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Manure Minute: The phosphorus cycle and manure


May 20, 2024  by Chryseid Modderman | University of Minnesota Extension

Manure is a first-rate, albeit smelly, treasure trove of nutrients, with all the macro- and micronutrients plants need to grow (though not necessarily in the right concentrations). 

While phosphorus often plays second fiddle to nitrogen, it is a vital nutrient for plant growth and worth managing.

Understanding how manure phosphorus behaves after application is important to know how best to manage manure. 

A crop farmer piped up from the back row of a meeting, “If phosphorus sticks to soil, why can’t I just bank it up in my soil and have it forever?”

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While it’s true that phosphorus is far less mobile in the soil than nitrogen, it can still be lost to the environment through runoff and erosion. In the case of high-phosphorus soils, leaching can also occur.

When excess phosphorus ends up in fresh water, it may cause harmful algal blooms and hypoxic zones where plants and animals can’t survive. 

Phosphorus management in manure poses a unique challenge as the ratio of nitrogen to phosphorus cannot be adjusted to precisely meet the needs of the crop.

Inevitably, phosphorus—and other nutrients—will be under- or over-applied when application rate is based on the crop’s nitrogen needs.

Most forms of manure are notorious for over-applying phosphorus, leading to phosphorus build-up in the soil and increasing the likelihood of phosphorus loss to the environment. 

Phosphorus forms in manure
Much like nitrogen, manure supplies two forms of phosphorus: inorganic and organic. The inorganic form is immediately plant available, while the organic form is not. Organic phosphorus is tied up and needs to be broken down by a process called mineralization to become plant available. Mineralization is driven by soil microbes, and the speed at which they work is heavily influenced by the environment.

Within the soil, there are three pools of phosphorus that contribute to plant-available phosphorus. Solution phosphorus is inorganic P and readily plant available; the inorganic P supplied by manure contributes to this pool. Active (also known as labile) phosphorus is stuck to soil particles and is released slowly into the solution P pool through desorption.

Fixed phosphorus is composed of mineral phosphorus and is released very slowly over the course of years.

While we tend to focus on the processes that release phosphorus for plant use, keep in mind that these pools are a two-way street and sometimes phosphorus may be tied-up rather than released.

Testing manure for phosphorus
While there are nutrient tables out there for nearly every type of manure, those are just estimates and we recommend sampling your manure for phosphorus as well as other nutrients. Actual nutrient content of manure can vary based on a whole host of management factors including feed, housing type, and storage system. Phosphorus should be reported as P2O5 to align with fertilizer recommendations.

If, for some reason, it is reported as elemental P, convert it to P2O5 by multiplying by 2.29. 

Also, when applying manure based on the phosphorus needs of the crop, remember that not all the phosphorus is immediately usable by the plant.

In most cases, a simple percentage is used when finding plant-availability for the purpose of application rate calculation.

In Minnesota, we use 80 percent. To calculate application rate, multiply the total phosphorus content – from the manure test – by 0.8 to find the plant-available content.

Availability factors may vary based on state or region, so be sure to use your local recommendations.•

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