Surface organic matter is defined as the living and dead organic material within the top inch of the putting green surface. Surface organic matter plays an important role in putting green performance by influencing the playability of the surface (e.g., firmness), its soil physical properties (e.g., infiltration rate and water-holding characteristics), and the occurrence and severity of pests like moss and disease.
Golf course superintendents regularly employ practices like sand topdressing and aeration to remove and dilute surface organic matter. Unfortunately, there are no widely accepted ranges for the level of putting green surface organic matter that will provide optimal playing conditions and rootzone properties. Therefore, decisions about how to manage surface organic matter are often not based on reliable data. This is partly because there is no standard method for measuring surface organic matter, and partly due to a lack of research on the topic of surface organic matter and its impact on putting green turf health and playability.
In 2019, the United States Golf Association (USGA) assembled a committee of turfgrass researchers from universities across the U.S. and charged the group with recommending a standard method for measuring surface organic matter of putting greens. The group conducted a literature review that summarized the existing knowledge and identified gaps in our understanding of surface organic matter measurement. Next, the committee conducted a series of experiments to answer questions about how to best quantify putting green surface organic matter. Below is a summary of the committee’s findings and our joint recommendations with the USGA Green Section for measuring putting green surface organic matter.
1. Sample preparation: Traditional organic matter analysis involves removing the turf (verdure) and then grinding and sieving the soil sample at a soil testing laboratory. These procedures remove as much as half of the total surface organic matter. The organic matter removed can influence surface playability and soil properties. The committee also found that removing this organic matter increased the measurement error. Therefore, we recommend leaving turf (verdure) intact as well as abstaining from grinding and sieving the samples at the laboratory prior to organic matter analysis. The intact samples should be ignited at 440 degrees Celsius for two hours. Many laboratories use a 360 C burn temperature, but we found 440 C is more appropriate for surface organic matter samples because of the higher organic matter content.
2. Sampling depth: Most of the surface organic matter accumulates and exists within the top inch (2.5 cm) of the surface. The organic matter levels within this surface layer significantly influence putting green playability and soil physical properties such as infiltration rate. Therefore, surface organic matter samples should be taken to a depth between 2.0 cm and 2.5 cm (0.8 to 1.0 inch). Deeper increments may be useful to measure in some circumstances (e.g., 2-4 cm, 4-6 cm or 1-2 inches, 2-3 inches), but these increments are not considered surface organic matter.
3. Sampling probe size: The committee compared surface organic matter samples taken with a standard soil probe (0.75-inch diameter) against samples taken with a 1.5-inch diameter probe and found that the large probe size provided no advantage over the standard probe size. Any probe diameter of 0.75 inch or larger can be used to collect intact cores for surface organic matter analysis. Repairing the sample hole is easiest with the 0.75-inch sampler. The committee acknowledges that “edge-effect” errors during sample acquisition and handling (e.g., loss of sand from the outer edges of a sample) will have a greater effect on smaller-diameter samples. Therefore, attention to consistent and careful extraction and handling is paramount to avoid introducing sampling error.
4. Number of samples: The committee studied the number of samples that should be taken from a putting green to adequately characterize the average surface organic matter by analyzing hundreds of samples from 15 different putting greens in three states. The results showed at least five samples are required to provide an accurate estimate of surface organic matter for most greens. The committee found no advantage in collecting more than 10 samples. Therefore five to 10 samples should be collected from each green to ensure accurate information. These samples should be taken at least 30 feet from each other. The number of putting greens to test is a decision that lies with the turf manager and may be based on variation in environmental conditions across the property, individual putting green performance, and a range of other factors. Within a putting green, avoid sampling areas that do not represent the majority of the surface – e.g., exit and entry points, mounds or low-lying areas, areas where sand accumulates from bunker shots, and shaded areas.
5. Sampling time: Surface organic matter was found to fluctuate throughout the year. Surface organic matter in cool-season putting greens was found to be lowest in summer and greater in spring and fall. Topdressing and cultivation events influenced surface organic matter as well. Therefore, to track changes in surface organic matter across years, samples should be taken at approximately the same time each year, with attention paid to topdressing and cultivation timing. For example, one could choose to sample in the spring every year, prior to the first topdressing and cultivation event. Another approach would be to test surface organic matter in August, prior to any topdressing or cultivation operations in that month.
Beyond these recommendations, there are several considerations that turf managers should be aware of:
1. As of this writing, most soil testing labs grind and sieve samples and use 360 C burn temperature for measuring organic matter. Ensure the lab you choose measures organic matter of the entire intact sample using 440 C without subsampling and without grinding or sieving.
2. There are two conventions for sampling depth: 0-1, 1-2, and 2-3 inches vs. 0-2, 2-4, and 4-6 cm. The committee did not address the differences between these two conventions, and both are likely appropriate for measuring and managing surface organic matter. Consistency will be most important.
3. Most of these recommendations were developed from samples of cool-season putting greens. Additional research on surface organic matter in warm-season putting greens is needed.
4. The next step for this committee is to create an ASTM (American Society of Testing Materials) standard by which all labs will utilize the same procedure for surface organic matter determination.
We are excited to have partnered with the USGA Green Section to develop this set of organic matter sampling and measurement recommendations. With widespread adoption, turfgrass managers will finally be speaking the same language when it comes to organic matter and it will become far easier to understand not only the trends at a particular course, but where those levels fit within the spectrum of desirable or undesirable quantities of surface organic matter. Following a consistent sampling and measurement procedure is an important step in helping superintendents know whether their greens have too much organic matter, not enough, or whether they’re just right.