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Verdure: Yep, we did that before

How do soil pH and various levels of fertilizer phosphorus affect competition between Kentucky bluegrass and creeping bentgrass?

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Several of the journals where turfgrass scientists publish their research are well over 100 years old. That includes Agronomy Journal, with Volume 1, Issue 1 covering agronomic topics published from 1907 to 1909. Turfgrass research (other than forage work) did not start to appear in these journals until the late 1930s, when explorations into the production of turfgrass for golf courses and lawns first appeared in what was then called the Journal of the American Society of Agronomy. In 1948, the great turfgrass pioneer H.B. Musser published a research paper titled “Effects of soil acidity and available phosphorus (P) on population changes in mixed Kentucky bluegrass-bent turf.” And that’s a topical horse we beat to this day.

Following up some research from the 1930s that had been published in the USGA Green Section, Musser wanted to examine how populations of Kentucky bluegrass and bentgrass changed because of their differing tolerances to soil pH and fertility. So, in 1938, he modified a research area in State College, Pa., with sand and peat moss to “ensure a good physical condition.” The area was halved, with one half treated with sulfur to lower soil pH, and the other treated with lime (CaCO3) to increase soil pH. This resulted in respective soil pH of 5.0 and 6.6 at the time of seeding. These blocks were further split into phosphorus rates of 0, 2 or 12 pounds P2O5/1,000 square feet (0, 9.8 or 58.6 grams P2O5/square meter), with organic nitrogen (activated sewage sludge) or inorganic nitrogen (nitrate of soda, pretty much our only inorganic nitrogen fertilizer at this time) applied at 3 pounds nitrogen/1,000 square feet (14.7 grams /square meter) and potassium (as KCl) at 3 pounds K2O/1,000 square feet.

In the fall of 1938, the blocks were seeded with mixtures of Kentucky bluegrass, creeping bentgrass and browntop (Agrostis tenuis). Equal percentages of the two bentgrasses combined to make up 5%, 10%, 15% or 20% of the mixture, and the rest was Kentucky bluegrass. In a statistical sense, the study was a 2 (levels of soil pH) × 5 (levels of fertilizer phosphorus) × 4 (different percent seed blends) factorial combination of treatments. The entire study was maintained at a 1-inch (2.5-cm) mowing height because “it conforms to the general practice used on golf course fairways.” For the next three years, the relative percentage of turfgrass species in each plot was counted.

The first finding was that the relative percentage of bentgrass or Kentucky bluegrass that was seeded had no effect on species composition over time. That is, seeding bentgrass at 5% of the mixture did not affect species composition change any differently than seeding the bentgrass at 20% of the mix. Additionally, acidity and soil phosphorus levels had little effect on species composition, except in the establishment year, when the Kentucky bluegrass used the higher levels of phosphorus to establish at a greater percentage. After that establishment year, however, the same effect occurred over the range of soil phosphorus and soil pH found in the experiment — that is, the percentage of the turfgrass that was Kentucky bluegrass declined steadily over the three years of measurement. The steady decline in Kentucky bluegrass occurred regardless of soil pH, the percentage of Kentucky bluegrass in the seed mixture, or the amount of applied phosphorus. It appeared that the selected bentgrasses used in this study (and one was browntop, which is aggressive) were simply able to outcompete the Kentucky bluegrass across a wide range of soil pH and phosphorus conditions. Under these 1938-type fairway conditions, the bentgrass was able to outgrow the Kentucky bluegrass, and it did this over the range of different acidities and soil phosphorus conditions created in the experiment.

Source: Musser, H.B. 1948. Effects of soil acidity and available phosphorus on population changes in mixed Kentucky bluegrass-bent turf. Journal of the American Society of Agronomy 40:614-620.


Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.

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