Green firmness and topdressing sand coefficient of uniformity

The coefficient of uniformity (C_u) is a value that indicates how similar the particle sizes of a material are.¹ If the C_u is a larger value, there is more variation in particle size; when the C_u is a lower value, the particles are more consistent in size. A wider particle size distribution can allow smaller particles to occupy spaces between larger particles, potentially increasing packing density.

This seems to have some relevance to the firmness of a putting green, and you can find the C_u in Table 3 of the USGA’s 2018 edition of Recommendations for a Method of Putting Green Construction. The recommended ranges for C_u are 1.8–3.5 for rootzone mixtures with peat, and 2.0–3.5 for rootzones without peat. For reference, a C_u of 1.0 would be material with all particle sizes (at least in the D₁₀ to D₆₀ range) the same diameter.

We have collected a lot of measurements of putting green firmness and of C_u, and this chart shows something a bit disappointing, if you are trying to increase firmness by using a topdressing sand with higher C_u. The particle sizes and C_u of the sand at the 0–2 cm layer of the rootzone are a result of topdressing sand, except on the newest greens.

From paired Clegg (firmness) and 0–2 cm surface layer C_u measurements from 19 different greens (tested greens shown in the image below), there was no correlation between the C_u of the sand at the surface layer of the green and the surface hardness measured with the Clegg.

A higher Clegg value is firmer—on this chart a 60 or 70 is a soft green, and 100 is a somewhat firm green. If higher C_u were consistently associated with greater firmness, we would expect Clegg values to tend to increase as C_u increased. Instead, greens with similar C_u values have substantially different Clegg measurements. For example, g14 and g19 in these data have a C_u around 2 but are among the firmest greens measured in this project. g4 is one of the softest greens, and it also has a C_u around 2. If C_u influenced firmness, we would expect greens with higher C_u values to have a Clegg measurement close to 100. But again, we can have either. g9 has a C_u of about 2.7 and a Clegg near 100; g1 and g6 also have relatively high C_u, but have Clegg measurements less than 70.

Why is there no correlation between C_u and firmness in these data? At the surface of the green, the sand particles are separated, not all touching each other. The volume of organic material in the 0–2 cm layer of the rootzone takes up so much space that sand particles are displaced. The C_u of the sand doesn’t really matter when sand is packed against organic material rather than packed against sand. That’s generally the case right at the surface of a putting green.

One other thing to note—the 2018 USGA recommendation for a rootzone material is for a C_u of 1.8–3.5. In these measurements from 19 greens, the C_u is 1.91–2.83. That’s a narrow range. There is only a certain set of sand particle sizes that are suitable for topdressing. The largest particles need to work their way into the canopy, and you typically won’t be using much silt or clay or very fine sand in the regular topdressing. Fifty percent of our measurements from putting greens have a 0–2 cm layer C_u of 2.2–2.6.

Within the typical range of C_u that topdressing sands produce once that sand is on the ground, and from these tested greens, C_u doesn’t explain the firmness of the surfaces.