By Chryseis Modderman, U of MN Extension Educator
Phosphorus is an essential macronutrient for most crops, so it’s needed in the soil. But too much of a good thing can be a not-so-good thing. Soils with high phosphorus levels can lead to runoff, putting the environment (especially waterways) at risk. Many areas have regulations requiring a plan to reduce high soil phosphorus levels over time. From a general nutrient management standpoint, we recommend refraining from applying phosphorus to the soil until the high levels have drawn down to a more moderate level. With commercial fertilizer, this is easy: simply don’t apply a phosphorus fertilizer. If manure is your most common nutrient source, this is trickier. The nutrient ratio of manure is fixed, so we can adjust the application rate up and down, but the overall ratio of nitrogen to phosphorus (and all other nutrients) will remain the same. As if that weren’t challenge enough, manure also tends to overapply phosphorus for most crops when applied at a nitrogen-based rate. Below are some ways to manage high phosphorus soils while using manure.
Refrain from manure application on high phosphorus areas. The best way to lower phosphorus levels is to avoid applying that nutrient for a few years. This may not be possible for those with large amounts of manure and few spreadable acres.
Acquire more land. Increasing manure-spreadable land base (while maintaining the same amount of manure) will allow more flexibility with application so high phosphorus fields can be skipped. Though this might not be financially feasible for many, it’s worth considering as a long-term goal for farms that have many high phosphorus fields.
- Sell manure. For many livestock owners, this is a more realistic, short-term solution to increasing their land base compared to buying more land. Though some would argue it’s a bit illogical to sell manure, a good nutrient source packed with macro- and micronutrients along with soil health benefits, just to buy commercial fertilizer for their high-phosphorus fields. Counterintuitive? Yes, but also an effective way to avoid phosphorus application.
- Apply manure at a phosphorus-based rate. Look up the phosphorus removal rate of the intended crop, and then calculate manure application rate to meet those phosphorus needs. This allows some manure to be applied while ensuring phosphorus levels are not built to even higher levels. However, soil phosphorus will not be reduced, only maintained, so this method is best for those who have limited land and need to apply some manure to each field. Also note that manure application based on crop phosphorus needs often underapplies nitrogen, which may mean adding a commercial nitrogen fertilizer.
- Plant crops with high phosphorus uptake. This will accelerate phosphorus draw-down and allow a higher application rate when using a phosphorus-based rate.