Phosphorus availability in root zones as affected by fertilizer type
What are the effects of organic fertilizers that supply excess phosphorus when they are applied in quantities that supply sufficient nitrogen to turf?
G.K. Stahnke, Ph.D.; E.D. Miltner, Ph.D.; C.G. Cogger, Ph.D.; R.A. Luchterhand and R.E. Bembenek
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Products used in the fertilizer trials that formed the basis for this study.
Photos by Eric Miltner
of concerns about phosphorus effects on eutrophication of surface waters, local
and/or state governments in New Jersey, Maine, Florida, Wisconsin, Minnesota
and Washington have adopted restrictions on residential use of phosphorus- containing
fertilizers (4,11,12,13). Urban and suburban lawns pose a specific concern for potential
phosphorus loss because managed turfgrass often abuts impermeable surfaces such
as sidewalks, driveways and curbs, which provide a direct conduit for
phosphorus transport to storm drains and surface water (1).
phosphorus-restriction legislation is considering exempting organic fertilizers
based on the premise that risk of phosphorus loss is reduced with these
materials (13). However, many natural organic-based fertilizers (particularly
manures and municipal biosolids) supply an excess of phosphorus when applied at
rates to meet plant nitrogen needs (6,7,8). When high-phosphorus organic fertilizers
are applied repeatedly, excess phosphorus accumulates in soil, potentially
increasing the risk of runoff and leaching loss (11,14).
risk of loss of phosphorus from natural organic sources depends on the
availability as well as the concentration of phosphorus in those sources.
Although phosphorus from organic sources is generally less available to leaching
and runoff than synthetic phosphorus sources (5), phosphorus availability
varies widely by source (5). Phosphorus from biosolids tends to be less available
than manure phosphorus, but even among biosolids sources phosphorus
availability can vary widely (3).
the effect of repeated applications of natural organic lawn fertilizers on soil
test phosphorus can provide guidance for the suitability of these materials in
phosphorus-sensitive areas. If organic fertilizers have sufficiently low phosphorus
availability, they might be used without increasing the risk of water-quality
shows that the risk of soluble phosphorus loss occurs at much higher soil-test
levels than those needed for agronomic sufficiency. Researchers have proposed
alternative soil tests to assess environmental risks, such as phosphorus saturation
(PSIox), dissolved phosphorus index or water-extractable phosphorus (3,7,10).
No environmental soil phosphorus test is widely recognized and in common use.
Agronomic tests also have some value as environmental indicators (7). Another
factor is the effectiveness of phosphorus fertilizers in changing soil test
phosphorus, with greater effectiveness indicating more rapid change in soil
test phosphorus per unit of fertilizer phosphorus applied (poorer buffering)
and greater long-term risk of phosphorus loss.
objective of this study was to determine how repeated applications of
nitrogen-based organic fertilizer sources to established turfgrass affected
soil test phosphorus and phosphorus saturation in native soil and a sand-based
root-zone mixture under field conditions.
Fertilizer applications and Measurements
this study, fertilizers were applied on a nitrogen basis, using natural organic
and synthetic fertilizer sources on perennial ryegrass (Lolium perenne L.) plots on
two root-zone media over three years (July 2008-June 2011). Soil samples from
the plots were analyzed to determine changes in phosphorus availability in each
treatment area after three years of applications (2,7). Application rates of
the fertilizers were based on their nitrogen content for the original
experimental design; therefore, phosphorus levels were not equalized among
ryegrass was grown on both a Puyallup fine sandy loam native soil (coarse-loamy
over sandy, isotic over mixed, mesic Fluventic Haploxerolls) and a sand/peat
90/10 (%volume/volume) root-zone mixture (following USGA recommendations) in
the Puyallup Valley of western Washington, 34 miles (55 kilometers) south of
Seattle. The plots on the native soil were maintained at 2.5 inches (6.25
centimeters) as a home lawn, and the plots on the sand/peat mixture were
maintained at 0.5 inch (1.25 centimeters) as a golf course fairway. All grass
clippings were returned to the plots.
experimental design for each site was a randomized complete block with five
fertilizer treatments and four replications. Plot size was 5 feet × 10 feet
(1.5 meter × 3 meters). Each plot was fertilized with one of five treatments
(Table 1). The treatments included two natural organic fertilizer sources at a
1× and a 1.5× nitrogen rate and a synthetic slow-release product at a 1×
nitrogen rate. The target annual nitrogen rate (1×) for the native soil plots
was 131.15 pounds/acre (147 kilograms/hectare), consistent with recommendations
for home lawns, and the target annual nitrogen rate (1×) for the sand/peat
plots was 218.58 pounds/acre (245 kilograms/hectare), consistent with golf
course fairway management. Fertilization was split into three equal
applications per year on the native soil plots and five applications per year
on the sand/peat plots. The 1.5× rate treatments received 50% more fertilizer
on each application date.
organic fertilizer sources were Organic 6-7-0, made from anaerobically digested
and heatdried municipal biosolids and a commercially available Organic 8-3-5,
made from mixed animal by-products (Table 1). In the field, the Organic 6-7-0
nitrogen application rate was slightly higher than the Organic 8-3-5 rate
(Table 2) because the product was originally labeled as 5% nitrogen (5-4-0),
but subsequent analysis showed it to be 6-7-0 (Table 1). Based on the
fertilizers applied to each treatment on a nitrogen basis, the amount of phosphorus
added per year in the organic fertilizers ranged from 49.06 to 123.12 pounds P2O5/
acre (55 to 138 kilograms/hectare) for the Organic 8-3-5 and from 183.78 to
459.47 pounds/acre (206 to 515 kilograms/hectare) for the Organic 6-7-0 (Table
The plots with a sand/peat root zone are shown at two weeks after treatment. The plots are located
at the R.L. Goss Research Farm at the Washington State University Research and Extension Center
in Puyallup, Wash.
synthetic slow-release control nitrogen source was a 20-5-10 formulation
containing polymer-coated, sulfur-coated urea (PCSCU). The phosphorus in this
formulation was monoammonium phosphate. It was applied at the same nitrogen
rate as Organic 8-3-5. Phosphorus rates for this material were 33 pounds P2O5/acre
(37 kilograms/hectare) per year for native soil managed as a home lawn and
54.42 pounds/acre (61 kilograms/hectare) per year for sand managed as a golf
the native soil plots managed as a home lawn, fertilizer application dates were
August and October 2008; May, June and October 2009; April, August and October
2010; and April 2011. For the sand-based plots managed as a golf course fairway,
fertilizer application dates were August, October and November 2008; April,
June, July, September and November 2009; March, May, August, September and
November 2010; and March and May 2011.
July 2011, six to eight soil cores, each 1 inch (2.5-centiimeter) in diameter
and 4 inches (10-centimeters) deep, were taken from each plot. Unfertilized
control samples were taken at the same time from untreated areas surrounding
the plots. Verdure and thatch were discarded, and soil samples were air-dried
and analyzed for Bray-1 phosphorus (this is the “weak” Bray test, which measures
phosphorus that is readily available to the plants) and
ammonium-oxalate-extractable iron, aluminum and phosphorus. These data were used
to determine phosphorus saturation (PSIox) in each treatment in each soil type
(9). We also compared the effectiveness of the phosphorus fertilizers in
changing Bray-1 phosphorus.
similar oxalate extraction and calculation was done on the two natural organic
fertilizers to determine the relative degree of phosphorus binding with iron
and aluminum in each material.
Phosphorus levels and potential Losses
for Bray-1 extractable phosphorus were significantly higher in most of the
Organic 6-7-0 treatments when compared to the PCSCU fertilizer treatment. In
the native fine sandy loam soil managed as home lawn, the plots receiving Organic
6-7-0 1.5× treatments were significantly higher in extractable phosphorus than
the PCSCU treatment (Table 3), and in the sand-based fairway soil, both sets of
plots receiving Organic 6-7-0 treatments were significantly higher in
extractable phosphorus than the PCSCU treatment (Table 4).
plots receiving Organic 8-3-5 treatments showed a trend for higher Bray-1
phosphorus than the plots receiving synthetic fertilizer, but differences were
not significant in either soil. Bray-1 test levels were in the low range in the
pre-fertilization control soils and the PCSCU treatment in native soil, but
were in the medium or high ranges following three years of application of
natural organic fertilizers. In the Pacific Northwest, turfgrass shows little
or no response to added phosphorus in soils that test in the medium or high range
(>20 milligrams phosphorus/kilogram soil).
determine if the potential risk of soluble phosphorus loss had increased,
oxalate extractions of aluminum, iron and phosphorus were run to determine if
the fertilizer applications had affected phosphorus saturation (PSIox) for each
treatment and soil type. The results of these calculations showed no
significant difference between PSIox values for any of the fertilizer
treatments on native soil after three years of fertilizer applications (Table
3). However, on sand, both Organic 6-7-0 treatments had significantly higher
PSIox values than the other fertilizer treatments (Table 4). The change in
Bray-1 phosphorus was much greater than the change in PSIox, showing that the soils
had exceeded the upper threshold for plant response to phosphorus, but had not
yet reached a level of concern for soluble phosphorus loss.
PSIox of the fertilizers alone was 16.6 for the Organic 8-3-5 compared with 3.8
for the Organic 6-7-0 biosolids product. The PSIox of Organic 8-3-5 is similar
to that of chicken manure (PSIox = 15) (3), but the PSIox for Organic 6-7-0 was
higher than reported for a range of biosolids products (PSIox = 0.47 to 1.4)
Organic 6-7-0 applications had a greater influence on Bray-1 phosphorus and
soil PSI than the Organic 8-3-5, despite having a greater phosphorus binding
capacity, because nearly four times as much phosphorus was applied in the Organic
6-7-0 than in Organic 8-3-5. Organic 6-7-0 applications added six to nine times
as much phosphorus each year as the synthetic control, resulting in a large
excess of applied phosphorus when products were applied to meet nitrogen needs
soil test phosphorus
also calculated the relationship between the change in Bray-1 phosphorus
applied for both natural organic fertilizers in both soils to compare the
effectiveness of the fertilizers in raising soil test phosphorus. The change in
Bray-1 phosphorus averaged 0.057 milligram/kilogram for every kilogram/ hectare
fertilizer phosphorus applied in the native soil, with no significant
differences between the 8-3-5 and 6-7-0 fertilizers. In the sand/peat root-zone
mix, the phosphorus effectiveness averaged 0.105 milligram/kilogram Bray-1
phosphorus for every kilogram/hectare fertilizer phosphorus applied, also with
no differences between fertilizer sources. This suggests that the organic
fertilizers had similar effects on soil test phosphorus per unit phosphorus
applied, despite differences in the PSIox of the two materials.
Organic fertilizer trials on the Home Course in DuPont, Wash. (data not shown).
appeared to have a greater influence on phosphorus effectiveness than
fertilizer, with the sand mix having a greater phosphorus effectiveness (less
buffering) than the native soil. Because each experiment had only one synthetic
phosphorus treatment, we could not calculate the phosphorus effectiveness of
the synthetic phosphorus fertilizer in our soils.
sand/peat experiment can be considered a worst-case for soil response to
phosphorus application, because the coarse-textured soil is poorly buffered and
phosphorus application rates were higher than those used for home lawns. When organic
fertilizer with high phosphorus concentration and high PSIox was applied to the
sand/ peat plots, significant increases in both Bray-1 phosphorus and soil PSIox
were observed after three years. Although it would take longer, similar changes
would occur in the native soil, eventually increasing the risk of phosphorus
leaching and runoff loss.
results show the importance of evaluating fertilizer sources for the amount and
availability of phosphorus. The soil test results show that Bray-1 phosphorus
was higher when using phosphorus-rich organic fertilizer, compared with
synthetic fertilizer containing phosphorus because of the greater rate of
phosphorus application from the organic fertilizer applied at rates to meet
nitrogen needs. The greatest increase in Bray-1 phosphorus occurred in the
sand-based fairway treatment. Changes in soil PSIox were smaller, indicating
only small changes in phosphorus saturation and the risk of phosphorus loss
from the soil over the three-year duration of this study.
organic fertilizers could have phosphorus concentrations and PSIox values that
are low enough that they could be used for years without risk of increasing
phosphorus loss from soil, but that did not appear to be the case for the
fertilizers in this study. Our results suggest that use of highphosphorus organic
fertilizers to meet turf nitrogen needs would not likely lead to increased risk
of phosphorus loss in the short run, but repeated use in the long run could
increase future phosphorus loss risk. This information can provide guidance for
legislation regarding turf fertilizer sources, fertilization practices and
was supplied by the Washington State Department of Agriculture Nursery
Surcharge Grant and the Northwest Turfgrass Association. Fertilizer product for
the study was donated by Pierce County Public Works & Utilities, Simplot
Partners and Wilco-Winfield Solutions.
article was originally published in the online journal Applied
Turfgrass Science on March 25, 2013, as “Phosphorus availability in
turfgrass root zones after applications of organic and synthetic nitrogen
fertilizers” by Gwen K. Stahnke, E.D. Miltner, C.G. Cogger, R.A. Luchterhand
and R.E. Bembenek.
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GwenK. Stahnke is an associate professor, E.D. Miltner is a
former associate professor and C.G. Cogger is a professor in the department of
crop and soil sciences, Washington State University, Puyallup, Wash. R.A.
Luchterhand and R.E. Bembenek are turfgrass research technologists at the Washington