The author is with the University of Wisconsin-Madison Division of Extension’s Discovery Farms program.
The agriculture community continuously adapts to fulfill the ever-growing desires and needs of society. This willingness to try innovative practices that protect soil and water resources, while maintaining the productive capacity of the land, is essential to sustainable agriculture. A thorough understanding of the complex interactions between nutrients, water, and plant growth is necessary to meet the growing environmental pressures placed on the farming community.
According to the USDA Census of Agriculture, from 2007 to 2017, the amount of commercial phosphorus (P) fertilizer applied and manure P generated continued to increase in many states. At the same time, farm acreage was shrinking.
Overapplication of P contributes to both economic inefficiencies and environmental concerns, including harmful algal blooms and eutrophication of water bodies. Although much effort has been put into conservation adoption and implementation to reduce the amount and impact of particulate P (moves with soil, Figure 1) lost from cropland, there continues to be issues with dissolved P (moves with water) entering waterways.
The impact of legacy phosphorus
Buildup of P in fields, whether due to overapplication of manure, unrealistic yield goals, or legacy accumulation of nutrients from fertilizers or manure, is causing negative impacts on streams, rivers, and lakes across the country. Accumulation of P in the surface soil is common in soils that receive surface applied fertilizer and manure. This accumulation, as indicated by high surface soil test results, can happen over years or even decades, and P can continue to mobilize even after additional inputs cease.
The mobility of legacy P in the dissolved form can act as a long-term source to surface waters. This unused P, which can leave fields dissolved in runoff, not only degrades water features but also represents an economic loss to producers. Residual P can persist for many years with the only significant drawdown coming from crop removal at rates of 3 to 5 ppm per year, or worse, from runoff and soil loss. The impacts of this legacy P are visible even after aggressive steps are taken to reduce P inputs and losses.
The National Oceanic and Atmospheric Administration’s (NOAA) National Climatic Data Center shows from long-term monitoring that Midwest states have seen a modest rise in both total precipitation and extreme precipitation events. With extreme precipitation events comes the potential for enhanced water runoff. The majority of annual nutrient and sediment runoff can occur during a few, or even just one, of these large events if proper precautions aren’t in place.
Although Discovery Farms monitoring has seen extreme weather events occur, fields with appropriate conservation practices in place have been resistant to detrimental losses during these outlier events. Timing of these large precipitation events, and whether or not there is cover on the soil, will often determine the extent of losses, underscoring the importance of keeping up with practices to buffer the impact.
The most evident impacts of large runoff events is seen in fields that do not have concentrated flow areas protected (Figure 2, left). A well-established, grassed waterway is critical to preventing losses via concentrated flow (Figure 2, right). With the potential of an uncertain climatic future, preparation and the ability to adapt will result in resilient farming systems.
Conservation practices that reduce erosion, improve infiltration, and slow the movement of runoff are widely encouraged to address soil and phosphorus loss from agricultural landscapes. Grassed waterways, cover crops, and conservation tillage, when used appropriately, have all been shown to improve the water quality impacts of agricultural practices.
Why, then, do we see continued impairment of waterways due to excessive levels of phosphorus? While this can be frustrating at times, we need to keep refining and developing management suggestions based on new and relevant information. There is an ever-growing need for knowledge on how management practices drive water quality outcomes at scales from the field to a whole watershed. There are many lessons to be learned from regional efforts at tackling water quality issues derived from nonpoint P sources.
Three case studies
Phosphorus load reductions into Lake Erie in the 1970s and 1980s were once touted as major environmental success stories. Yet in the mid 2010s, even after the implementation of conservation practices to target nonpoint source P loading, some of the largest and most widespread algal blooms ever witnessed in Lake Erie occurred.
Some potential culprits of this re-eutrophication problem seem to be the elevation in dissolved P loading, as well as more precipitation. This leads to a greater volume of water entering the tributaries leading to Lake Erie.
Early efforts at tackling these issues targeted particulate P, which is just one potential pathway of P loading. This may have had some unintended consequences of greater P stratification and buildup in the surface soil layers, leading to high levels of dissolved P during runoff events.
Extreme precipitation events and long-term increases in the volume of water flowing in tributaries may also be pushing the dissolved P further into the Lake. This causes a wider distribution of impacts.
The Chesapeake Bay is North America’s largest estuary and has over 12 million acres of farmland within the watershed that drains into the bay. Despite widespread efforts to reduce P loading, including a 2010 implementation of total maximum daily load (TMDL) effort, there continues to be water quality issues.
As in the Lake Erie watersheds, the Chesapeake Bay has seen some progress in the reduction of P loading from conservation practices aimed at reducing soil and particulate P loss. However, an acceleration of dissolved P has continued to cause issues in the bay.
Again, a likely cause of this rise in dissolved P is the buildup of legacy P in the soil, which can potentially take decades to draw down. The impact of legacy P can oftentimes conceal the results from implemented conservation practices, as the release of this P can continue to cause issues long after conservation efforts are put in place. Long-term monitoring regularly sees continued high P loss from fields complying with nutrient management standards.
The implementation of Wisconsin’s 2010 Phosphorus Rule developed total maximum daily loads (TMDLs) for surface waters in the state. While not directly regulating agricultural nonpoint P sources, the rule led to the development of important tools for best management practices and nutrient management planning.
The Wisconsin P Index is a planning and assessment tool for managing runoff P losses from cropland. The P Index utilizes weather information, soil loss equations, and field-specific data about topography, slope, and proximity to waterways to estimate runoff P losses for a particular field.
The P Index is integrated into Wisconsin’s nutrient management planning software SnapPlus. The P Index allows producers to evaluate how modifications to their fertilizer and/or manure management plans may reduce P losses and improve water quality impacts.
The development of these tools has led to smaller, albeit significant, reductions in total P loading, yet the persistence of legacy P continues to push dissolved P out of fields, causing impaired water quality. Since the mid 1990s, consistent progress in point source reductions has been made through the use of Wisconsin Pollutant Discharge Elimination System permits, which has seen a 70% reduction in wastewater P from municipalities.
Research suggests that no single practice holds the key to reducing environmental impacts of P loading. Recent efforts highlighted the need for a multi-prong approach through bundling or layering strategies. This includes altering the timing and delivery of fertilizer and manure applications, use of cover crops, restoration of headwater wetlands, streambank fencing, and vegetated riparian buffers to control drainage.
There are limited short-term approaches to tackling the dissolved P issue associated with legacy P buildup, but some innovative strategies are being considered and tested. The use of so-called P sorbing materials, those that bind to P and reduce the solubility, have shown some promising results. Application of these amendments directly to the soil has risks and may only lead to short-term reductions. The use of these materials on the edge of field structures, which filter runoff, may be one innovative strategy for capturing dissolved P.
Other opportunities may be found in social programs, such as nutrient credit markets or water quality trading programs. These types of programs allow source polluters to offset their contributions by paying into a fund or by buying or trading credits in a marketplace from other producers or farmers who are making reductions in their pollutants.
Voluntary opportunities such as environmental quality incentive programs and enrollment in conservation reserve programs are other indirect forms of reducing P source and loading. The establishment of producer-led watershed groups allows for collaboration among farmers to implement and evaluate innovative management strategies through funding opportunities and research partnerships.
Take action now
A major lesson learned is that widespread adoption of multiple practices aimed at addressing P loss, as well as continued monitoring of their impact, is necessary to continue to understand, advance, and refine management recommendations. Everyone wants clean water, pollutant-free recreational opportunities, and natural habitats for aquatic creatures.
Addressing the environmental impacts of agriculture need not wait or rely on regulations. Through collaboration and innovation, proactive efforts from all stakeholders will further the goals of improving water quality.
This article appeared in the May 2022 issue of Journal of Nutrient Management on pages 6-8. Not a subscriber? Click to get the print magazine.