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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August 2013 Phosphorus research: fertilizer

Products used in the fertilizer trials that formed the basis for this study.
Photos by Eric Miltner

Because 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).

Some 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).

August 2013 Phosphorus research: table 1

The 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).

Soil test phosphorus

Understanding 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 degradation.

Evidence 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.

The 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

For 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 treatments.

August 2013 Phosphorus research: table 2

Perennial 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

The 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 fertilizers

The 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 2).

August 2013 Phosphorus research: trial 1

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 fertilizers

The 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 course fairway.


For 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.

Phosphorus analysis

In 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.

August 2013 Phosphorus research: table 3

A 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

Values 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).

The 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).

Potential phosphorus loss

To 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.

August 2013 Phosphorus research: table 4

The 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) (3).

The 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 (Table 2).

Raising soil test phosphorus

We 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.

August 2013 Phosphorus research: trial 2

Organic fertilizer trials on the Home Course in DuPont, Wash. (data not shown).

Soil 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.

The 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.

These 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.

Some 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 water quality.


Funding 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.


This 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 State.