Turf roots in putting green rootzones tend to grow where the soil is richest. If there is a darker-colored layer at the top 3-4 inches of the profile, roots are often confined to this area regardless of the turf species. Roots tend to stop at the interface between the area of elevated organic matter and the original sand rootzone installed during greens construction. This observation becomes even more pronounced as the discrepancy in moisture and nutrient content increases between the upper layer of the rootzone and the original sand below. You can’t hardly blame the roots because the darker sand contains more water and nutrients than the underlying sand that has limited moisture and nutrient retention, but much higher aeration porosity. The original mix is often selected to provide fast water infiltration, but the result is a droughty rootzone and very low cation exchange capacity.
At courses that apply enough topdressing to keep pace with organic matter accumulation, roots almost always grow deep and past the interface of the original greens mix and the sand and organic matter accumulated after construction. In such rootzones, there is no abrupt change in water- and nutrient-holding capacity. The rootzone is more consistent from top to bottom and supports a healthier, deeper root system – even for turf species like Poa annua and ultradwarf bermudagrasses, which are known for shallow roots. Moreover, rootzones with more consistent moisture retention produce drier surfaces and better playing conditions. Drier surfaces also are less likely to see disease and the infiltration rate is better than in rootzones with elevated surface organic matter.
If the putting green rootzones at your facility have a darker layer with high organic matter content at the surface, it is not too late to make a positive impact. I have worked with courses that have made significant improvements in as little as two growing seasons. First, submit samples for loss on ignition organic matter analysis of the top 1 inch or 2 centimeters of the soil by collecting at least five cores that are about 1.5 inch in diameter and sending them to a lab. If you want help with the sampling process or how to interpret and act on the soil test results, contact your regional USGA agronomist. The soil test information can then be used to guide aeration and sand topdressing inputs. More than likely, you will need to increase the rate and frequency of sand topdressing.
In summary, avoid making the top 3-4 inches of greens too comfortable for roots due to the excessive moisture- and nutrient-holding capacity that comes from elevated organic matter content. Roots in these situations rarely grow past the richer sand layer and down into the original rootzone mix. Apply enough sand to keep pace with organic matter production to maintain a rootzone with more consistent physical properties from top to bottom.
West Region Agronomists:
Brian Whitlark, senior consulting agronomist – bwhitlark@usga.org
Cory Isom, agronomist – cisom@usga.org