While 2020 has been anything but a conventional year, harvest season brings a sense of normalcy with it. Cool mornings and nights, combines on roads and in fields, and the smells of crop residue in freshly harvested fields are staples of this time of year. While we enjoy the fruits of another year of hard work, it is also important to think about next year. To be specific, I want to jog your memory on phosphorus fertilizer management strategies that can simultaneously benefit your farm and help promote healthy, clean water in our local watersheds and downstream.

I like to think about this topic in the framework of the 4R principles of nutrient stewardship. The 4Rs remind us of best management practices related to fertilizer applications. What is the right source, right rate, right time, and right place for my next round of fertilizer on a particular field? In order to utilize the 4Rs to their full potential in the context of phosphorus, it is important to first know what the Soil Test Phosphorus (STP) levels are in the fields to which you are considering applying phosphorus fertilizer. The Illinois Agronomy Handbook recommends soil sampling at least every four years in order to determine if and how much of a nutrient should be applied. On average in Illinois, when STP values are at 65 pounds per acre, there is no agronomic advantage to applying phosphorus fertilizer. Check the Illinois Agronomy Handbook for regionally specific values.

While MAP (11-52-0) and DAP (18-46-0) are popular sources of phosphorus because of their additional nitrogen nutrient content, Triple Superphosphate (0-45-0) is one source to consider if you desire the simplicity of applying phosphorus without additional nitrogen being factored into your nutrient plan, which might lead to over application of nitrogen. For manure sources, sending a sample to a lab to perform a nutrient analysis can help you determine how much phosphorus, nitrogen, and other nutrients will be spread with an application.

From a water quality standpoint, it is important to remember that no matter what source you use, applying greater rates of phosphorus is generally associated with greater losses of phosphorus. Applying phosphorus at crop removal rates helps prevent excessive loss. If you are only using nitrogen-based rates when applying manure, you might be over applying phosphorus. Again, performing regular soil testing can help determine your fields’ phosphorus needs.

When thinking about the timing of your next phosphorus application, avoid the one to two days prior to a forecasted large precipitation event. Applying directly before precipitation leaves the phosphorus fertilizer more vulnerable to loss. Additionally, never apply phosphorus fertilizer or manure to snow-covered or frozen ground, as this greatly increases its susceptibility to running off. Spring applications of phosphorus could lower risk of loss compared to fall or winter timings.

In regards to water quality, placing phosphorus under the soil via subsurface application is preferred over surface broadcast without incorporation. Many of the fertilizer sources we use today are highly soluble in water, so if phosphorus is placed below the soil surface it has higher potential to bind to soil rather than risking loss via water flowing across the surface in broadcast application situations. However, subsurface placement of phosphorus may not stop soluble phosphorus losses through tile lines if the application is at a rate above crop needs, or if it is applied before the crop needs it (i.e. fall applications).

If you want to read more about these considerations, check out a new fact sheet at go.illinois.edu/TilePLoss created by University of Illinois Extension in partnership with several other universities. For more information on the 4Rs, visit nutrientstewardship.org/4rs. You can contact me at woodyar2@illinois.edu.

Jennifer Woodyard Jones is a University of Illinois Extension Watershed Outreach Associate