proceedings - Phosphorus Symposium

January 30, 2015
Wye Mills, Maryland
The Role of Phosphorus Management in the Green Pastures and Blue Waters Paradox
Dr. Andrew Sharpley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Impact of Phosphorus on Water Quality
Dr. Walter Boynton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Agricultural Phosphorus Sources – The Obvious and the Obscure
Dr. Peter Kleinman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
The Role of Hydrology in Connecting Agricultural Phosphorus Sources to Surface Water
Dr. Anthony Buda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Current Computer Models for Agricultural Phosphorus Management
Dr. Pete Vadas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Legacy Phosphorus
Dr. Douglas Smith
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Agricultural Best Management Practices to Address Phosphorus Loss
Dr. Joshua McGrath
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Implementation of Agricultural Phosphorus Management Policy in Maryland
Dr. Frank Coale
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Presenter Biographies
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Hosts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Andrew Sharpley
Department of Crop, Soil and Environmental Sciences, Division of Agriculture,
University of Arkansas, Fayetteville, Arkansas
Recent high profile harmful algal bloom outbreaks and an inability to meet water quality
goals and targeted load reductions have increased attention on agriculture’s role in contributing
phosphorus (P) to surface water impairment and the effectiveness of current and future
conservation strategies designed to mitigate such loads. Research on the sources and pathways
of P loss predates most of us, yet we still deal with unintended consequences of some
conservation measures. To a large extent, these concerns are fueled by a public debate of the
wise use of conservation funding, an underlying desire for “quick fixes,” and an underestimation
of the legacies of prior nutrient management. These problems are not unique to the Chesapeake
Bay Watershed nor are our options for future P management. As a result, lessons can be learnt
and regional and national research, demonstration, and extension efforts can guide approaches
towards sustainable P management and conservation systems.
Until the much publicized occurrences of Pfiesteria in the Chesapeake Bay in the mid1990’s, there was a continuing emphasis on managing the land application of manures and
compound fertilizers for nitrogen (N) to meet annual plant needs and secondarily to limit nitrate
leaching to ground waters. This management philosophy was supported by the 60’s school of
thought that land application of P was akin to putting money in the bank, and that P was bound to
soil and not going anywhere, except being there for the next crop or forage plant to take it up.
Our knowledge and message that as soils became more saturated with P, P could more be easily
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 2
released to surface runoff waters or leach through the soil, was not heeded by land managers nor
did we properly translate the information to the farming community. The end result, however,
was the continued application of manure P at levels greater than crop uptake, increased soil P
and consequent risk of off-site movement. However, as recent events in several watersheds
across the U.S. highlight, there are many other site, weather, and management factors that
influence these losses.
Here, there is a brief description of recent efforts to manage agricultural P in the face of
water use impairment for three locations in the U.S., spanning a wide range in scale from the
Mississippi River Basin (MRB) to Lake Erie Watershed to the Eucha-Spavinaw Illinois River
Watershed in northwest Arkansas. These three efforts illustrate lessons pertinent to current and
future P management in the Chesapeake Bay Watershed.
Figure 1. “4R” source management and conservation management of P in agricultural
production systems.
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 3
The MRB drains 41% of the 48 contiguous U.S. states (or 1,245,000 miles2). Thus,
spatially expansive sources of P and the relative contributions of municipalities, industries, and
agriculture, create system complexities that should not limit action by any one entity. Recent
model estimates (USGS-SPARROW) suggest that up to 85% of the P and N entering the Gulf of
Mexico originate from agriculture. While these estimates are based on large-scale modeling
within the MRB, there have been few farm-scale studies of P and N loss from agricultural
production systems in the Basin. Even so, NRCS used model estimates to prioritize allocation of
conservation cost-share funding under the 2009 Healthy Mississippi River Basin Initiative
(MRBI; $320 million over 5 years) to 41 of the top contributing 12-digit watersheds in 13 states
along the Mississippi River corridor. One unique aspect of this program was the provision for
financial incentives to producers to conduct edge-of-field monitoring under NRCS Conservation
Practice Standard 201 and 202.
Since MRBI, NRCS has implemented similar initiatives in other watersheds. These
initiatives have been very successful in getting conservation systems approaches implemented on
a large acreage of agricultural lands to help producers avoid, control, and trap P, N, and sediment
and address water quality concerns. However, a transparent framework is needed to document
and verify on-the-ground conservation practices to ensure agricultural stakeholders are credited
with accurate reductions. This need will only grow in importance and urgency over the coming
years, in light of recent litigation against individual farms and farming communities in MRB
states of Arkansas, Iowa, Minnesota, and Wisconsin, with the intent of halting perceived source
water impairment related to P and N inputs (see Related Reading for additional information).
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 4
The richly documented history of Lake Erie water quality, outbreaks of harmful algal
blooms and land management in the Lake Erie Watershed over the last 50 years, provides an
excellent example of how well-meaning conservation strategies, can result in intended and
unintended consequences on P fate and transport within the watershed continuum. Steady
declines in P inputs from predominantly agricultural watersheds were measured between 1980
and 1995 with the adoption of best management practices (BMPs) such as increased nutrient
management planning (NMP) that reduced fertilizer and manure applications to corn and
soybeans and a transition to no-till cropping.
However, increased P inputs from agricultural runoff, mainly in the biologically available
dissolved form, over the last decade, have resulted from complex, dynamic, and yet predictable
factors. These include the accumulation of P at the soil surface, fall application of fertilizer,
continued broadcasting of P, a focus on implementing BMPs for particulate P loss, rapid rise in
tile drainage fuelled by higher grain prices, and release and remobilization of fluvial P. For
instance, more fields with tile drainage that connect to ditches and streams have increased,
contributing source areas of legacy P to Lake Erie. The combination of these factors created a
“perfect P loss storm,” which along with more intense summer rains increased P inputs to Lake
Erie to record levels in 2010, culminating in the 2014 toxic bloom and water crisis in Toledo,
OH. However, scientifically valid remedial strategies were not likely to be readily adopted by
farmers due to several logistical, practical, and cost limitations. Clearly, the research community
needs to work closely with the farming community to generate innovative support, stewardship,
reward, and trading program that will empower change.
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 5
Figure 2. Conceptual representation of legacy P processes.
The Eucha-Spavinaw and Illinois River Watersheds (ESIRW) have the unfortunate
distinction of having the application of any form of P to agricultural land, tightly litigated. A
rapid, five-fold increase in the population in Northwest Arkansas over the last 20 years has
coincided with the expansion of confined poultry broiler operations, which now produce over 2
billion birds annually, nearly 25% of the total broiler production in the U.S. In 2001, the City of
Tulsa, Oklahoma and in 2004 the Attorney General of Oklahoma filed lawsuits to mitigate the
accelerated eutrophication of municipal water supplies and Eucha-Spavinaw reservoirs, and Lake
Tahlequah, respectively. As part of settlement for the 2001 lawsuit, in 2004 the Judge mandated
the use of a P Index developed specifically for the watershed, required at least a third of the
generated litter be exported out of the watershed, and imposed a soil P ceiling for litter
application, preventing application to soils with a Mehlich-3 > 300 mg kg-1 (see Additional
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 6
Reading for litigation and Index information). As part of a court settlement agreement in 2013,
this threshold was cut in half, making it much more restrictive: no P can now be applied to soils
with a Mehlich-3 > 150 mg kg-1. A decade on from the initial agreement, P management and
water quality outcomes provide examples of the intended and unintended consequences of these
actions, which are relevant to the Chesapeake Bay Watershed.
Since the required NMP process was set in place, land application of poultry litter has
decreased from an average of 2.5 tons ac-1 before litigation to 1.2 tons ac-1 in 2014. In addition,
80 to 90% of the produced litter has been transported out of the Eucha-Spavinaw Watershed
(~75,000 tons year-1) and about 40% out of the larger Illinois River Watershed (~100,000 tons
year-1) since 2006. In other words, a lot less poultry litter is now being applied to pastures,
which has greatly reduced the amount and risk of P runoff. As is usually the case, there is a
consequence to using so much less litter on pastures in ESRIW, where beef grazing operations
have had a symbiotic relationship with poultry operations, using litter as a low-cost source of N
and P, allowing a more profitable cattle production than before the chickens came. In fact, the
ability to land apply less litter has led to a slow decline in beef herd size and pasture
Despite initial concerns that restrictions placed by the court case would force poultry
growers out of the litigated watersheds, poultry farmers have adapted to the P-based regulations,
in part through subsidies supporting manure export. Even with subsidies, a key component of
the success of the manure export program are the ties made between the fertilizer industry,
particularly distributors, and the livestock industry so that mutual goals are achieved. In order to
maintain the economic viability of all farming enterprises, not just the poultry farms, it has
become clear that the NMP process must go beyond addressing poultry litter application rates
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 7
and environmental risk and include educational efforts to help farmers develop sustainable
farming operations.
Have all these changes in P and litter use in ESIRW translated in an improvement in
water quality? Given that lower point P inputs from wastewater treatment plant upgrades have
also occurred in the last 10 years, it is not possible to definitely say yes. Further, annual
variations in stream flow, means lower concentrations of P have not led to a statistically
significant decrease in annual P flux. However, the implementation of a threshold total P
concentration of 0.037 mg L-1 as water flows from Arkansas into Oklahoma in the Illinois River,
currently dictates the success or failure of water management and conservation efforts. While
the 0.037 mg P L-1 standard is not yet met on an annual mean flow-weighted basis,
concentrations have decreased by a third compared with pre-2003 levels (i.e., 0.29 mg P L-1 in
2002 to 0.07 mg P L-1 in 2013). Clearly, progress is occurring.
In the three examples given here, there has been little attempt to quantify legacy sources
of P from past management practices and to determine their relative contribution to current
fluxes of P and the potential of legacy P sources to mask conservation benefits. Thus, this leads
us to P management paradoxes.
Common threads interweave among the examples given, which show the pressures
placed on farmers to maximize yields in response to increasing demand for cheap food, feed, and
fuel. To a large extent this has occurred through the development and use of fertilizer products.
At the same time, there is increased pressure being placed on farmers to be environmental
stewards. However, despite a long history of soil and water P research, management questions
still exist and water-use impairment continues as a result of P enrichment of soil-water systems.
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 8
Thus, we need to accept that current research knowledge should be translated and transferred
better to farmers, while at the same time realizing that farm management decisions are largely
driven by competitive economics. This leads to four current P paradoxes relevant to this
1) Blue – green paradox: An increasingly affluent population is becoming more
demanding of cheap, reliable food sources and wanting inexpensive clean, safe water
for many essential and recreational uses. As we have moved from nutrient
management that improves crop production to the environmental quality arena, we
face many challenges in balancing competing demands for protecting and restoring
water quality and aquatic ecology, with sustainable and efficient agricultural
production. It is important to recognize that market prices do not always motivate
farmers to manage nutrients in an environmentally sustainable way. Consumers can
be given a choice about which products they buy, with premiums paid to farmers who
provide more environmentally friendly products. However, after the low hanging
fruit of remedial measures are adopted, remaining BMPs become increasingly less
cost beneficial and raise the old dilemma “who benefits and who pays?”
2) Conservation legacy P paradox: Many conservation practices have been implemented
to trap and retain P on the landscape rather than enter waterways. Yet, the capacity of
those practices to retain is finite and there are more and more examples of
conservation practices (e.g., buffers, wetlands, reservoirs - Conowingo Dam)
transitioning from P sinks to P sources. Research that better quantifies the sinks and
sources of nutrients as they are transported through a watershed, and the legacies and
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 9
lags from past land use, will help develop realistic expectations for BMP use and the
timescales for aquatic ecosystem recovery.
3) Soil health paradox: Many important NRCS initiatives are rightly promoting
improved soil health as a major goal of future agricultural management practices.
However, some of the claims that improved soil health will stop nutrient runoff and
leaching are misguided. For instance, practices such as no-till can lead to a surface
accumulation of applied P, which can enrich dissolved P runoff, as well as a greater
potential for leaching through intact macropores, unless there is either a concomitant
change in fertilizer and manure management or occasional soil destratification.
4) The grain for fuel paradox: With increasing pressures to meet biofuel mandates, 42
and 25% of the corn and soybean grown in the U.S. was used to produce biodiesel in
2012. In some areas, CRP and environmentally sensitive lands were allowed to go
back into grain production; large tracts of land have been tiled drained, increasing
source areas and connectivity of soils directly to streams and bypassing the soil
matrix where P might have otherwise been sorbed; and in other areas crop residue is
removed as biomass fuel increasing the potential for runoff and erosion.
Clearly, agricultural P issues facing the Chesapeake Bay Watershed are neither new nor
specific to the Bay Watershed and lessons can be gleaned from other remedial efforts in the U.S.
This Symposium “The State of the Science of Phosphorus,” will highlight some of the Peffective BMPs, where and when on the landscape they would be most effective, and how
response to change will occur.
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 10
Mississippi River Basin:
Dale, V.H. et al. 2010. Hypoxia in the Northern Gulf of Mexico. Springer Series on
Environmental Management. Springer Science, New York, NY. 284 pages.
USGS SPARROW model - Alexander, R.B., R.A. Smith, G.E. Schwarz, E.W. Boyer, J.V.
Nolan, and J.W. Brakebill. 2008. Differences in phosphorus and nitrogen delivery to the
Gulf of Mexico from the Mississippi River Basin. Environmental Science and
Technology 42:822-830.
USDA-NRCS Mississippi River Basin Initiative
Examples of farming litigation:
Wisconsin :
Lake Erie:
Baker, D.B., and R.P. Richards. 2002. Phosphorus budgets and riverine phosphorus export in
northwestern Ohio watersheds Journal of Environmental Quality 31:96-108.
Richards, R.P., D.B. Baker, and J.P. Crumrine. 2009. Improved water quality in Ohio tributaries
to Lake Erie: A consequence of conservation practices. Journal of Soil and Water
Conservation 64:200-211.
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 11
Daloglu, I., K.H. Cho, and D. Scavia. 2012. Evaluating causes of trends in long-term dissolved
reactive phosphorus loads to Lake Erie. Environmental Science and Technology, 46:
Michalak, A.M. et al, 2013. Record-setting algal bloom in Lake Erie caused by agricultural and
meteorological trends consistent with expected future conditions. Proceedings of the
National Academy of Sciences, 110: 6448-6452.
Joosse, P.J., and D.B. Baker. 2011. Context for re-evaluating agricultural source phosphorus
loadings to the Great Lakes. Canadian Journal of Soil Science 91:317-327.
Eucha-Spavinaw and Illinois River Watersheds:
Sharpley, A.N., R.P. Richards, S. Herron, and D.B. Baker. 2012. Can production and environmental
goals coexist in phosphorus-based farm management? Journal of Soil and Water Conservation
Herron, S., A.N. Sharpley, S. Watkins, and M. Daniels. 2012. Poultry litter management in the Illinois
River Watershed of Arkansas and Oklahoma. Cooperative Extension Service, Division of
Agriculture, University of Arkansas. Fact Sheet FSA 9535. 4 pages.
DeLaune, P.B., B.E. Haggard, T.C. Daniel, I. Chaubey, and M.J. Cochran. 2006. The Eucha/Spavinaw
phosphorus index: A court mandated index for litter management. Journal of Soil and Water
Conservation 61: 96-105.
Robertson, G.P., V.H. Dale, O.C. Doering, S.P. Hamburg, J.M. Melillo, M.M. Wander, et al.
2008. Agriculture - Sustainable biofuels Redux. Science 322: 49-50.
Sims, J.T., and A.N. Sharpley. 2005. Editors. Phosphorus; Agriculture and the Environment.
American Society of Agronomy Monograph. American Society of Agronomy, Madison,
WI. 1121 pages.
Sharpley, A.N. 2000. Agriculture and Phosphorus Management: The Chesapeake Bay. CRC
Press, Boca Raton, FL.
Sharpley, A.N., T. Daniel, T. Sims, J. Lemunyon, P. Stevens and R. Parry. 2003. Agricultural
Phosphorus and Eutrophication. United States Department of Agriculture - Agricultural
Research Service. ARS-149. 38pp.
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 12
Sharpley, A.N. T.C. Daniel, G. Gibson, L. Bundy, M. Cabrera, T. Sims, R. Stevens, J.
Lemunyon, P.J.A. Kleinman, and R. Parry. 2006. Best management practices to
minimize agricultural phosphorus impacts on water quality. USDA-ARS Publication
163. U.S. Government Printing Office, Washington, DC.,%20ARS%20163%202006
Dr. Andrew Sharpley | The State of the Science of Phosphorus Symposium | January 2015 | Page 13
Walter Boynton, University of Maryland Center for Environmental Science Chesapeake
Biological Laboratory, Solomons, Maryland
This presentation summarized the current understanding of the role of phosphorus in
estuarine eutrophication, a process largely driven by excess amounts of both phosphorus and
nitrogen entering these ecosystems. Relatively simple conceptual models have been developed
to capture the various water and habitat quality impacts of nutrient enrichment. More recently,
strong feedback mechanisms have been identified wherein ecosystem health can change rapidly
when these feedback systems come into play. In the case of phosphorus in aquatic ecosystems,
low dissolved oxygen conditions, elevated water pH and salinity all play a role enhancing
phosphorus mobilization from sediments. Field and laboratory studies and simulation modeling
all indicate rapid phosphorus responses to changing environmental conditions and a longer
“system memory” for phosphorus than for nitrogen.
Key points:
The basic model of nutrient enrichment and restoration is solid…stay with it!
The Duel Nutrient reduction strategy is sound…both Phosphorus and Nitrogen play
powerful roles in Bay water and habitat quality
Substantial reductions of Nitrogen and Phosphorus result in improved water quality
and better habitat conditions
Dr. Walter Boynton | The State of the Science of Phosphorus Symposium | January 2015 | Page 14
The pathways estuaries follow during degradation and restoration often involve time
delays (lags), abrupt changes (thresholds) and other things not yet known or fully
Restoration trends (and hints of trends) have been observed in both small and large
Chesapeake systems…very good signs!
Dr. Walter Boynton | The State of the Science of Phosphorus Symposium | January 2015 | Page 15
Pete Kleinman, USDA-ARS, Pasture Systems and Watershed Management Research Unit,
University Park, Pennsylvania and Doug Beegle, Penn State University
The persistence of agricultural phosphorus management concerns can be attributed to
many causes, but, at its core, is the product of fundamental processes that are often undeterred by
conventional conservation practices. When viewed through the lens of agricultural development,
or basic soil fertility management, phosphorus can be seen as a limiting macronutrient that is so
inherently sticky in nature that large amounts are needed just to overwhelm the basic buffering
capacity of soils. Indeed, the majority of our soils still need to be approached with this
perspective in order to ensure that our crop production keeps track with demand. It is from this
perspective that the quantity-intensity relationship was first understood: for every unit of
phosphorus that we hope to reside in the soil solution (where crops can readily access it), at least
ten times that amount must be sacrificed to the various binding agents in soils. All food, organic
or conventional, owes its phosphorus to our ability to overcome the fundamental inequality of
phosphorus chemistry. Indeed, overcoming this inequality is at the foundation of our civilization,
regardless of whether it is recognized by society.
It is therefore not surprising that many of us were educated with the generalization that
the water quality problems of phosphorus stem from erosion – the single largest reserve of
phosphorus in our landscapes is that which resides in soils. Conserve the soil and one prevents
phosphorus from running off of our lands. To a large extent, this generalization remains true.
However, a closer inspection of phosphorus in soils reveals complexity in its distribution that
readily roasts old chestnuts. Phosphorus is most concentrated in the finest particles of soils, those
with the greatest surface area and most prone to erosion. These particles are increasingly selected
Dr. Pete Kleinman | The Science of the State of Phosphorus Symposium | January 2015 | Page 16
as erosion is controlled, a phenomenon known as phosphorus enrichment. Improve soil
conservation and the sediment that does erode is more concentrated with phosphorus than the
sediment that dislodges from poorly conserved soils. Go figure.
Most vexing is the problem of dissolved phosphorus release to runoff. Here, focusing on
the “intensity” component of quantity-intensity relationships offers perspective, and hope, in
management. At its simplest, the more phosphorus in the soil the greater the concentration in
runoff. This can be described with new “environmental” soil tests or with old fashioned
agronomic tests. Although agronomic tests tend to offer sufficient perspective into soil sources of
P, the concept of soil phosphorus saturation can be an important educational device that may
even be estimated from existing sources of data without requiring new expense.
Soils are three dimensional resources that must be managed accordingly. Vertical
stratification, i.e., the accumulation of phosphorus in a thin veneer at the soil surface, occurs so
readily that it can be observed over just a few crop rotations under the right conditions. As a
result, standard soil testing protocols are prone to underestimating the edaphic sources that are
available to runoff waters. Furthermore, in the wrong location, a small amount of soil
phosphorus will enrich runoff waters with every event. When phosphorus leaching is a concern,
this may mean that low concentrations at deeper depths, too deep to manage and too low to be of
normal concern, can be a source.
The fact that many of our greatest watershed phosphorus concerns are tied to areas of
intensive livestock production and land application of manure is no coincidence. While manures
may enrich soils and indirectly contribute to runoff concerns of soil phosphorus release, they
may also contribute directly to runoff, particularly over the short term. “Wash off” or “incidental
transfer” of manure phosphorus is related to many factors, including the inherent concentration
Dr. Pete Kleinman | The Science of the State of Phosphorus Symposium | January 2015 | Page 17
and form of phosphorus in manure. Great strides have been made in lowering manure P
concentrations for all livestock, arising from feed additives and formulations. However, these
advances have not necessarily transferred to hobby and equine operations, which account for a
substantial quantity of the manure produced in the Chesapeake Region (almost 10% of the
manure dry matter in the watershed derives from horses). Adjusting the solubility of phosphorus
in manure has proven to be productive, particularly with poultry where it can be tied to ammonia
management in barns.
Manures do not only have to be brown. One of the least appreciated sources of
phosphorus is vegetation. When plants senesce or are killed with herbicides, they release
dissolved phosphorus from their cells that can enrich runoff water. Case studies from
Scandinavia to Ohio raise concern with dissolved phosphorus from cover crops. But cover crops
help to conserve soils and capture soil nitrate. Considering these trade-offs, particularly in areas
where dissolved phosphorus is a primary concern to water quality, is a challenge that must not be
taken lightly.
King, K.W., M.R. Williams, M.L. Macrae, N.R. Fausey, J. Frankenberger, D.R. Smith, P.J.A.
Kleinman, and L.C. Brown. 2014. Phosphorus Transport in Agricultural Subsurface
Drainage: A Review. J. Environ. Qual.: DOI:10.2134/jeq2014.04.0163.
Kleinman, P.J.A., A.N. Sharpley, R.W. McDowell, D. Flaten, A.R. Buda, L. Tao, L. Bergstrom
and Q. Zhu. 2011. Managing Agricultural Phosphorus for Water Quality Protection:
Principles for Progress. Plant and Soil 349: 169-182.
Kleinman, P.J.A., A.N. Sharpley, A.R. Buda, R.W. McDowell, and A.L. Allen. 2011. Soil
controls of phosphorus runoff: management barriers and opportunities. Canad. J. Soil Sci.
91: 329-338.
Dr. Pete Kleinman | The Science of the State of Phosphorus Symposium | January 2015 | Page 18
Kleinman, P., D. Beegle, K. Saacke Blunk, K. Czymmek, R. Bryant, T. Sims, J. Shortle, J.
McGrath, D. Dostie, R. Maguire, R. Meinen, Q. Ketterings, F. Coale, M. Dubin, A.
Allen, K. O’Neill, M. Davis, B. Clark, K. Sellner, M. Smith, L. Garber, and L. Saporito.
2012. Managing manure for sustainable livestock production in the Chesapeake Bay
Watershed. J. Soil and Water Conserv. 67: 54A-61A.
Kleinman, P.J.A., C. Church, L.S. Saporito, J.M. McGrath, M.S. Reiter, A.L. Allen, S. Tingle,
G.D. Binford, K. Han and B.C. Joern. 2014. Phosphorus leaching from agricultural soils
of the Delmarva Peninsula, USA. J. Environ. Qual.
Sharpley, A., H. P. Jarvie, A. Buda, L. May and P. Kleinman. 2013. Phosphorus legacy:
Overcoming the effects of past management practices to mitigate future water quality
impairment. J. Environ. Qual. 42: 1308-1326.
Dr. Pete Kleinman | The Science of the State of Phosphorus Symposium | January 2015 | Page 19
Anthony R. Buda, Research Hydrologist, USDA - Agricultural Research Service,
Pasture Systems and Watershed Management Research Unit, University Park, Pennsylvania
Minimizing the risk of phosphorus (P) loss from land to water represents one of the most
important priorities of nutrient management in the Chesapeake Bay watershed. Simply put, for P
to pose a water quality problem, there must be a source of P that can readily be connected to
surface water by hydrologic transport processes. Areas with high P sources that lack hydrological
connectivity do not typically constitute a water quality risk. By the same token, areas with high
hydrological connectivity that do not link to high P sources also pose little threat. While the role
of hydrological connectivity in P loss is simple to articulate, uncertainty arises when we try to
observe the hydrological processes and landscape features linking P sources to receiving waters,
to model and represent these processes in risk assessment tools, and to control them with targeted
management measures. Indeed, recognizing and addressing these uncertainties is central to
advancing the science of P management across the Chesapeake Bay watershed.
The critical source area concept is perhaps the most recognizable illustration of how
hydrological connectivity has been incorporated into P management, most notably the P Index.
The critical source area concept posits that areas of the landscape posing the greatest risk to P
loss are those where high P sources and high transport potential coincide (as shown below).
Dr. Anthony Buda | The State of the Science of Phosphorus Symposium | January 2015 | Page 20
In upland areas of the Bay watershed, variable source area hydrology dominates such that
small areas of watersheds are responsible for the majority of surface runoff. These runoff
generating zones, typically found near streams and in lower landscape positions, expand and
contract during storms and over seasons as a result of interactions between ground and surface
water, storm characteristics, and soil properties. It is from this perspective that distance to
receiving water has oft been viewed as one of the best proxies for hydrological connectivity in
nutrient management.
While the greatest risk of P loss in upland regions is usually confined to variable source
areas near streams, a host of hydrological processes can modify and enhance hydrological
connectivity, thus activating distant P source areas that otherwise would not normally contribute
to P loss in agricultural watersheds. Chief among these are preferential flow pathways, including
macropores, soil pipes, and fractures, as well as shallow lateral flows induced by the presence of
soil or bedrock confining layers. While digital soil mapping, near-surface geophysics, and tracer
studies can offer insight into the spatial arrangement and extent of preferential flow paths, the
activation of these pathways in time and space remains dynamic and threshold-dependent (e.g.,
affected by variable rainfall characteristics and antecedent conditions), which complicates our
ability to adequately represent these flow paths in P risk assessment tools. Even when we have
good knowledge of preferential flow networks and their hydrological behavior, determining
Dr. Anthony Buda | The State of the Science of Phosphorus Symposium | January 2015 | Page 21
whether they are linked to P source areas is often hindered by fact that P sources (edaphic and
applied) are rarely, if ever, mapped in detail across agricultural landscapes.
Farm infrastructure and daily farming activities also play a role in shaping the
hydrological connectivity of agricultural landscapes. For example, impervious surfaces such as
barnyards, roads, and roofs can rapidly generate stormwater runoff and connect P sources on the
farm with nearby receiving waters. This problem may be especially acute in areas where farms
are intensifying operations by adding significant infrastructure. Soil compaction by heavy farm
machinery and animal activity, as well as practices that orient soil roughness features parallel
with topographic slope (e.g., the direction of plowing and cropping patterns) also create
concentrated flow pathways that increase the risk of P loss. Animal heavy-use areas, including
streamside dairy and beef cattle loafing areas, offer a notable example of how compacted soils
and high P sources combine to produce a significant risk of P loss in concentrated runoff. While
some success has been achieved in applying filters with P sorbing materials to remove P from
stormwater generated in loafing areas, low hanging fruit such as improved water management on
the farm may be the best long-term strategy to reducing offsite P losses in barnyard runoff.
Artificial drainage represents an area of heightened concern with regard to hydrological
connectivity and P loss, particularly on the flat, poorly-drained soils of the Delmarva Peninsula.
Here, networks of open ditches, and to a lesser extent, buried tiles lines, are commonly used to
drain fields for crop production. In many cases, fields on the Delmarva Peninsula possess soil P
levels well in excess of crop requirements. Recent evidence highlights the importance of
macropores and soil cracks as conduits for preferential flow, allowing P from surface soils to
leach to shallow groundwater, where it can then move laterally to nearby ditches or streams. At
present, most water quality simulation models are ill equipped to simulate these shallow
Dr. Anthony Buda | The State of the Science of Phosphorus Symposium | January 2015 | Page 22
preferential flow pathways, pointing to the need for new experiments that provide insight into P
contributing areas and hydrological connectivity in near-ditch zones. At the watershed scale,
determining the intensity of artificial drainage (depth and spacing) is critical to assessing the
connectedness of the landscape and its capacity for delivering P to surface waters. Unfortunately,
information on the geometry and spatial patterns of field-scale artificial drainage networks are
rarely mapped, precluding our ability to reliably capture this risk in P site assessment tools.
Looking to the future, we would be remiss to ignore the fact that hydrological
connectivity affects and is affected by changes that occur well beyond the farm or small
watershed scales that dominate our P management concerns. One such example is the
Conowingo Dam, which has served to trap P-rich sediments in the Susquehanna River for the
past 85 years, thereby reducing downstream losses of particulate P to the Bay. As the capacity of
the dam has been reached, particulate P losses have begun to trend upward again, particularly
during large storm events, suggesting that the pool of legacy P stored in sediments behind the
dam will once again be reconnected with the Chesapeake Bay. Projected changes in climate also
portend potential changes in hydrological connectivity throughout the Bay watershed as extreme
storms, shifting rainfall patterns and intensities, and longer dry spells alter the hydrological
processes that mobilize and transport P. Short and long-term hydroclimatic forecasting tools will
be needed to help farmers and nutrient managers anticipate changes in hydrological connectivity
that increase the risk of incidental and chronic P losses in runoff, and adapt their management
King, K.W., M.R. Williams, M.L. Macrae, N.R. Fausey, J. Frankenberger, D.R. Smith, P.J.A.
Kleinman, and L.C. Brown. 2014. Phosphorus Transport in Agricultural Subsurface
Drainage: A Review. J. Environ. Qual.: DOI:10.2134/jeq2014.04.0163.
Dr. Anthony Buda | The State of the Science of Phosphorus Symposium | January 2015 | Page 23
Sharpley, A.N., H.P. Jarvie, A.R. Buda, L. May, B. Spears, and P.J.A. Kleinman. 2013.
Phosphorus legacy: overcoming the effects of past management practices to mitigate
future water quality impairment. J. Environ. Qual. 42(5): 1308-1326.
Buda, A.R., G.F. Koopmans, R.B. Bryant, and W.J. Chardon. 2012. Emerging technologies for
removing nonpoint phosphorus from surface water and groundwater: introduction. J.
Environ. Qual. 41: 621-627. DOI:10.2134/jeq2012.0080.
Buda, A.R., P.J.A. Kleinman, M.S. Srinivasan, R.B. Bryant, and G.W. Feyereisen. 2009. Effects
of hydrology and field management on phosphorus transport in surface runoff. J.
Environ. Qual. 38: 2273-2284. DOI:10.2134/jeq2008.0501.
Buda, A.R., P.J.A. Kleinman, M.S. Srinivasan, R.B. Bryant, and G.W. Feyereisen. 2009. Factors
influencing surface runoff generation from two agricultural hillslopes in central
Pennsylvania. Hydrol. Process. 23: 1295-1312. DOI:10.1002/hyp.7237.
Dr. Anthony Buda | The State of the Science of Phosphorus Symposium | January 2015 | Page 24
Peter Vadas, USDA-Agricultural Research Service,
Dairy Forage Research Center, Madison, Wisconsin
Since the 1970’s, non-point pollution of surface waters by agricultural P has been studied
to understand interactions between land management and natural hydrology and biogeochemistry
processes. The number of possible interactions that deserve study are far more than what can be
physically measured given our typical time and resources limits. Computer models have been
developed to simulate interactions that cannot be studied and to predict P loss for future
management and weather scenarios. Now, models are frequently used to assess P loss in the
absence of measured data. Models are also important because they force us to formalize and test
our understanding of P loss processes, and identify knowledge and data gaps. Given the role that
models are playing in P research and management decisions, we need to be sure they are up to
date with the science, are well tested and proven, and can be improved in the future.
There are many models that predict P loss at the field edge, transport through a stream or
drainage network, and delivery to surface waters. Some models are simple and simulate only
edge-of-field P loss, while others are very complex and simulate very large watersheds. Models
can differ widely in how they calculate P loss, how they represent physical and management
differences in the landscape, whether they simulate single storms or weather across many years,
and whether they simulate changes with time or long-term, average conditions.
Dr. Peter Vadas | The State of the Science of Phosphorus Symposium | January 2015 | Page 25
On one end of the modeling spectrum are simple, user-friendly P Indexes. P Indexes are
generally tables of rules or guidelines that assess the average risk of P loss from a single field.
They assess P loss risk based on P sources (soil P, fertilizer, and manure), and transport
processes (erosion and runoff). Only a few studies have used measured P loss data to test if P
Index output is reliable, while other studies have identified short-comings of P Indexes. The P
Index is best at showing land managers if certain practices will increase or decrease P loss, but
may not accurately quantify the magnitude of P loss. Also, most P Indexes deliver a relative risk
of P loss but cannot predict water quality beyond the field edge.
There are a few examples of field-scale models that have the user-friendly approach of a
P Index but use process-based equations to quantify edge-of-field P loss. These include the
Annual Phosphorus Loss Estimator (APLE), the Wisconsin P Index, the PPM model from
Oklahoma, and the TBET model in Texas. These models quantify P loss from sediment bound
and dissolved P in runoff at the field edge, but may not simulate management practices outside
of the field. These models have generally been better tested than P Indexes because their output
can be directly compared to measured P loss data.
Beyond these user-friendly examples, P loss models are typically much more complex
and difficult to operate. More complex models like EPIC, APEX, GLEAMS, and IFSM use
similar equation as simpler models like APLE, but can simulate more processes and can operate
at the farm or small watershed scale. To quantify the impacts of P loss at larger scales or to
assess management implemented outside of fields, watershed scale models such as SWAT or
HSPF are necessary. Watershed scale modeling is substantially more complex and challenging.
Significant experience is needed, and models need to be calibrated, which requires a lot
monitoring data to compare with model output. Because these complex models can be calibrated
Dr. Peter Vadas | The State of the Science of Phosphorus Symposium | January 2015 | Page 26
to agree with measured P loss data, it may seem like they correctly simulate P transport and fate
processes. However, because watershed models have many parameters that can be adjusted, it is
easy to get the “right” answer for the “wrong” reasons. Therefore, it can be difficult to accurately
assess the real effects of management practices, especially those that require specific landscape
positioning to be effective. Instead, current watershed models are best for assessing impacts of
large-scale changes (e.g., conversion from corn to alfalfa, or changes in P application rates), but
may not be appropriate to determine what practices to implement at the field scale to reduce P
loss or to assess practices that require specific locations in the landscape.
The 10 points below can help guide someone to know which model to use or if model
output is reliable. Important model characteristics include:
1) Does the model accurately represent true P loss and reductions?
2) Does the model operate at an appropriate scale and resolution?
3) Can the model simulate local conditions and agricultural practices?
4) Do model data requirements match availability?
5) Are model sensitivity and uncertainty appropriate relative to the magnitude of
desired P loss reduction?
6) Does the model represent current science, and has it been well developed and
7) Does the model deliver information in the units and on a timescale needed?
8) Is model user-friendly enough and will it give consistent results across multiple
users for the same scenarios? Is the model practical and economical to set up and
Dr. Peter Vadas | The State of the Science of Phosphorus Symposium | January 2015 | Page 27
9) Is the model transparent enough so model equations are understandable and
simulations can be followed?
10) Does the model have adequate support to be applied and updated as needed?
No single model may meet all of these criteria, and a model user will have to decide
which ones are absolutely critical. Also, while a model may appear to meet certain criteria, it
may not do it very well. Failure to meet many of the criteria may mean a particular model should
not be used or needs to be improved. Failure of most available models to consistently meet
criteria may mean a whole new modeling approach is needed.
Models can always be improved, and weaknesses should not necessarily prevent a
model’s use, especially because using a model is one of the best ways to learn how to improve it.
If model weaknesses are understood, then a P loss program can be designed to account for them.
Weaknesses are understood by knowing how models work, their equations and algorithms, and
not just how to make them run and give output. While this is obvious, some models can be so
complex that it is difficult for anyone except a model developer to know how they work. This is
truer for watershed models than field models. In such cases, it is up to developers to
communicate how models work, and their strengths and weaknesses. However, it is up to model
users to know how reliable their models are.
Below is a discussion about principles of model development and function, which can
help show what kinds of errors models have. There are four areas of model development:
1) The perceptual model, which is our understanding of how a system behaves.
2) The conceptual model, which is the mathematical equations used to describe the
3) The procedural model, which is how equations are written into computer code.
Dr. Peter Vadas | The State of the Science of Phosphorus Symposium | January 2015 | Page 28
4) Model calibration and validation, which is how we test how well a model works.
Model errors can exist in all four areas. A poor perceptual model may be when we do not
understand what controls P fate and transport, or what we think controls them is not correct.
Even when a P model is perceptually correct, the way it is translated into equations, or the
conceptual model, can be incorrect, perhaps because of a lack of data or modeler inexperience.
This may be true of P Indexes. Also, mathematical equations may be used differently in different
models, giving rise to errors in the procedural model. This can happen if model developers
understand an equation differently or have to make it fit differently into an existing model. This
can make two models give different predictions for the same scenario. Finally, how we decide to
run and test a model in calibration and validation can determine how well we think it performs
and expose or hide model weaknesses. Testing a model with limited data over short periods or
few scenarios may not show its weaknesses. Models that have too many errors should be
avoided, even though they are advertised as robust and dependable. Some priorities for model
improvement include:
1) Current models are better at field scale than watershed scale P loss predictions,
especially for fast, affordable estimates that minimize the expertise and resources
needed to run the models. Linking field practices to watershed outlet impacts is
at the edge of scientific understanding, but remains a priority for P modeling.
2) The translation of science into models often lags years and even decades behind
current scientific understanding. Some model weaknesses can be readily and
rapidly improved by addressing this situation.
3) The computing foundation of many P loss models has not been updated for 20+
years. Model formats need to be kept up to date with computing technology,
Dr. Peter Vadas | The State of the Science of Phosphorus Symposium | January 2015 | Page 29
including spatial and online possibilities, and user-friendly interfaces. However,
fancy technology does not improve a model’s accuracy.
4) Other specific model improvements include:
a. Runoff: Alternatives to the Curve Number approach should be developed
and implemented (e.g. TOPMODEL).
b. Erosion: Improvements to the USLE family of models are needed.
c. In-stream processes: Better nutrient cycling and especially sediment
transport predictions are needed.
d. Nutrient cycling in soil is generally well simulated, but incremental
improvements can be made, especially to reflect technology changes in
farm management and scientific understanding.
e. Estimates of uncertainty are needed for full confidence in model
Current efforts to establish policies and make management decisions require being able to
predict P loss at field to watershed scales. The best way for model development to proceed is
through interdisciplinary collaborations and communication between experimentalists, model
developers, and model users. An interconnected framework of experimentation and model
development should advance agricultural P management and environmental protection beyond
what the two proceeding alone can achieve.
Dr. Peter Vadas | The State of the Science of Phosphorus Symposium | January 2015 | Page 30
Vadas, P.A, C.H. Bolster, and L.W. Good. 2012. Critically evaluating select issues of
agricultural phosphorus model development. Soil Use. Manage. 29:36-44.
Olander, L., T. Walter, P. Vadas, J. Heffernan, E. Kebreab, and T. Harter. 2014. Refining models
for quantifying the water quality benefits of improved animal management for use in
water quality trading. NIR 14-03. Durham, NC: Duke University.
Krueger, T., Freer, J., Quinton, J.N., Macleod, C.J.A., 2007. Processes affecting transfer of
sediment and colloids, with associated phosphorus, from intensively farmed grasslands: a
critical note on modeling of phosphorus transfers. Hydrological Processes 21:557-562.
Bolster, C., P.A. Vadas, A.N. Sharpley, and J.L. Lory. 2012. Using a phosphorus loss model to
evaluate and improve phosphorus indices. J. Environ. Qual. 41:1758-1766.
Dr. Peter Vadas | The State of the Science of Phosphorus Symposium | January 2015 | Page 31
Douglas R. Smith, USDA-Agricultural Research Service
Grassland, Soil and Water Research Laboratory, Temple, Texas
In recent years, phosphorus (P) loadings have led to eutrophication of water bodies such
as Lake Erie and Chesapeake Bay. Since the 1980’s, great efforts have been made to decrease
sediment, and many thought by proxy the P to these waters. In the case of Lake Erie, total P
loads have declined since the 1980’s while soluble P showed an initial decline over this period.
However, in the mid-1990’s soluble P began to increase again.
The lack of improved water quality may be in part due to legacy P (Jarvie et al., 2013;
Sharpley et al., 2013), defined as that P that has accumulated in soils, water, or sediments within
a watershed. Much of the P within an agricultural landscape can be considered legacy P until it is
removed from the watershed through crop or meat production or removed hydrologically in
water or sediment.
Many conservation programs within watersheds have been implemented with short term
monitoring programs (i.e. 2-5 years), often with no discernable differences between the pre- and
post-conservation implementation periods. This can be in part due to lag times in P transport
(Meals et al., 2010). Lag times depend upon the source of P (i.e. direct deposit of animal manure
to a stream versus residual high soil test P, STP), the pathway (surface runoff versus
groundwater transport), the distance the conservation activities occur from the monitoring point
and the scale of the monitoring (field versus large watershed).
Legacy P in soil includes the fraction in excess of what is necessary for crop production.
In many regions where animal production has occurred, the chronic application of manures at
waste disposal rates as opposed to agronomic rates has led to very high STP levels. The area
around manure storage facilities can also be highly elevated in STP. Phosphorus release from
Dr. Douglas Smith | The State of the Science of Phosphorus Symposium | January 2015 | Page 32
soils can be related to the ratio of P to iron plus aluminum (Maguire and Sims, 2002), referred to
as the P sorption ratio, degree of P saturation (DPS) or P saturation ratio (PSR). Initially, P
release from soils should be relatively low, until the PSR exceeds 20-25%, at which time a
change point occurs and much greater P losses can occur. Drawdown strategies can reduce the
STP levels to an agronomically acceptable range if P applications are ceased; however, this can
take from several years to decades.
Groundwater P concentrations sufficient to induce eutrophication have been observed on
the eastern shore of Maryland (Kleinman et al., 2007). Groundwater residence times in the
Chesapeake Bay can range from months to decades (Phillips and Lindsey, 2003). Unless
practices are designed to intercept and treat this source (i.e. Penn et al., 2007), groundwater may
continue to mask the water quality benefits of other practices for the decades to come.
Stream and ditch sediments are also known to serve as a source or a sink for P in the
water, although this source is often ignored. For example, P related water quality problems were
being blamed solely on the poultry industry. However, one study showed waste water treatment
plants were not adequately treating effluent for P and that one such plant was discharging
sufficient P to saturate the stream sediments and elevate stream P concentrations 30 km
downstream (Haggard et al., 2001). With P saturated sediments in the streams or lakes, it may
take years to remove enough P to return the stream water quality to an acceptable level. In
another example, a hurricane in North Carolina resulted in swine lagoons flooding, which
saturated stream sediments with P. Two months after the lagoon breech, P concentrations in the
affected river were an order of magnitude higher than samples collected upstream of the affected
reach (Burkholder et al., 1997), which further illustrates the importance of sediments as a P
Dr. Douglas Smith | The State of the Science of Phosphorus Symposium | January 2015 | Page 33
In artificially drained landscapes, drainage districts are obligated to ensure adequate
drainage throughout the land base in their region. One of the practices employed by drainage
districts to ensure adequate drainage has been dredging or dipping agricultural ditches. This
process has been shown to affect nutrient transport within the ditch networks (Smith et al., 2006).
In Indiana and Ohio, immediately after dredging, the sediments in the ditch bottom were less
able to remove P from the water column than prior to dredging. However, after a few months and
up to a year after dredging, a dredged drainage ditch in Indiana was able to remove P from the
water such that there was a decrease in the P mass as it passed through this 3 mile reach (Smith
and Huang, 2010). P removed by the ditch and contained in its sediment in that study would be
considered legacy P. Generally, sediments or spoils dredged from ditches are left on the ditch
banks or spread in adjacent fields. These spoils can contain hundreds or thousands of pounds of
P, which is potentially available for transport back into the ditch network unless removed from
the watershed.
Burkholder, J.M., M.A. Mallin, H.B. Glasgow, Jr., M. Larsen, M.R. McIver, G.C. Shank, N.
Deamer-Melia, D.S. Briley, J. Springer, B.W. Touchette, and E.K. Hannon. 1997.
Impacts to a coastal river and estuary from rupture of a large swine waste holding lagoon.
J. Environ. Qual. 26(6):1451-1466.
Haggard, B.E., D.E. Storm and E.H. Stanley. 2001. Effect of a point source input on stream
nutrient retention. J. Am. Water Resourc. Assoc. 37(5):1291-1299.
Jarvie, H.P., A.N. Sharpley, B. Spears, A.R. Buda, L. May, and P.J.A. Kleinman. 2013. Water
quality remediation faces unprecendented challenges from “Legacy Phosphorus.”
Environ. Sci. and Technol. 47:8997-8998.
Kleinman, P.J.A., A.L. Allen, B.A. Needelman, A.N. Sharpley, P.A. Vadas, L.S. Saporito, G.J.
Folmar, and R.B. Bryant. 2007. Dynamics of phosphorus transfers from heavily manured
coastal plain soils to drainage ditches. J. Soil Water Conserv. 62:225-235.
Meals, D.W., S.A. Dressing, and T.E. Davenport. 2010. Lag time in water quality response to
best management practices: A review. J. Environ. Qual. 39:85-96.
Dr. Douglas Smith | The State of the Science of Phosphorus Symposium | January 2015 | Page 34
Penn, C.J., R.B. Bryant, P.J.A. Kleinman, and A.L. Allen. 2007. Sequestering dissolved
phosphorus from ditch drainage water. J. Soil Water Conserve. 62:269-276.
Phillips, S.W., and B.D. Lindsey. 2003. The influence of ground water on nitrogen delivery to
the Chesapeake Bay. USGS Fact Sheet FS-091-03. Available at U.S. Geological Survey.
Maguire and J.T. Sims. 2002. Soil testing to predict phosphorus leaching. J. Environ. Qual.
Sharpley, A. H.P. Jarvie, A. Buda, L. May, B. Spears, and P. Kleinman. 2013. Phosphorus
Legacy: Overcoming the effects of past management practices to mitigate future water
quality impairment. J. Environ. Qual. 42:1308-1326.
Smith, D.R., and C. Huang. 2010. Assessing nutrient transport following dredging of agricultural
drainage ditches. Trans. ASABE. 53:429-436.
Smith, D.R., E.A. Warnemuende, B.E. Haggard, and C. Huang. 2006. Dredging of drainage
ditches increases short-term transport of soluble phosphorus. J. Environ. Qual. 35:611616.
Dr. Douglas Smith | The State of the Science of Phosphorus Symposium | January 2015 | Page 35
Joshua M. McGrath, Associate Professor, University of Kentucky, Lexington, Kentucky
The Chesapeake Bay Model identifies agriculture as the leading source of phosphorus (P)
to the Chesapeake Bay (USEPA, 2015). This has led to considerable investment in policies and
practices to reduce agricultural P loading. Much of the discussion has focused on local P imbalances
and ways to increase P use efficiency. However, increases in P recovery efficiency are not likely to
result in reduced P loading to surface water – particularly in the short term. Improving agricultural P
recovery efficiencies might help avoid future legacy P losses that result from elevated soil P
concentrations. However, meeting short term water quality objectives will also require a holistic
approach that aims to interrupt the field to water transport continuum by targeting P management
practices and conservation measures to site-specific P sources and transport conditions.
Site characteristics including hydrology, soil characteristics, and manmade features (e.g.
tile drainage, ditching, terracing) combine with P source (e.g. fertilizer, manure, soil P) to determine
the potential for P loss from an agricultural field. Phosphorus management must aim to maximize
crop uptake and soil storage of applied P, while minimizing P exposed to transport processes. One
example of a management practice that might limit P loss and increase P use efficiency is the use of
starter fertilizer banded below the soil surface. In this situation the quantity-intensity relationship that
controls P availability to crops is manipulated by placing less total P in a concentrated band, thereby
limiting fertilizer exposure to soil sorption processes. However, there are many examples in the
literature where management impacts P loss independently of P use efficiency (Daverede et al., 2003,
2004; Buda et al., 2009; Kaiser et al., 2009; McGrath et al., 2010). In most situations management
reduces P loss by decreasing the exposure of a source to transport processes or directly interrupts
Dr. Joshua McGrath | The State of the Science of Phosphorus Symposium | January 2015 | Page 36
transport pathways, while having little effect on P use efficiency. Examples include incorporation or
direct injection of P fertilizer sources (inorganic and organic), tillage to dilute surface soil P
concentrations, or maintaining soil cover to minimize soil-water interaction.
Repeated P applications beyond crop removal can result in elevated soil P concentrations.
This “legacy” P source can be particularly difficult to manage, especially in areas with high
hydrologic connectivity to surface water. Strategies should focus on minimizing water export
and reducing P concentrations in runoff and drainage water. Practices such as drainage control
(in tile or ditch drains), land-application of P sorbing materials, or direct removal of P from
drainage waters using treatment structures can help meet short-term water quality goals (Penn
and Bryant, 2006; Grubb et al., 2011; Buda et al., 2012; Penn and McGrath, 2014; Penn et al.,
2014) Over the long-term, continuous crop production can remediate high soil P concentrations,
however, this can take decades or longer in many situations.
The 4R Nutrient Stewardship approach (The Fertilizer Institute, 2015) provides a
framework for implementing practices to meet multiple performance objectives. It centers on the
use of the right rate, right timing, right source, and right placement of nutrients and management
practices. However, it is important to keep in mind that in agriculture there is often a push pull
relationship between multiple objectives. For example, often the highest net profits are realized
when some nutrients are discharged to the environment, particularly in areas with concentrated
livestock production. Furthermore, spatial and temporal variability in management practice
effectiveness demands a site-specific approach to implementation. These tradeoffs, along with
uncertainty surround practice efficacy must be weighed in the context of multiple, competing
performance objectives when developing management strategies that optimize crop production
and minimize P loss to surface water.
Dr. Joshua McGrath | The State of the Science of Phosphorus Symposium | January 2015 | Page 37
Buda, A.R., P.J.A. Kleinman, M.S. Srinivasan, R.B. Bryant, and G.W. Feyereisen. 2009. Effects
of Hydrology and Field Management on Phosphorus Transport in Surface Runoff. J.
Environ. Qual. 38(6): 2273.
Buda, A.R., G.F. Koopmans, R.B. Bryant, and W.J. Chardon. 2012. Emerging Technologies for
Removing Nonpoint Phosphorus from Surface Water and Groundwater: Introduction. J.
Environ. Qual. 41(3): 621.
Daverede, I.C., A.N. Kravchenko, R.G. Hoeft, E.D. Nafziger, D.G. Bullock, J.J. Warren, and
L.C. Gonzini. 2003. Phosphorus Runoff. J Env. Qual 32(4): 1436–1444.
Daverede, I.C., A.N. Kravchenko, R.G. Hoeft, E.D. Nafziger, D.G. Bullock, J.J. Warren, and
L.C. Gonzini. 2004. Phosphorus Runoff from Incorporated and Surface-Applied Liquid
Swine Manure and Phosphorus Fertilizer. J Env. Qual 33(4): 1535–1544.
Grubb, K.L., J. McGrath, C. Penn, and R. Bryant. 2011. Land application of spent gypsum from
ditch filters: phosphorus source or sink? Agric. Sci. 02(03): 364–374.
Kaiser, D.E., A.P. Mallarino, M.U. Haq, and B.L. Allen. 2009. Runoff Phosphorus Loss
Immediately after Poultry Manure Application as Influenced by the Application Rate and
Tillage. J. Environ. Qual. 38(1): 299.
McGrath, J.M., J.T. Sims, R.O. Maguire, W.W. Saylor, and R. Angel. 2010. Modifying Broiler
Diets with Phytase and Vitamin D Metabolite (25-OH D): Impact on Phosphorus in
Litter, Amended Soils, and Runoff. J. Environ. Qual. 39(1): 324.
Penn, C.J., and R.B. Bryant. 2006. Application of phosphorus sorbing materials to streamside
cattle loafing areas. J. Soil Water Conserv. 61(5): 303–310.
Penn, C., and J. McGrath. 2014. Chemistry and Application of Industrial By-products to Animal
Manure for Reducing Phosphorus Losses to Surface Waters. p. 211–238. In He, Z.,
Zhang, H. (eds.), Applied Manure and Nutrient Chemistry for Sustainable Agriculture
and Environment. Springer Netherlands.
Penn, C., J. McGrath, J. Bowen, and S. Wilson. 2014. Phosphorus removal structures: A
management option for legacy phosphorus. J. Soil Water Conserv. 69(2): 51A–56A.
The Fertilizer Institute. 2015. Nutrient Stewardship | The Right Time for Nutrient Stewardship is
Right Now. Available at (verified 26 January 2015).
USEPA. 2015. Agriculture - Chesapeake Bay Program. Available at (verified 26 January 2015).
Dr. Joshua McGrath | The State of the Science of Phosphorus Symposium | January 2015 | Page 38
Frank J. Coale, Ph.D., Professor, University of Maryland, College Park, Maryland
For decades, it has been recognized that many surface waters in Maryland are impaired
by excessive inputs of nutrients. Inputs of both nitrogen (N) and phosphorus (P) have stimulated
algal growth in the degraded waters, thus decreasing water clarity and depleting dissolved
oxygen levels. Both N and P contribute to water quality impairment, with freshwater systems
being particularly sensitive to P inputs. Currently, data from water quality monitoring programs
combined with assessment of the sources of nutrient inputs have identified drainage from
agricultural landscapes as the largest source of P inputs to the Chesapeake Bay.
Based on the U.S. EPA Chesapeake Bay Program watershed models, P input to the
Chesapeake Bay from municipal wastewater treatment has declined by 70% since 1985. Today,
according to Chesapeake Bay Program models, wastewater discharge now accounts for 18% of
the P that enters the Chesapeake Bay as a result of human activities and is roughly equal to the P
loading from urban storm water runoff (19%). Over the same time period, P inputs from
agricultural sources also have been reduced, but at a much slower rate (6% reduction) and,
currently, the Chesapeake Bay Program models attribute 64% of the human-influenced P that
enters the Chesapeake Bay as originating from agricultural landscapes.
Beginning in the late 1980’s, the State of Maryland adopted various policies and
developed voluntary agricultural nutrient management programs aimed at reducing P loading of
surface waters. In swift response to a popularized Chesapeake Bay fish kill during the summer
of 1997 that was attributed to the suspected nutrient-stimulated toxicity of the dinoflagellate
Pfiesteria piscicida, the State of Maryland passed the Water Quality Improvement Act of 1998,
Dr. Frank Coale | The State of the Science of Phosphorus Symposium | January 2015 | Page 39
which phased in mandatory N and P-based nutrient management planning regulations for
Maryland farmers. The P management provisions of these aggressive regulations were fully
implemented by 2005.
In an effort to further alleviate water quality impairments and accelerate reductions of P
inputs to the Chesapeake Bay from agricultural sources, President Obama issued Executive
Order 13508 in May 2009 that declared the Chesapeake Bay a “national treasure” and ushered in
a new era of federal oversight and accountability. In 2010, under the existing provisions of the
Federal Clean Water Act of 1992, the U.S. EPA developed Total Maximum Daily Load (TMDL)
limits for P entering the Chesapeake Bay. The Chesapeake Bay TMDL prescribed the amount of
P input that can be tolerated by the Bay ecosystem and not result in impaired water quality. A
2025 deadline was established by which time each of the Chesapeake Bay watershed states was
legally obligated to achieve the TMDL P load reductions necessary to alleviate water quality
impairments. By 2025, total P loading to the Chesapeake Bay must be less than 14.5 million
pounds P/year and P loading from Maryland’s tributaries to the Chesapeake Bay must be no
greater than 2.8 million pounds P/year. The TMDL implementation plan allows for half of
Maryland’s total load, or 1.4 million pounds P/year, to originate from agricultural sources. In
order to achieve the 2025 TMDL mandate, overall P loading from Maryland tributaries will need
to be reduced by 15% and P loading from agricultural sources will need to be reduced by 12%,
relative to today’s estimated loading rates.
Phosphorus in eroded sediments, runoff water and subsurface drainage is a function of
the concentrations and forms of P present in the soil, the type of soil, field management, and
hydrologic connectivity. While Maryland soils do have a large capacity to retain P, at some
point a specific soil’s P retention capacity may become saturated and dissolved P losses with
Dr. Frank Coale | The State of the Science of Phosphorus Symposium | January 2015 | Page 40
field drainage water may rapidly increase. Assessment of all potential P sources, including
newly applied P, residual soil P and long-term legacy P, and evaluation of off-field transport
pathways are essential elements of a comprehensive P management strategy.
In 1994, research began on the development of a tool designed to identify site-specific
risk for P loss from farm fields and provide guidance for adoption of management practices to
reduce the risk for P loss. The resulting risk assessment tool was tailored to Maryland’s soils,
agricultural management practices, climate, topography, and hydrology. Phosphorus loss risk
assessment tools have been implemented widely in Maryland’s agricultural nutrient management
planning process since 2000 and have been revised, updated and improved along the way, most
notably in 2005 and 2013. The current version, the Phosphorus Management Tool (PMT), has
incorporated the most reliable science into a method that has improved the ability to identify
sites for potential P losses from the agricultural landscape and identify targeted management
practices for mitigating P loss.
The science of P dynamics in the agro-ecosystem will continue to evolve. Undoubtedly,
new and refined field management practices will be developed to help minimize P losses from
agricultural production systems. In some specific locations where P losses are particularly
egregious due to combinations of past management, elevated legacy P concentrations, and
accelerated hydrologic connectivity, adjustment of current field management practices may not
be sufficient to result in a significant reduction in P losses. In such cases, active remediation
techniques such as purposeful crop drawdown of soil P reserves, chemical immobilization of soil
P, installation of P trapping filters in drainage ditches, and intensified drainage water
management may be required to reduce P losses from identified high risk sites.
Dr. Frank Coale | The State of the Science of Phosphorus Symposium | January 2015 | Page 41
Dr. Walter Boynton, University of Maryland Center for Environmental Science
Dr. Walter Boynton is a Professor at the Chesapeake Biological laboratory (CBL),
University of Maryland Center for Environmental Science and has been a faculty member at
CBL since 1975. Boynton’s research expertise is estuarine ecology, particularly issues related to
eutrophication and ecosystem restoration. He has published over 100 scientific papers and many
more technical reports related to water quality, habitat and restoration issues. Dr. Boynton
currently has funding from Sea Grant, Maryland Department of Natural Resources, Maryland
County and city government and the National Science Foundation. All of this research involves
coastal and estuarine eutrophication and restoration of these ecosystems. Dr. Boynton serves on
boards of the Patuxent Riverkeeper, the Maryland-DC Chapter of The Nature Conservancy and
the Patuxent River Commission. He has served on several EPA Science Advisory Board panels
reviewing the state of the hypoxic zone in the Gulf of Mexico, Florida nutrient criteria, an EPA
workgroup developing national water quality standards for estuarine systems and, more recently,
worked with the Department of Justice on Gulf of Mexico issues. He served on Maryland
Governor O’Malley’s transition team for environmental issues and is currently a member of the
science advisory panel for the Chesapeake Bay Trust Fund. He was awarded the Odum Award
for Lifetime Achievement from the Coastal and Estuarine Research Federation and was elected
president of this scientific society. More locally, he served as the vice-chair of the Calvert
County Zoning Appeals Board for more than a decade and in this position has been involved in
many Maryland Critical Area decisions. He teaches a graduate ecology course and seminar that
ties together the ecosystems of Maryland from the western mountains to the coastal ocean.
Biographies | The State of the Science of Phosphorus Symposium | January 2015 | Page 42
Dr. Anthony Buda, USDA Agricultural Research Service
Dr. Anthony Buda is a hydrologist with the USDA Agricultural Research Service (ARS)
Pasture Systems and Watershed Management Research Unit (PSWMRU) in University Park,
PA. He joined the unit in 2007 after receiving a PhD in Forest Hydrology from Penn State
University. Dr. Buda works collaboratively with teams of physical, natural, and social scientists
to address critical water resource challenges facing agriculture. His field research program
applies a variety of tools, including conservative hydrologic tracers, near-surface geophysics, and
hydrometric monitoring, to understand factors connecting nutrient sources on the landscape with
surface water and groundwater. He works with action agencies and farmers to develop decision
support tools for nutrient management, most notably the Fertilizer Forecaster, which leverages
hydrologic modeling and weather forecasting to guide daily decisions by farmers on the timing
and placement of nutrients. He also conducts research to understand how changes in land
management and climate impact hydrology and water quality in agricultural watersheds. Dr.
Buda serves on the editorial board of Soil Science Society of America Journal, and has received
research recognitions from the American Society of Agronomy, Soil Science Society of
America, American Water Resources Association, and USDA’s North Atlantic Area.
Biographies | The State of the Science of Phosphorus Symposium | January 2015 | Page 43
Dr. Frank Coale, University of Maryland
Dr. Frank J. Coale is Professor and Extension Specialist for agricultural nutrient
management in the Department of Environmental Science & Technology at the University of
Maryland. Dr. Coale also serves as Director of the Gemstone Honors Program in the Honors
College at the University of Maryland. The Gemstone Honors Program is a unique and
prestigious multidisciplinary four-year research experience for selected undergraduate Honors
students of all majors. Dr. Coale received his B.S. degree in Agronomy from the University of
Maryland and his Masters’ degree in Crop Physiology and his Ph.D. in Soil Fertility and Plant
Nutrition from the University of Kentucky. After 7 years on the faculty at the University of
Florida, Dr. Coale joined the faculty at the University of Maryland in 1993. His research and
extension programs concentrate on efficient agronomic and environmental management of
applied nutrients and nutrient management policy development in the Chesapeake Bay
watershed. Dr. Coale has published 50 refereed journal articles and 167 Extension publications.
He has delivered nearly 200 scientific presentations with published abstracts and has given over
500 Extension Education presentations. Dr. Coale has mentored 31 graduate students and has
supported his programs with over $15 million in external grant funding. Dr. Coale served as
Chair of the Department of Environmental Science and Technology at the University of
Maryland for six years. He has received recognition awards for his contributions from his
university, the State of Maryland, and federal agencies. Dr. Coale is Fellow of the American
Society of Agronomy (2012) and Fellow of the Soil Science Society of America (2014).
Biographies | The State of the Science of Phosphorus Symposium | January 2015 | Page 44
Dr. Peter Kleinman, USDA Agricultural Research Service
Dr. Peter Kleinman is a soil scientist with USDA’s Agricultural Research Service and the
research leader of the Pasture Systems and Watershed Management Research Unit in State
College, PA. He obtained his PhD from Cornell University in 1998. His research aims provides
tools to farmers, resource managers and policymakers to improve the stewardship of nutrients in
agriculture. He has worked to advance the Phosphorus Index and other decision support tools,
including the Fertilizer Forecaster which provides daily forecasts of when and where to apply
nutrients. He has spear-headed efforts to bring new manure application technologies to farmers,
from liquid manure injectors to dry manure applicators. He and his team have developed new
filtration technologies to remove phosphorus from manures and from runoff waters. He leads and
advocates for collaborative, consensus based science at regional, national and international
scales. He has learned that, as a scientist and as an advisor to watershed and farming programs,
conveying the trade-offs of management options is more important than promoting any single
approach. He is a recent member of the Chesapeake Bay Program’s Science and Technology
Committee. He is a fellow of the Soil and Water Conservation Society, American Society of
Agronomy and Soil Science Society of America, serves on the editorial boards of Journal of
Environmental Quality and Journal of Soil and Water Conservation.
Biographies | The State of the Science of Phosphorus Symposium | January 2015 | Page 45
Dr. Joshua McGrath, University of Kentucky
Dr. Josh McGrath joined the Department of Plant and Soil Sciences, University of
Kentucky as Associate Professor and Soil Management Specialist in July of 2014. Dr. McGrath’s
research and extension activities focus on agricultural productivity and environmental quality as
they relate to soil fertility, nutrient management, and water quality. He has published and
conducted research on in-situ treatment of agricultural drainage; sensor-based variable-rate
nitrogen; manure management in no-till; manure storage to reduce nutrient losses; environmental
persistence of manure-borne anti-microbial compounds; and phosphorus forms and cycling in
manure and soil. Prior to joining the faculty at the University of Kentucky, Dr. McGrath was an
Associate Professor and Extension Specialist at University of Maryland and was heavily
involved in Chesapeake Bay water quality issues and policy development. Dr. McGrath routinely
speaks throughout the United States on issues related to agriculture and the environment. He has
secured over five million dollars in external funding for his research and extension activities, has
advised and mentored numerous graduate students, post-doctoral researchers, and
undergraduates, and has authored or co-authored 39 peer-reviewed publications and five book
chapters in addition to presenting at numerous scientific meetings. Dr. McGrath was born and
raised in Smyrna, Delaware, graduated with a Bachelor of Arts from Johns Hopkins University
in Environmental Earth Sciences, and earned his Ph.D. in Plant and Soil Sciences from the
University of Delaware.
Biographies | The State of the Science of Phosphorus Symposium | January 2015 | Page 46
Dr. Andrew Sharpley, University of Arkansas
Dr. Andrew Sharpley joined the Department of Crop, Soil and Environmental Sciences,
University of Arkansas, Fayetteville in 2006. He is Distinguished Professor of Soil and Water
Sciences, Director of the Discovery Farms for Arkansas Program, and Chair of the Division of
Agriculture’s Environmental Task Force. He received degrees from the University of North
Wales and Massey University, New Zealand and spent 25 years with the USDA-ARS in
Oklahoma and then Pennsylvania. His research investigates the cycling of nutrients (primarily
phosphorus) in soil-plant-water systems in relation to soil productivity and water quality and
includes the management of animal manures, fertilizers, and crop residues. He also evaluates the
role of stream and river sediments in modifying phosphorus transport and response of receiving
lakes and reservoirs. He helped developed decision making tools for agricultural field staff to
identify sensitive areas of the landscape and to target management alternatives and remedial
measures that have reduced the risk of nutrient loss from farms. He is the Editor-in-Chief of the
Soil Science Society of America, in 2008 was inducted into the USDA-ARS Hall of Fame and in
2012 received the Christopher Columbus Foundation Agriscience Award. Dr. Sharpley serves
on National Academy of Science Panels and EPA’s Scientific Advisory Board.
Biographies | The State of the Science of Phosphorus Symposium | January 2015 | Page 47
Dr. Doug Smith, USDA Agricultural Research Service
Dr. Doug Smith is a soil scientist with USDA-ARS at the Grassland, Soil and Water
Research Laboratory. He obtained his BS (1997) and MS (1999) degrees from Texas A&M
University - Commerce and a PhD from the University of Arkansas (2002), where he worked on
dietary modification and manure amendment strategies to decrease P losses from swine and
poultry production. Dr. Smith has worked on P fate and transport in the tile drained landscapes of
the Western Lake Erie Basin for 12 years while a soil scientist with the USDA-ARS National
Soil Erosion Research Laboratory. He has conducted novel research at bench, plot, farm field
and small watershed scale to elucidate the impacts of management activities on nutrient losses in
agricultural landscapes. He has authored or coauthored more than 60 peer-reviewed publications
and mentored 18 graduate students and postdocs. Dr. Smith is incoming Chair of SERA-17, an
organization for agricultural phosphorus management, and has served on the editorial board of
the Journal of Environmental Quality.
Biographies | The State of the Science of Phosphorus Symposium | January 2015 | Page 48
Dr. Peter Vadas, USDA Agricultural Research Service
Dr. Peter Vadas is a soil scientist with USDA-Agricultural Research Service at the Dairy
Forage Research Center in Madison, WI. He obtained M.S. (1996) and Ph.D. (2001) degrees
from the University of Delaware, where he worked on field and modeling research for
phosphorus management on the Delmarva Peninsula. His research focuses on nutrient
management in agricultural production systems, and especially with developing tools and models
for phosphorus management. He has conducted lab and field research to generate the data needed
to improve existing field, farm, and watershed scale models that are used to help manage
agricultural phosphorus. Notable modeling advances include better simulation of phosphorus
loss from field-applied manures and fertilizers, and the Annual Phosphorus Loss Estimator
(APLE), which is a user-friendly model that quantifies annual phosphorus loss from cropped
fields, grazed pastures, and cattle barnyards and feedlots. APLE is being used to help improve
Phosphorus Indexes in several states, including Maryland. Dr. Vadas is active in the SERA-17
organization for agricultural phosphorus management, and has served on the editorial boards of
the Journal of Environmental Quality and the Soil Science Society of America Journal.
Biographies | The State of the Science of Phosphorus Symposium | January 2015 | Page 49
Hosts | The State of the Science of Phosphorus Symposium | January 2015 | Page 50