When it comes to managing a farm, both daily decisions and long-term strategies can have an impact on water quality and specifically phosphorus movement. Farmers prioritizing soil health and conservation practices on their farm are likely doing a good job of reducing the total number of large, acute loss events (Figure 1) and reducing the amount of soil and nutrient loss in the runoff events that do happen.
Practices like no-till, cover crops, vegetated buffers, and grassed waterways work to keep soil, and the phosphorus attached to those soil particles, in place and out of surface waters. However, even within these soil health systems, more opportunities exist for farmers to make tweaks and adjustments that further impact water quality in a positive way.
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Decisions drive annual losses
For example, consider a field in southwestern Wisconsin monitored through the University of Wisconsin-Madison Division of Extension’s Discovery Farms edge-of-field network. The timeline of field activities included the following:
- Manure was surface applied in mid-March and early April.
- A cover crop was planted in May.
- Manure was applied again in mid-June.
Runoff events immediately following the March manure application resulted in relatively large phosphorus losses of 1.05 pounds per acre, accounting for 77% of the total annual losses (Figure 2). This mid-March application occurred on frozen soils with about 30% to 50% corn residue and when crops were not actively growing to take up the nutrients in the manure. This left the manure vulnerable on the surface when runoff conditions arose. Later manure applications in early April and mid-June under more favorable soil and crop conditions were not associated with elevated phosphorus losses in subsequent runoff (Figure 2).
Major acute loss scenarios associated with unprotected soil can deliver larger phosphorus losses than this Discovery Farms example; however, a little phosphorus can go a long way in aquatic systems. Even at these low levels, phosphorus can put an aquatic system out of balance and stimulate harmful algae blooms. Daily decisions on the farm, like choosing when to spread manure or fertilizer according to runoff risk, frozen soil, and actively growing crop conditions can drive annual losses and subsequent water quality, even when farms have prioritized soil health practices.
Keep up with maintenance
Beyond daily decision making, it is also important to consider the impact of long-term maintenance on phosphorus losses to surface water. Popular conservation practices like no-till, cover crops, vegetated buffers, and grassed waterways primarily target the particulate phosphorus loss pathway, or the portion of phosphorus attached to soil particles. These practices can significantly reduce soil loss and associated phosphorus losses when implemented and maintained appropriately.
Conservation practice maintenance includes using visual assessments to confirm gullies are not forming in grassed waterways and monitoring cover crop establishment and biomass production from year to year. If cover crops are planted for erosion control, prioritize species that reliably overwinter like cereal rye for soil protection during early spring storms along with seeding rates and methods that result in good stand establishment. Consistent attention to maintenance and yearly implementation ensures conservation practices function properly for their intended use; in this case, it is for controlling soil loss.
A long-term commitment
While controlling soil loss is essential to reducing total phosphorus loading, many recent studies indicate that these practices do not address dissolved phosphorus losses, resulting in total phosphorus loading above eutrophication benchmarks. Dissolved phosphorus is the portion of phosphorus dissolved in water; therefore, it does not require soil erosion to be lost from the field.
Soil test phosphorus (STP) is used to inform nutrient management decisions; it also has the potential to serve as a water quality risk assessment tool. Research shows elevated STP levels are associated with phosphorus losses in both surface and tile water. However, like most other facets of farming, the relationship between STP and edge-of-field water quality is site-specific and influenced by factors like field hydrology and management. Despite these secondary confounding factors, long-term research from Canada shows that drawing down STP levels can reduce the concentration of dissolved phosphorus in edge-of-field water samples without negatively impacting yield.
Consider drawing down phosphorus levels on high-testing fields to target the dissolved phosphorus loss pathway. This includes first ceasing phosphorus applications on high testing fields because it is unlikely there will be a crop response from adding phosphorus. Next, consider editing rotations to include double cropping. Harvesting two crops from one field maximizes the amount of yearly phosphorus removal. If this does not fit in your system, consider choosing crops with the greatest phosphorus removal potential, such as corn silage.
Ultimately, these practices work by reducing the source of phosphorus in the soil that might be vulnerable to runoff losses. However, it is important to remember that phosphorus drawdown is slow. For example, a typical corn crop only removes approximately 3 to 5 parts per million (ppm) of phosphorus per year. It will take long-term commitment to these phosphorus drawdown strategies to realize water quality benefits.
Pay attention to your farming system’s impact on water quality. The most impactful strategies layer daily decisions — like when to spread nutrients — with practices that take long-term commitment, including conservation practice maintenance or fine-tuning and phosphorus drawdown strategies on high testing fields.
This article appeared in the November 2024 issue of Journal of Nutrient Management on pages 20-21. Not a subscriber? Click to get the print magazine.
