Strategies for reducing phosphorus loss

Stack conservation practices to mitigate harmful algal blooms in Lake Erie.

Illustration showing the cycle of phosphorus loss through surface runoff and drainage discharge.
Figure 1. There are two main pathways for phosphorus (P) loss: surface runoff (particulate P loss) and drainage discharge (soluble P loss). Photo by Ehsan Ghane, MSU.

Phosphorus (P) is mainly lost from subsurface-drained farms through two primary pathways of surface runoff and drainage discharge (Figure 1). Surface runoff mainly moves particulate P (soil-attached P) and drainage discharge mainly moves soluble P. The soil-attached P can become soluble by detaching from the soil particle. When too much P enters surface water bodies like Lake Erie and Saginaw Bay, it causes algal blooms (Figure 2). In this article, we discuss four strategies for reducing P loss from subsurface-drained farms.

Photos from algal blooms in Lake Erie.
Figure 2. Photos from algal blooms in Lake Erie from Aug. 19, 2019. Photo by Ehsan Ghane, MSU.

Strategies for reducing phosphorus loss from a subsurface-drained farm

The substantial contribution of event flows to P loss calls for a system of stacked practices to reduce P loss from both surface runoff and drainage discharge. Generally, there are four strategies to address both water quality and crop production needs:

  • Reduce surface runoff leaving the field by improving soil health.
  • Reduce the soil test P and apply fertilizer as needed with 4R (right source, right rate, right time and right place) nutrient management.
  • Reduce the amount of water leaving the field with water management.
  • Capture phosphorus leaving the field with a phosphorus removal structure.
Figure 3.jpg
Figure 3. System of stacked practices to address both water quality and crop production needs. Photo by Ehsan Ghane, MSU.

Reduce surface runoff by improving soil health

Implement minimum or no soil disturbance, permanent cover crop and diverse crop rotation. There are resources that can help you plan and implement these practices, such as One Good Idea, Soil Health Nexus and Midwest Cover Crops Council. These practices help build soil organic matter, thereby improving soil structure. The better soil structure increases infiltration and leads to a better performing tile drainage system, thereby reducing surface runoff and particulate P loss. The buildup of organic matter helps protect against drought by keeping more moisture in the soil, according to Bagnall et al. 2022.

Reduce the soil test P and apply fertilizer as needed with 4R nutrient management

Implement the 4R nutrient management approach (right source, right rate, right time and right place). Reduce the excess soil test P by performing soil testing and only applying as needed to maintain a soil test P as low as possible, according to Osmond et al. 2019. Go with subsurface placement (subsurface banding, injection) of fertilizer instead of surface broadcasting to reduce P loss. If subsurface placement is unfeasible, incorporate the fertilizer after surface broadcast. Check the weather forecast to put as much time as possible between the time of fertilizer application and rainfall.

Reduce the amount of water leaving the field with water management

Reduce drainage discharge with water management

Implement water management practices to reduce the drainage discharge leaving the field, thereby reducing the soluble P loss. Examples of these practices are controlled drainage, saturated buffer and drainage water recycling. Even with those conservation drainage practices, some of the water reaches the ditch. That’s where two-stage ditches are useful because they have potential to reduce P loss from the stream flow. Two-stage ditches help lessen the erosive power of water in ditches, which, in turn, decreases erosion along the streambanks. This reduction in streambank erosion also leads to less phosphorus coming from eroding banks.

Controlled drainage and saturated buffers allow for water management. These two practices reduce drainage discharge by managing the outlet pipe of the drainage system. When water removal is needed for spring planting and fall harvest, these two practices allow for temporary free drainage to provide field trafficability. This is where two-stage ditches prove valuable, as they can decrease P loss from the stream's flow through the process of sediment settling.

Reduce surface runoff with water management

Implement conservation practices to minimize water movement on the soil surface (that is, reduce surface runoff), thereby reducing particulate P loss. Buffer strips, terraces and grassed waterways are water management practices targeting surface runoff because they intercept the water moving on the soil surface. Reducing surface runoff reduces the loss of particulate P (soil-attached P).

Capture phosphorus with a phosphorus removal structure

Even with nutrient and water management practices, some fields can have high P loss. In that case, a P removal structure can be used to capture the excess P in drainage discharge using a P sorption material, according to Penn et al. 2020. Because event flows dominate P loss (89% in our study at Michigan State University Extension), P structures should be designed to capture as much P as possible from event flows.

When a P removal structure is coupled with controlled drainage, the control structure dampens the peak flow and temporarily stores water in the field, thereby treating more water and capturing more P from the large event flows. Like in a controlled drainage system, dampening the peak flow can be achieved during most of the year except for when the system needs to be under free drainage for spring planting and fall harvest. Overall, stacking of practices can reduce more P loss than a single practice alone.

Take-home messages

By adopting a well-considered blend of strategies focusing on soil health, nutrient management, and water management, you can improve nutrient retention, boost crop production, and concurrently protect water quality.

Reach out to your local Conservation District, MAEAP technician, or NRCS office for expert guidance in crafting a tailored strategy and implementing the most effective farming practices for your specific needs.

For more information about water management and drainage, visit the MSU Extension Drainage website.

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