Phosphorous plays a key role in germinating grasses. Whether extra P should be added has been a point of a number of research projects that have examined rates of P and turfgrass germination. GCM file photos
I get asked this question a lot: “I’ve always been told that germinating grasses need extra phosphorus (P). Given that, should I apply extra P fertilizer when seeding, regardless of the current soil-test P levels?” My general answer
is, “No — that P is already there in the soil, and your seedlings will find it.”
But I had never really explored the published research on this area, to see what had been done to answer this question. Given that phosphorus application may now be constrained by environmental regulation, I thought it was worth my time to take a look
to see what had been done to study rates of P and turfgrass germination.
So, we will start back in 1969. A pioneer of turfgrass science, C. Richard Skogley, Ph.D., published one of the first papers that studied rates of P for the establishment of a mixed Kentucky bluegrass/red fescue lawn. The background soil-test P was considered
“low” (22 pounds P per acre; 25 kilograms per hectare), but with revised soil-test guidelines, that likely would be considered more of a “medium” soil-test level today, with some P likely recommended.
Phosphorus was applied at rates of 44, 88, 176 and 352 pounds of P per acre (50, 100, 200 and 400 kilograms P per hectare, as superphosphate), with the P either raked into the top 0.4 inch (1 centimeter) of soil or incorporated throughout a 4-inch (10-centimeter)
depth prior to seeding. There were both spring and fall seedings, and researchers collected data such as color, quality and clipping weight. Applying P with shallow incorporation provided initially better turf than when the P was fully mixed in the
soil, but after one month there was no difference. And this was only in the fall planting, as the spring planting was never affected by P placement. Phosphorus rate never consistently affected color, quality, clipping yield or root weight, and no
recommendations for P fertilizer rates were made. So, in this study, a P soil test of 22 pounds P per acre was enough for establishment of Kentucky bluegrass.
What about Poa?
In 1986, the folks at the University of Florida examined P rates for Poa annua establishment. Using a very sandy soil that had an initial P content of 5.4 pounds per acre (6 kilograms per hectare), P was added at rates of 0, 36 or 107 pounds per acre
(0, 40 or 120 kilograms P per hectare), as calcium phosphate. Establishment and clipping yield of Poa annua increased as P rate increased, as initial soil-test P in this work was very low. This paper was one of the first to clearly demonstrate the
relationship between P and Poa, as the authors stated in the final paragraph of their paper: “In turf stands where annual bluegrass is a problem, P applications should be limited, if not omitted, depending on soil test values and the P requirement
of the desired turfgrass.”
In a greenhouse study that used a wide variety of soils and three soil extractants (Bray-1, Mehlich-1 and Mehlich-3), researchers at Rutgers found that the critical level of soil-test P for establishment of Kentucky bluegrass, perennial ryegrass and tall
fescue (as measured by clipping yield) were high, from 170 milligrams per kilogram to 280 milligrams per kilogram. However, when they then moved outside to a series of field experiments, the results were variable, and often the turfgrasses did not
respond to added P, as they did in the greenhouse trials. In the first field trial, Kentucky bluegrass establishment increased as applied P increased, while establishment of tall fescue and perennial ryegrass were unaffected by the rate of added phosphorus.
In the second field trial, none of the grasses established faster when P was added. Varying with the extractant and soil, soil-test P ranged from 28 to 86 milligrams per kilogram, indicating that no additional fertilizer P was needed for establishment
of soils tested in this range. The authors noted that this was an example of greenhouse studies incorrectly predicting fertilizer needs, likely a factor of changing environmental influences in the field.
In 2014 and 2015, a different soil-test extract was evaluated (the Olsen, used for calcareous soils), on a calcareous soil (pH 7.8) with initial P soil tests of 10.3 to 10.7 milligrams per kilogram. Phosphorus was added at rates of 0, 3.4, 10.0, 30.2,
50.2 and 66.9 pounds per acre (0, 3.8, 11.3, 33.8, 56.3 and 75 kilograms per hectare) as liquid phosphorus acid, with the fertilizer tilled into the top inch (2.5 centimeters) of soil. One week later, individual plots of Kentucky bluegrass, perennial
ryegrass, tall fescue, creeping bentgrass and red fescue were seeded. Establishment, shoot density, aboveground biomass and root volume were all measured throughout the trials. In Year 1, adding P sped the establishment of all the turfgrasses, with
variation between them for the amount of P that was needed. However, none of them needed more than 50.2 pounds P per acre for fastest establishment, and creeping bentgrass only needed the lowest P rate of 3.4 pounds per acre. In Year 2, perennial
ryegrass, tall fescue and red fescue had no response to P application, and in fact higher rates (> 30.2 pounds per acre) seemed to slow growth. So, a general soil-test (Olsen) range of 15 to 25 milligrams per kilogram (13-22 pounds per acre) was
found to be the critical range for establishing cool-season turfgrasses. If your Olsen soil-test values are below those, some P fertilizer might help to improve establishment of a newly seeded turfgrass stand, but there’s no need to apply more
if your soil-test P exceeds 25 milligrams per kilogram (22 pounds per acre).
Soil testing is a good method of determining the amoung of P in your soils and whether additional P should be added through fertilization on germinating turfgrasses.
If P is high, no need to add more
Most recently, a series of studies were conducted at Penn State to specifically determine if extra P was needed at seeding, even when soil-test P was high. Researchers conducted a series of four field experiments, all with seeded tall fescue. Initial
Mehlich-3 soil tests indicated soil P levels ranging from 38 to 53 milligrams per kilogram, and P was added (as monoammonium phosphate) at rates of 0, 21.9, 43.7 and 65.6 pounds P per acre (0, 24.5, 49.0 and 73.5 kilograms per hectare). Collected
data included percent groundcover and clipping yield. Over four experiments, there were few times when the addition of P increased the rate or amount of groundcover. Groundcover was measured three times at various dates in each experiment (for a total
of 12 data collections), and there was only one time in the 12 when the addition of P increased groundcover, and that was in soil with the lowest P soil-test (38 milligrams per kilogram). For this study, when Mehlich-3 soil test P exceed 38 milligrams
per kilogram, adding additional P had no effect on establishment.
So, if your soil-test P is already on the higher side, do you need to add additional P fertilizer when seeding? And, what exactly is that soil-test P level? The answer to the first question is largely “no.” With the exception of some greenhouse
research, P fertilization of soils that already had adequate P did not improve establishment of turfgrasses (in this article, everything studied were cool-season grasses). The soil-test P levels that seemed to be sufficient were somewhat variable,
but in general soil P values that ranged from 28 to 86 milligrams per kilogram for acid-based extracts (Bray and Mehlich 1 and 3) seem sufficient, and in calcareous soils, the critical values for soil test P (using the Olsen extract) were 15-25 milligrams
per kilogram.
For conversion, a few numbers should be mentioned. Your soil-test reports will likely report extractable P in parts per million (ppm). For conversion, a milligram per kilogram is the same as a ppm, so that’s easy. For a 6-inch-deep soil sample (which
is what most labs use for calibration), the conversion for ppm to pounds per acre is to multiply ppm by 2 (that’s a rule of thumb, but it works pretty well). And last, if you want to convert from P to P2O5 (because fertilizer bag numbers are
in P2O5), multiply your P number by 2.29 to get that P2O5.
Literature cited
- Chang, Z., X. Jin and D. Li. 2014. Phosphorus responses vary among cool-season turfgrasses during establishment from seed. Agronomy Journal 106(6):1975-1980 (https://doi.org/10.2134/agronj14.0225).
- Hamel, S.C., and J.R. Heckman. 2006. Predicting need for phosphorus fertilizer by soil testing during seeding of cool season grasses. HortScience 41(7):1690-1697 (https://doi.org/10.21273/hortsci.41.7.1690).
- King, J.W., and C.R. Skogley. 1969. Effect of nitrogen and phosphorus placements and rates on turfgrass establishment. Agronomy Journal 61(1):4-6 (https://doi.org/10.2134/agronj1969.00021962006100010002x).
- Liu, B., and P.J. Landschoot. 2018. Influence of phosphorus in starter fertilizer on the establishment of tall fescue. HortScience 53(12):1897-1906 (https://doi.org/10.21273/hortsci13416-18).
- Varco, J.J., and J.B. Sartain. 1986. Effects of phosphorus, sulfur, calcium hydroxide and pH on growth of annual bluegrass. Soil Science Society of America Journal 50(1):128-132 (https://doi.org/10.2136/sssaj1986.03615995005000010025x).
Beth Guertal, Ph.D., is the program director for the Center of Excellence on Mitigation, Adaptation and Resilience to Climate-Change in Haiti at Kansas State University’s Sustainable Intensification Innovation Lab. A past president of the Crop Science Society of America, she is a 25-year member of GCSAA.