Putting Green Rootzone Selection

Sandy soils have long been recognized as highly suitable for putting greens since the earliest days of golf course development on the links land bordering the sea in Scotland (Alister Mackenzie, 1995). Today, most putting green rootzones are constructed with high sand contents, sometimes 90% or more by volume, and amended with an organic material such as peat, an inorganic amendment, soil, or a combination of the above. These types of rootzone mixtures are ideal for putting greens because they support healthy turf growth, resist compaction, drain well, and maintain air-filled porosity. However, there are numerous examples of greens built using materials that drain too quickly or are not tailored to their specific site; both of which can lead to long-term challenges and limit the performance of a putting green. This article outlines the keys to success for selecting a putting green rootzone that considers climate, water quality, surface conditioning expectations, labor, and overall maintenance and construction budget.

Construction Methods

Before we dive into rootzone selection, let’s start by discussing the most common construction types being utilized by golf course architects and builders. These include USGA greens, USGA style greens with variable depth rootzones, California Greens, sands-based greens with sand channel drainage, and native sand greens. The basic differences between these construction methods are outlined in Table 1.

Table 1 The most common construction methods used for modern putting greens. 

Variable Depth

Of the construction methods listed, USGA greens have been the standard for decades. However, building USGA style greens with variable rootzone depths – e.g., 8-10 inches of mix on high spots, 12-inches of mix on flat areas, and 14-16 inches of mix in low-lying areas – is becoming more popular. Research from Michigan State demonstrated how variable depth construction can provide more uniform soil moisture in the rootzone compared to a uniform 12-inch deep rootzone across an entire green with surface contours. The researchers found that the improvements in moisture uniformity were most evident when a pure sand rootzone was installed. However, field observations suggest there is noticeable improvements in moisture uniformity when amendments are used with variable depth construction. A cost-benefit analysis should be considered since this construction method is likely to require more rootzone mix and is more time-consuming than a traditional USGA green. Superintendents are advised to work closely with a skilled agronomist, an experienced builder, and accredited soil testing lab when considering variable depth construction.

Variable depth construction is becoming more popular because it can result in improvements in moisture uniformity, but it is paramount to work with a skilled agronomist and builder to ensure proper construction. 

Is a Gravel Layer Necessary?

The biggest difference between USGA greens and other methods in Table 1 is the presence of a gravel layer. There are several functions of the gravel layer including:

• Creates a perched water table at the interface of the rootzone and gravel
• Allows the rapid movement of water into the drainage pipe once the rootzone reaches field capacity
• Preventing fine particles from migrating into the drainage pipes
• Serves as a barrier to salts that could move into the rootzone from the subsoils

The most noteworthy impact of a gravel layer is the perched water table. This occurs because of the significant change in texture between the gravel and the rootzone materials. The perched water table increases the rootzone’s ability to retain moisture, allowing superintendents to irrigate deep and infrequently. Research has shown that a gravel layer can provide more uniform soil moisture near the surface compared to a California green. In addition, when a gravel layer is absent in a California green, moisture levels are strongly influenced by the proximity of the underlying drainage pipe (Pettyman and McCoy, 2002).

Despite the documented benefits, the value of installing a gravel layer is often debated. The added cost of the gravel is easily scrutinized, and the benefits are not always obvious. Recently, low-pH rootzone mixtures placed over high-pH gravel layers have been linked to the formation of iron-cement layers at the interface between the rootzone mix and gravel (O’Bear and Soldat, 2014). These iron-cement layers might impede drainage, particularly in low-lying areas and when high levels of iron fertilizers are applied. The USGA specifications were modified in 2018 and now include the recommendation to select a neutral-pH gravel where practical to reduce the potential for iron-cement layers to form, but it’s too early to tell if this will eliminate the risk entirely.

Most putting greens are built with a gravel layer, but some feel that the perched water table isn’t needed in select locations, especially considering the potential for iron-cement layers.

Some industry professionals feel that the gravel layer – i.e., perched water table – isn’t needed in select locations, especially considering the potential for iron-cement layers. For instance, if the subgrade beneath the rootzone mixture is sandy and drains well, a perched water table may not provide as much value compared to sites with poorly draining subsoils. In temperate climates where it rains frequently, some feel they’d rather have moisture continuously flowing directly into drainage pipe instead of having to wait for the rootzone to become saturated. Ultimately, most feel the benefits of a gravel layer exceed the potential challenges, which is why it is included in most projects.

Rootzone Mixtures

Coarse Sands versus Fine Sands

Most modern putting greens are built using sands that fall within the USGA specifications (Table 2). These specifications are purposefully wide so USGA greens can be utilized around the world with different climates, water quality, and locally available materials. If we think of the USGA sand particle size specifications as a spectrum ranging from coarse to fine, many favor rapidly draining coarser sands given the agronomic and playability issues that occur when greens stay too wet. However, there are associated challenges that must be considered with coarse sands. For instance, rapidly draining coarse sands tend to have less capillary porosity, which means they can be droughty and costly to maintain. Finding a suitable topdressing sand that is compatible with the underlying rootzone can also be challenging. In situations where frequent rains are common or water quality is poor, sand selection is often driven by the desire to have rapid drainage and moisture retention is secondary. Flushing salts can still be accomplished with finer sands on the low end of the drainage spectrum – i.e., 6 inches per hour, so the desire to utilize coarse sands to maximize drainage isn’t always warranted.

Many favor rapidly draining coarser sands for putting green rootzones given the agronomic and playability issues that occur when greens stay too wet, but there are associated challenges that must be considered with coarse sands.

Sands that fall on the finer end of the USGA spectrum tend to have higher water holding properties and slower infiltration compared to coarse sands. However, fine sands or even sands slightly finer than the USGA specifications can still make for an excellent rootzone. Murphy et al. (2001) reported that turfgrass establishment and turf quality was improved when rootzones constructed with medium-fine sands compared to medium-coarse sand. More recent research from Rutgers (Wang et. al., 2020) demonstrated that putting greens topdressed with a medium-fine sand, which was slightly outside of USGA specifications, produced better overall turf quality than plots topdressed with a medium-coarse sand because of the slightly increased moisture holding capacity. This isn’t suggesting the particle sizes required for USGA greens won’t yield acceptable results, but rather that increasing the amount of fine sand particles into the rootzone while maintaining a reasonable percentage of medium and coarse sands may result in a better performing green.

In addition to holding slightly more moisture, rootzones with a closer balance between fine, medium, and coarse sands will raise the coefficient of uniformity (Cu). The Cu indicates the uniformity of the sand particle sizes of a rootzone. Mixes with a low Cu have most particles within a narrow size range and could yield unstable, soft playing surfaces while rootzones with a high Cu will pack, potentially providing firmer surfaces. Most courses with a strong desire to produce firm conditions favor rootzones with a Cu between 2.5 and 3.5. Rootzones with a Cu between 1.8 and 2.5 can provide firm conditions but may require significantly limiting soil moisture compared to rootzones with higher Cu values. Rootzones with wide distribution of particle sizes, particularly those with more fine sand and very fine sand along with a reasonable percentage of medium and coarse sands are garnering interest for rootzone mixtures because they have a higher Cu. Sand particle shape plays a role in firmness as well, with rounded sands yielding less stable surfaces compared to angular sands. When rounded sands are the only practical option, be sure to have wide gradation in particle sizes to minimize stability issues.

Graphs can make it easier to visualize where a potential rootzone falls in relation to the USGA specifications. Both rootzone mixes in this example meet USGA specifications but they would perform significantly differently.  

The USGA specifications call for no more than 20% of fine sand particles (0.15 – 0.25 mm) and no more than 5% of the very fine sand particles (0.05 – 0.15 mm). Considering the research conducted by Jim Murphy and field observations, it’s clear that in the right climate we can use sands that have more very fine and fine sands in a rootzone mix so long as there is a reasonable balance between medium and coarse sands. If you are considering using a sand that is finer, you’ll want to ensure adequate surface drainage exists.

Table 2. Recommended particle size distribution of USGA greens.

Dirty Mixtures

Some university soil scientists and superintendents feel we can even increase the amount of allowable silt and clay in putting green rootzones. Dirty mixtures generally describe sands that have more silt, clay, and very fine sand compared to sands that meet USGA specifications. For instance, a 6-2-2 or 7-2-1 sand-soil-peat mixture are often described as dirty sands. These mixes have performed well when used to backfill sand channel drainage trenches and green expansion projects in native soil putting greens, so there is increased interest of their use as a rootzone mix. The caveat with these types of materials is they can vary significantly in their particle size analysis, infiltration rate, silt/clay content, and moisture retention (Hummel, 2014). The benefits in moisture retention, a high Cu, and reasonable drainage are intriguing. For dirty mixtures to be successful, it is paramount that internal drainage is installed into the subgrade on 6 foot centers – e.g., sand channel drainage. In these instances, the higher capillary porosity and slower infiltration of a dirty mix probably offset the value of a gravel layer.

More research is needed in this area to better understand where the true limits of fine sand particles and silt/clay exist, but there are clear benefits to using sands on the fine side of the spectrum that might have been previously eliminated from consideration. Obviously, there is a point where too many fines will contribute to excess moisture holding which is why it is important to work closely with a skilled agronomist and accredited lab to evaluate sand options.

Dirty mixes have performed well when used to backfill sand channel drainage trenches and green expansion projects in native soil putting greens, so there is increased interest of their use as a rootzone mix.  

Rootzone Amendments

Soil

Amending sands is common to increase water and nutrient retention of sands. Certain amendments may also help increase the Cu, especially a small about of soil. USGA greens allow for a soil to be added to sand so long as the final mixture meets the performance criteria in the specifications. The important thing to consider when using soil in a rootzone mixture is that it is a screened sandy loam soil. Quality control issues have been cited often by superintendents that wanted to use soil to amend sands but couldn’t ultimately find a source that would work for their project.

Peat

Sphagnum and reed sedge peats have been the industry standard amendment for decades given their affordability and predictability in increasing water and nutrient retention. However, there are growing concerns over the carbon footprint associated with harvesting peat, supply, and to a lesser extent the potential for degradation over time.

Inorganic Amendments

Inorganic amendments are gaining in popularity because they can provide similar nutrient and water retention benefits as peats without some of the concerns associated with peat. In some instances, inorganic amendments have been preferred because they can raise water retention without decreasing infiltration. Another frequently cited benefit of inorganic amendments is quality control and having availability worldwide.

Alternatives

Compost and biochar are other amendments that have been used for amending putting green rootzones, though biochar has been used less often. Quality control and consistency with these products can be a limiting factor depending upon the source and amount of material to be used. That said, when amended in small quantities, usually less than 10 percent by volume, they can be valuable additions to a rootzone mixture to increase water and nutrient retention and microbial activity. Long-term, wood-based biochar products seem like a promising amendment, but continued research is needed before widespread use can be expected. Some biochar experts speculate that it would just be a matter of experimentation and time to develop specifications for the right processing of the material.

Rootzone Performance Indicators

Working with an accredited lab is critical to understand the performance indicators of potential rootzone mixtures and how the putting green will perform over time (Table 3). Arguably the most valuable performance indicators are the Cu, porosity, infiltration, and water release curves. The Cu was discussed earlier, so the remaining sections will cover porosity, infiltration, and water release curves.

Table 3. Recommended performance indicators for USGA greens.

Porosity

Total porosity, air-filled porosity, and capillary porosity influence water retention characteristics, gas exchange, and infiltration rate. Like particle sizes, the USGA specifications provide a broad range for total porosity, air-filled porosity, and capillary porosity. When examining rootzones with different porosity characteristics, consider local climate and rainfall patterns. When rainfall occurs occasionally, rootzones with a lower total porosity, higher air-filled, and medium capillary porosity are likely to provide the best balance of moisture retention and irrigation requirements. For arid climates, higher capillary porosity and lower air-filled pores makes sense while the opposite would be ideal in climates with frequent, heavy rains.

Infiltration

Superintendents have long recognized the importance of having adequate infiltration. However, there has been an overemphasis on this performance indicator. A minimum infiltration rate of 6-incher per hour is recommended for USGA greens. We know the infiltration rate will slow down by as much as 74% after grow-in and as a green ages (Gaussoin et al., 2007), so selecting rootzone mixes with infiltration rates exceeding 25 inches per hour can be appealing. Unfortunately, this can result in the need for daily irrigation and droughty conditions. An important part of the research cited above is that while infiltration slowed down over time, it remained acceptable because of sound agronomic practices. Rootzones with infiltration rates between 15 and 20 inches per hour are adequate for most putting greens. However, the infiltration rate doesn’t tell the full story of how a mix is going to perform. For instance, two potential rootzones might have a similar infiltration rate, but they could have drastically different water-holding properties. This is where water release curve data and capillary porosity values become more valuable than infiltration rate data alone.

Water Release Curves

Capillary porosity values indicate the water-holding potential of a rootzone mix, but it doesn’t tell you how much of the water is available to the plants (Hummel, 2014). A water release curve predicts the plant-available water in a rootzone. This test also can identify the best depth to install each rootzone material, which is particularly important when building USGA greens with variable depths or using dirty mixes over sand channel drainage. A water release curve is also helpful when trying to determine if native sands can be used to build greens.

Water release curves are helpful at determining how different depths – e.g., tension (-cm) – impacts porosity characteristics and moisture retention. (Source/Turf & Soil Diagnostics).

Conclusion

Ultimately, determining a construction method and selecting a rootzone mix needs to be site specific. Climate, water quality, surface conditioning expectations, labor, and overall maintenance and construction budget need to be considered. The USGA specifications are proven to be an effective method for building putting greens, but it’s important to recognize they are intentionally broad. Using rootzone mix on the fine end of the USGA spectrum, or even slightly outside of those specifications might yield better performing greens. Similarly, using a different construction method may be more appropriate in some situations. Working closely with a skilled agronomist and an experienced builder is the most effective way to ensure the rootzone is tailored to your site and the construction process is successful.

References

Frank et. al., 2005. Effect of Rootzone Material and Depth on Moisture Retention in Undulating USGA Putting Greens. USGA Turfgrass and Environmental Research. June.

Gaussoin et al., 2007. Soil physical and chemical characteristics of aging golf greens. Golf Course Management. January. p: 161-165.

Hummel, 2014. Understanding water-relations in mixes for sand channel drains. USGA Green Section Record. August. 52 (16) p: 1-5.

Mackenzie, A. 1995. The Spirit of St. Andrews. Sleeping Bear Press, Chelsea, Mich.

Pettyman, G. and E. McCoy. 2002. Effect of profile layering, rootzone texture, and slope on putting green drainage rates. Agronomy Journal. March/April. 94(2): p. 358-364.

Murphy et. al. 2001. Creeping bentgrass establishment on root zones varying in sand sizes. Int. Turfgrass Soc. Res. J. 9:573-579.

Obear, G. and D. Soldat. 2014. Iron-Cement Layers in Putting Green Soils. Golf Course Management. April. p: 96-98, 100.

Wang et. al., 2020. Velvet bentgrass putting green quality, water retention, and infiltration as affected by topdressing sand size and rate. Agronomy Journal. September/October. 113(5): p. 3857-3870.

About the Author

Adam Moeller

Director of Agronomy – North America

Adam Moeller is the former director to the USGA Green Section Education and editor-in-chief of the USGA Green Section Record digital magazine. He has consulted with over 330 golf courses and collaborated with host superintendents to prepare 22 USGA national championships, including prestigious events like the U.S. Open and U.S. Women’s Open.

Adam is a highly accomplished individual with a Bachelor of Science degree in Horticulture from the University of Wisconsin and a Master of Science degree in Agronomy from Purdue University.

He is widely recognized as an industry leader and authority in his field, developing educational content for golf course superintendents, course officials, and golfers.

Due to his sought-after expertise, he frequently presents on golf course maintenance topics at turfgrass conferences in both the USA and at an international level.

In his free time, Adam loves to spend time with his wife and two sons. As an avid snowboarder and golfer, he enjoys seeing his young sons become interested in these sports.