matter in putting greens. USGA Green Section Record, 54(21), 1–7..

Organic Fertilisers.

Organic fertilisers add C to the soil. In contrast, synthetic N fertilisers deliver plant available N with no organic content.

Organic fertilisers like composted manures, deposit large quantities of organic matter into the rootzone that slowly decomposes. This is in addition to the OM already being generated by the turf itself through the turnover of roots, leaf aging, and stoloniferous growth.

You can regard thatch levels that exceed 13 mm as excessive⁴Christians, N.E. (1998). Fundamentals of Turfgrass Management. Ann Arbor Press.⁵Miller, R.W. (1965). Nature and process of thatch formation. USGA Green Section Record, 3(1), 18–21.⁶McCarty, L.B. & Miller, G. (2002). Managing Bermudagrass Turf. Ann Arbor Press..

When there is a build up of thatch it is results in lower cold tolerance, more disease and insect activity, and a reduction in pesticide efficacy⁷Cornman, J.F. (1952). Effects of thatch on turf. Proceedings of the Northeastern Weed Control Conference, 6, 96–98.⁸Miller, R.W. (1965). Nature and process of thatch formation. USGA Green Section Record, 3(1), 18–21.⁹Thompson, W.R. (1967). Turfgrass thatch. USGA Green Section Record, 5(4), 2–4..

Several factors accelerate this build-up of OM in well managed turf:

• Low heights of cut.
• High N.
• Frequent irrigation and
• The type of fertiliser used (Weaver et al., 2022).

Organic fertilisers contribute to two of these simultaneously by adding more N and more carbon substrate.

Soft Playing Surfaces.

Excessive OM at a green surface:

• Produces soft conditions.
• Increases ball marks.
• Gives inconsistent green speeds and
• Increases the susceptibility of turf to scalping¹⁰Barton, L., Wan, G.G.Y., Buck, R.P. & Colmer, T.D. (2009). Effectiveness of cultural thatch-mat controls for young and mature Kikuyu. Agronomy Journal, 101(1), 67–74.¹¹Carrow, R.N. (2001). Thatch and organic matter management on turfgrass. USGA Green Section Record, 39(2), 18–21.¹²Hurto, K.A., Turgeon, A.J. & Spomer, L.A. (1980). Physical characteristics of thatch as a turfgrass growing medium. Agronomy Journal, 72, 165–167..

On sports pitches:

• It leads softer surfaces.
• Inconsistent ball bounce and
• More divots.

Research at several US universities confirms that coring increases surface hardness by around 19% on creeping bent greens. This is in contrast to untreated greens with high OM levels that became progressively softer¹³McCarty, L.B., Gregg, M.F. & Toler, J.E. (2005). Thatch and mat management in an established creeping bentgrass golf green. Agronomy Journal, 99(6), 1530–1537..

When you topdress with sand it dilutes the OM layer and firms the surface. However, this is a race. If organic inputs outpace dilution and OM breakdown, the surface becomes softer.

If you manage greens with heavy organic inputs these need more aggressive and frequent sand topdressing, aeration, and verticutting to compensate. That means more disruption, more cost, and more days where surfaces are unavailable for play.

The turf manager’s dilemma is clear. Companies pushing organics promote microbial activity and “soil health.” But if you add more OM to a system that already produces OM faster than it decomposes it works against the engineering intent of the rootzone.

Disease Development with Organic Fertilisers.

The relationship between N, OM, and disease is not straightforward, so you should challenge claims that organics are “better for turf health”.

Low N makes turf more susceptible to dollar spot, anthracnose, red thread, and rust.

Although adequate N levels are a common cultural tool to suppress low N diseases, the flip side is also well known. Too much N makes turf prone to brown patch, Pythium blight, Fusarium patch, and leaf spot/melting-out diseases¹⁴Landschoot, P.J. & McNitt, A.S. (1997). Effect of nitrogen fertilizers on suppression of dollar spot disease of Agrostis stolonifera L. International Turfgrass Society Research Journal, 8, 905–911.¹⁵Clarke, B.B., Vincelli, P., Koch, P. & Chou, M-Y. (2024). Chemical Control of Turfgrass Diseases 2024. University of Kentucky, Rutgers University, University of Wisconsin-Madison..

Organic N sources release N by microbial activity. This process is temperature dependent and difficult to accurately predict. In warm, moist conditions, release rates can spike, and N is released at precisely the wrong time for diseases like brown patch and Pythium.

In fact. the results do not support the common belief that organic N suppresses disease better than synthetics.

• Davis and Dernoeden (2002)¹⁶Davis, J.G. & Dernoeden, P.H. (2002). Dollar spot severity, tissue N, and soil microbial activity in bentgrass as influenced by N source. Crop Science, 42, 480–488. found that organics only controlled disease in the spring when there is low to moderate disease pressure. Any benefit was attributed to an increase in plant N rather than better microbial suppression.
• Kaminski et al. (2004)¹⁷Kaminski, J.E., Dernoeden, P.H. & Mischke, S. (2004). Organic amendment effects on bentgrass establishment and dollar spot. International Turfgrass Society Research Journal, 10, 1028–1036. found no effect from organics on dollar spot in comparison to standard fertilisers.

Organics as a Habitat for Disease Pathogens.

The addition of OM can be a great habitat for pathogens. Thatch harbours fungae, retains moisture, and hinders fungicide penetration¹⁸McCarty, L.B., Huber, A., Tucker, B. & Toler, J.E. (2016). Thatch-mat management on ultradwarf bermudagrass greens. Agronomy Journal, 108(3), 1075–1084.; Weaver et al., 2022).

A 2022 study¹⁹Beckley, P.M. et al. (2022). Utilizing organic amendments for general suppression of dollar spot on creeping bentgrass turf. International Turfgrass Society Research Journal, 14, 532–541. found that N reduced dollar spot in comparison to unfertilised turf. However, the organics did not perform better than conventional fertilisers.

The evidence is consistent. N plays an vital role in disease management.

• The source of N is not as important as the rate and timing.
• Organics have an unpredictable release and
• They are substrates for pathogens.

Poa annua invasion and Organic Fertilisers.

Winter grass (Poa annua) thrives in organic-heavy nutrition programs. Research from Oregon State University²⁰Brauen, S.E. & Goss, R.L. (2008). Annual bluegrass, Poa annua L. Oregon State University Extension. shows that Winter grass increases with rising soil P and high N rates.

Organic fertilisers in contrast to synthetic N sources tend to have high P levels relative to their N content . As a result they raise soil P over time regardless of whether any extra P is needed.

The USGA Green Section identifies that when you apply excessive N and irrigation for aesthetics they act as drivers of Winter grass encroachment on bent greens²¹Managing putting greens that are a mix of Poa annua and creeping bentgrass. USGA Green Section Record, 60(10). (USGA, 2022).

High N rates favour this weed over creeping bentgrass. Bent may need only 100-150 kg N/ha/yr, while Poa demands 150-200 kg N/ha/yr to maintain quality²²The ins and outs of managing Poa annua putting greens. USGA Green Section Record, 59(10). (USGA, 2021).

Organic programs that use high N for “soil health” benefits can easily push annual rates above what bent needs. This tilts the balance toward Winter grass.

Scandinavian research on fescues found that rootzones amended with compost had higher levels of Winter Grass than those amended with peat. This is despite compost having several turf benefits such as better tiller density²³Aamlid, T.S. & Andersen, T.E. (2012). Composted garden waste as organic amendment to the USGA-rootzone and topdressing sand on red fescue (Festuca rubra) greens. International Turfgrass Society Research Journal..

The high P content of the compost was identified as a likely contributing factor.

On USGA-spec greens where you are aiming to maintain a desirable sward composition, organic programs that deliver surplus P and unpredictable N work against this.

Water Retention and Hydrophobicity.

In agricultural soils, increases in OM improve the water-holding capacity. This is regarded as being beneficial²⁴Li, Z. et al. (2018). Long-term effects of organic fertilization on soil organic carbon and its fractions. Soil & Tillage Research, 178, 50–58. ²⁵Beck-Broichsitter, S. et al. (2020). Soil organic carbon determines soil water retention. Soil & Tillage Research, 196, 104426.

In sand-based rootzones, the opposite is true. The entire purpose of a sand-based construction is rapid drainage.

Water Retention impacts on Turfgrass.

If you increase water retention in the upper profile it means saturated conditions persist long after rainfall or irrigation. This then reduces oxygen availability to roots, extends the length of time that leaf tissue remains wet and promotes fungal disease.

Simultaneously, as OM accumulates and decomposes in sandy rootzones, hydrophobic coatings can form on sand particles from root exudates and decomposing organic tissue²⁶Cisar, J.L., Williams, K.E., Vivas, H.E. & Haydu, J.J. (2000). The occurrence and alleviation by surfactants of soil-water repellency on sand-based turfgrass systems. Journal of Hydrology, 231–232, 352–358.²⁷Doerr, S.H., Shakesby, R.A. & Walsh, R.P.D. (2000). Soil water repellency: its causes, characteristics and hydro-geomorphological significance. Earth-Science Reviews, 51(1–4), 33–65.

Research in Western Australia demonstrated that water repellency is much more severe in turfgrass soils with high OM content (32%) than low OM (8.6%)²⁸Lyons, E.M., Barton, L. & Colmer, T.D. (2009). Granular wetting agents ameliorate water repellency in turfgrass. Plant and Soil, 348, 411–424.²⁹Barton, L. & Colmer, T.D. (2011). Ameliorating water repellency under turfgrass of contrasting soil organic matter content. Irrigation Science, 29, 443–455.

This means that the rootzone can retain too much water, and also repel water where hydrophobic conditions develop. This creates wet, disease-prone areas adjacent to dry, stressed areas.

Our own soil wetting agent research at Bonnie Doon GC in Sydney confirms that soil moisture contents and water penetration times are significantly affected by OM content, and that excessive water produces soft, disease-prone surfaces with increased maintenance costs (Gilba Solutions, 2024).

Managing this requires wetting agents, more precise irrigation scheduling, and aggressive OM dilution through sand topdressing.

Ectotrophic Root-Infecting (ERI) Fungi and Couch Decline: A Case Study in Compounding Risk.

Couch decline on ultradwarf putting greens deserves separate attention because it shows how every risk factor discussed above converges in a single disease system.

Couch decline (BD) was first described by Elliott (1991)³⁰Elliott, M.L. (1991). Determination of an etiological agent of bermudagrass decline. Phytopathology, 81, 1380–1384. and is caused by a complex of ectotrophic root-infecting fungi³¹Vines, P.L. (2015). Evaluation of ultradwarf bermudagrass cultural management practices and identification, characterization, and pathogenicity of ectotrophic root-infecting fungi associated with summer decline of ultradwarf bermudagrass putting greens. PhD dissertation, Mississippi State University.³²Stephens, C.M. & Kerns, J.P. (2020). First report of Gaeumannomyces graminicola causing bermudagrass decline of ultradwarf bermudagrass putting greens in North Carolina. Plant Disease, 104(6), 1762.³³Stephens, C.M. et al. (2023). In vitro fungicide sensitivity and effect of organic matter concentration on fungicide bioavailability in take-all root rot pathogens isolated from North Carolina. Plant Health Progress, 24(1), 57–65.

These organisms colonise the outer surfaces of roots, stolons, and rhizomes and gradually destroy root function. You can see damage as irregular chlorotic patches that can lead to total turf loss on affected greens³⁴Tucker, M.A. & Tomaso-Peterson, M. (2019). Bermudagrass decline. Mississippi Turfgrass, Spring 2019.

No published peer-reviewed trial has directly compared organic vs synthetic fertiliser programmes and measured ERI fungal load on ultradwarf couch greens.

However, the known predisposing factors for ERI disease align precisely with the consequences of organic fertiliser use, creating a strong inferential case that warrants serious caution.

Soil pH and Organic Fertilisers.

ERI fungi, like their cereal crop relative Gaeumannomyces tritici (the cause of take-all in wheat), are favoured by neutral to alkaline soil conditions.

• The Alabama Cooperative Extension identifies neutral soil pH as a key risk factor for both spring dead spot and couch decline³⁵Hagan, A.K. (2000; updated 2025). Control of spring dead spot and bermudagrass decline. Publication ANR-0371. Alabama Cooperative Extension System..
• The Georgia Extension confirms that take-all root rot is most severe at a soil pH of 6.5 or above³⁶Waltz, C. & Martinez-Espinoza, A.D. (2023). Turfgrass diseases in Georgia: identification and control. UGA CAES Publication B-1233.
• NC State recommends maintaining pH at 5.8–6.2 and using acid forming fertilisers³⁷Tredway, L.P. (2024). Spring dead spot in turf. NC State Extension Publications.
• Penn State explicitly recommends acidifying nitrogen sources and avoiding nitrate-based fertilisers for ERI disease management³⁸Landschoot, P.J. & McNitt, A.S. (1997). Effect of nitrogen fertilizers on suppression of dollar spot disease of Agrostis stolonifera L. International Turfgrass Society Research Journal, 8, 905–911.

This is where the use of organic N sources becomes a problem. Ammonium sulphate, delivers a sustained pH-lowering effect through the nitrification of ammonium combined with the residual sulphate anion.

Organic nitrogen sources mineralise through microbial pathways that produce ammonium initially. However, in warm soils where you tend to find couch this converts rapidly to nitrate.

This means that any acidifying effect is transient and unpredictable. Some organic fertilisers will actually raise the soil pH through the release of calcium and magnesium from the decomposing organic matrix.

On ultradwarf greens irrigated with alkaline bore water, organic N program will not provide the rootzone acidification that is required.

Thatch management and Organic Matter in Putting Greens.

The Alabama Extension directly links “exceptionally low mowing heights, poor drainage, thatch accumulation, and high soil organic matter” with increased couch decline on greens and tees³⁹Hagan, A.K. (2000; updated 2025). Control of spring dead spot and bermudagrass decline. Publication ANR-0371. Alabama Cooperative Extension System..

Vines⁴⁰Vines, P.L. (2015). Evaluation of ultradwarf bermudagrass cultural management practices and identification, characterization, and pathogenicity of ectotrophic root-infecting fungi associated with summer decline of ultradwarf bermudagrass putting greens. PhD dissertation, Mississippi State University. documented that ultradwarf couch cultivars, with their extremely high shoot densities, already accumulate excessive OM that can impact plant health.

NC State recommends aerification and frequent topdressing to reduce OM as a key practice to manage Take-all Root Rot (TARR).

When you add organic fertiliser carbon to a system that already overproduces OM it adds fuel to the fire.

Fungicide bioavailability.

Perhaps the biggest impact on turf management comes from Stephens et al. (2023)⁴¹Stephens, C.M. et al. (2023). In vitro fungicide sensitivity and effect of organic matter concentration on fungicide bioavailability in take-all root rot pathogens isolated from North Carolina. Plant Health Progress, 24(1), 57–65. at NC State. They investigated the effect of organic matter on fungicide bioavailability against TARR pathogens.

Their work showed that high OM can limit the bioavailability of fungicides. This potentially reduces the efficacy of DMI and QoI chemistries that turf managers rely on for TARR control.

This means that high OM rootzones may need higher rates or more frequent applications to achieve the same degree of disease control.

Organic fertiliser programs that elevate rootzone OM tend to favour the pathogen and negatively impact the main tool that turf managers use to fight it.

The ERI complex complicates matters further.

Tucker and Tomaso-Peterson (2019)⁴²Tucker, M.A. & Tomaso-Peterson, M. (2019). Bermudagrass decline. Mississippi Turfgrass, Spring 2019. and Vines et al. (2020) showed that couch decline is not a single pathogen disease but involves a complex of four or more ERI species that can co-infect roots.

Their work showed that 59% of samples that showed symptoms contained multiple ERI species. Of greater importance was that the ERI complex was present in both greens with and without symptoms.

This strongly suggests that this disease depends on environmental and cultural conditions tipping the balance, with thatch, OM, pH, and moisture all being tipping points.

An organic fertiliser programme pushes all four in the wrong direction.

Australian relevance.

While most ERI research has been in the southeastern US, couch decline and TRR are seen on couch greens in eastern Australia, where ultradwarf cultivars are found.

Australian couch greens face many of the same risk factors: sand-based USGA rootzones, alkaline irrigation water, humidity, and aggressive OM accumulation from high shoot density.

The use of organic fertilisers in these pre-existing conditions creates an elevated risk profile for ERI that cannot be dismissed simply because the fertiliser-type trial hasn’t been published.

What Does This Mean in Practice for Organic Fertilisers?

None of this is an argument that organics have no place in turf management. Composts and natural organic products can improve microbial diversity and provide slow-release nutrition on low-maintenance turf.

The problem arises when you apply organics to intensively managed, sand-based surfaces where the physical properties of the rootzone are engineered for a specific purpose.

For superintendents and sports turf managers, the evidence points toward a balanced approach:

• Use synthetic N sources as the backbone to a precision fertility program on greens and high-performance surfaces. This allows N rate, timing, and form to be precisely controlled.
• Reserve organic fertilisers for low maintenance areas (fairways, roughs, surrounds) where OM accumulation is less important. Monitor rootzone OM, aiming for below 4% by weight in the upper 50 mm of sand-based greens⁴³Moeller, A. & Lowe, T. (2016). Managing organic matter in putting greens. USGA Green Section Record, 54(21), 1–7.. Make any fertility decisions based on soil and tissue testing, not ideology.

The marketing of organics often conflates “natural” with “superior.” The peer-reviewed research does not support this on managed turfgrass surfaces. Organic matter management is the single most important long-term determinant of putting green quality, and organic fertiliser programs can make that job harder.