June 1, 2012

The phosphorus factor

Study shows properly fertilized turf can reduce phosphorus runoff


Turfgrass managers continue to face scrutiny on the types of inputs used to manage landscapes and playing surfaces. Questions and comments include: What type of fertilizers are you using, organic or synthetic? Are those fertilizers slow release? Why don’t you just use compost from the local recycling centre? I prefer not to fertilize my lawn because I don’t want to pollute the environment. As stewards of the environment, turfgrass managers are responsible for proper application of inputs, developing conservation strategies, and communicating those strategies with customers and the greater public.

Phosphorus is an essential element required for plant growth and development of turfgrass. In soils, it is relatively immobile except in runoff water and erosion of soils. Off-site transport of phosphorous to natural fresh waters can result in accelerated eutrophication. Eutrophication is a process that increases aquatic plant growth and subsequent reduction of water clarity, habitat loss, and decreased levels of dissolved oxygen.  When properly maintained, turfgrass does an excellent job at minimizing soil erosion. The concern in a turfgrass system results from runoff of dissolved (not particulate-bound) phosphorus where there are high amounts of organic residue from clippings and thatch, high soil test phosphorus, or recently applied phosphorus fertilizers.

Research has clearly demonstrated that added phosphorus fertilizer should not be applied following establishment when soil-test phosphorous levels are high.

There is no simple answer
Phosphorus fertilizer-use restrictions are popping up across the upper Midwest of the U.S. Michigan, Wisconsin, Illinois and Minnesota all have statewide use restrictions on fertilizer applied to turfgrass. These laws typically exempt golf courses and sod farms and provide an opportunity to apply phosphorous when establishing a new lawn or when a soil/tissue test states a deficiency.

Minnesota’s fertilizer restriction has been in place since 2004 (metro) and 2005 (statewide). The primary outcome from this law is that the amount of phosphorous fertilizers sold has been significantly reduced and most fertilizer manufacturers have formulated a zero-phosphorus fertilizer. The expectation in Minnesota was that surface water quality would improve now that phosphorous fertilizers applied to lawns have been removed from the market. No changes in water quality due to the Minnesota law have been documented. Unfortunately, it isn’t that simple.

Research nuts and bolts
A study was conducted on a silt loam soil at the University of Minnesota Turfgrass Research, Outreach and Education Center, with a 5 per cent slope and high soil test   phosphorous (27ppm Bray P-1) to evaluate phosphorous fertilization and clipping management effect on phosphorous runoff from Kentucky bluegrass. Phosphorus runoff was evaluated separately under frozen and non-frozen soil conditions. The study was conducted for five years following initiation of experimental treatments in September 2004.

Four fertilizer treatments were compared: (1) no fertilizer; (2) nitrogen and potassium only (0-P treatment); (3) complete fertilizer N + 1xP+ K; and (4) complete fertilizer N + 3xP+ K. For treatments 1 to 3, nitrogen was applied at 147 kg ha-1 (3 lbs. per 1000 ft.2). Potassium was applied based on soil test recommendations. Phosphorus rates were: 1xP = 49 kg ha-1 and 3xP = 147 kg ha-1 the first year (1 lb. and 3 lbs. P2O5 per 1000 ft.2, respectively) and 1xP= 16 kg ha-1 and 3xP = 49 kg ha-1 for the following four years (0.33 lbs. and 1 lb. P2O5 per 1000 ft.2, respectively).

The 1xP treatment in the first year was equivalent to the recommended phosphorous rate for turfgrass in Minnesota in the establishment year, at the measured soil test level. The 1xP treatment in the following four years was typical of the amount of phosphorous commonly applied before the widespread availability of zero P turf fertilizers. Each fertilizer treatment was evaluated with clippings removed or clippings recycled back to the turf for a total of eight experimental treatments.

Data were collected for total phosphorous (TP) and dissolved reactive phosphorous (RP) concentrations in runoff, runoff depth, amounts of TP and RP transported in runoff, turfgrass growth and quality, phosphorous concentrations in plant tissue, phosphorous uptake, and soil test P levels (Bray P-1) at two soil depths. In the fourth and fifth years of the study, TP measurements were not made and only RP was measured in runoff.

After five years of data collection, our results can be summarized as follows:
  • Phosphorus in water runoff, the soil, and turfgrass tissue increased linearly with increasing phosphorous fertilizer application rate
  • 86 per cent of phosphorus runoff occurred when the soil was frozen
  • 78 per cent of the water runoff (total volume of water running off the turfgrass) occurred when the soil was frozen
  • 72 per cent of runoff phosphorous was dissolved reactive P
  • No clipping management effect was found
  • In year one, highest phosphorous runoff occurred from the 3xP treatment
  • In years two to five, highest phosphorous runoff occurred from the no fertilizer treatment
  • Phosphorous runoff can be reduced without affecting turf quality by not applying phosphorous fertilizers when soil test P levels are high
  • Properly fertilized turf can reduce phosphorous runoff.
The story
Considering the results just presented, in parts of North America with prolonged frozen soil conditions, the majority of off-site movement of phosphorous from the landscape occurs during the winter when plants are not actively growing and the entire landscape is impervious. Stating this differently, management practices to mitigate or reduce off-site movement of phosphorous from the landscape can impact 14 per cent of the total P lost. Plants that are actively growing and actively managed reduce phosphorous runoff.

Further consideration of these results should also lead you to the fact that turfgrass, when properly fertilized, will actually improve runoff water quality from the landscape. At this research location, properly fertilized would be N and K only, because of adequate supply of phosphorous in the soil after establishment. Soil testing is the only way to identify what “properly fertilized” turf means at your location.

The public should not extrapolate or infer the following: If Phosphorus is bad for the environment then all fertilizer is bad for the environment. This is simply not true. From these results and in a separate five-year study on golf course fairway turf, any strategy to reduce runoff volume will reduce off-site movement of nutrients in runoff water. These strategies would include hollow-tine aerification to reduce compaction, nitrogen fertilizers to increase plant density, vertical mowing to reduce thatch accumulation, and proper fertilizer selection. When comparing hollow-tine and solid-tine aerification, hollow-tines reduce runoff volumes and phosphorous in runoff by 55 and 44 per cent, respectively (second after aerification). Follow the water!

With regards to proper fertilizer selection, late season fertilizers should not contain P. There is no published research that suggests any benefit from late-fall or dormant applications of phosphorous fertilizers, especially in parts of North America that experience frozen soil conditions.

Properly fertilized turf will improve water quality. Soil testing will determine phosphorous fertilizer needs. Not applying phosphorous when an adequate supply is available in the soil will reduce phosphorous runoff. Any management practice implemented to keep water on the landscape will reduce nutrient loading into surface waters.