As the quantity and quality of farm data increases over time, the understanding of the impact of agriculture on water quality has evolved. As research and monitoring technology have advanced, the scientific community is interested in more than runoff and soil loss, but rather in different forms of phosphorus (P).
In the past, total P was discussed, but now it is broken down into particulate P and dissolved P. This quantification and distinction complicates the story (Figure 1) but also helps distinguish the concepts of amount of surface runoff versus soil loss.
Twenty years of UW Discovery Farms research has shown that soil loss and surface runoff don’t necessarily peak in the same months. The majority of soil loss occurs in May and June in Wisconsin (Figure 2). It is then that P is lost in particulate form. Particulate phosphorus is bound to soil particles and is therefore lost with soil movement.
Reduced tillage and cover crops help hold the soil in place by promoting soil aggregation and providing residue that protects the soil from wind and water during this vulnerable time. Increased adoption of soil conservation practices began to address particulate P loss during the critical months of May, June, and July across Wisconsin.
Dissolved P is the portion of P in solution with water and does not need soil movement to be lost. The majority of these losses occur in January, February and March, during periods of frozen ground and/or snowmelt (Figure 3). Clear runoff does not always mean that no P is leaving the field.
When water interacts with organic and water-soluble forms of P, dissolved P can enter runoff. Generally, recent manure spreading can be considered as the source of dissolved P. But it can come from the interaction of water with any source of P, such as fertilizer applied to the surface in the fall, residue from the last crop or cover crop, or P in the soil.
Dissolved phosphorus is generally lost in systems where the phosphorus is not incorporated and concentrates on the surface, such as direct seeding or generally when nutrients are surface applied. As the ground is frozen in winter, during episodes of precipitation or snowmelt, water infiltration is limited or prevented, leading to the evacuation of dissolved phosphorus by runoff.
A practical solution consists in reducing these sources of P, in particular those concentrated on the surface where it can interact with runoff. Soil Test Phosphorus (STP) can remain high in fields where P additions exceed crop removals. This can occur when the nutrient content of the manure differs from the estimated average values and when the expected crop yields may be unrealistic. Harvesting cover crops as forage and decreasing the amount of phosphorus applied can reduce elevated STP by slowly correcting historical imbalances.
Spring nutrient application, closer to when crops are taking up nutrients, can eliminate the risk of loss during snowmelt. But regardless of timing, prioritizing placement below the surface is essential to combat phosphorus buildup in surface soil and to protect nutrients from long-term annual runoff events.
In a strip till system, only narrow strips are plowed and nutrients can be placed 4 to 8 inches below the surface. By placing nutrients below the surface, they can attach themselves to the soil, and then proven conservation strategies like residues and winter cover crops can protect the soil and its newly applied fertility.
The loss of dissolved phosphorus is a difficult problem to solve because weather conditions cannot be controlled, but the community can work to reduce the amount of phosphorus available to the surface. The Extension Ag and Water Quality program plans to work with Discovery Farms at new monitoring sites, as well as researchers in Wisconsin and other states to explore mechanisms to reduce dissolved phosphorus loss in no-till systems. .
After reducing soil erosion, reducing dissolved P losses is the next step to minimize the impact on water quality and keep P where plant roots need it. Just a few years ago, cover crops after cereal crops seemed impossible, but look how far aerial seeding and intercropping have come. This industry can certainly use that same innovation and creativity to tackle dissolved phosphorus. As the understanding of the industry evolves, management should also move closer to the ever-shifting goal of being the primary stewards of the land.
Chelsea Zegler is the Agriculture and Water Quality Educator for the UW-Madison of Extension Division