Why Earlier Fall Manure Applications Are More Prone to Nitrogen Loss

When it comes to fall manure application, timing matters. Applying manure too early in the fall creates more risk of nitrogen (N) loss before your crop can use it. Two main factors drive this: temperature and time.

The nitrogen in manure is primarily in two forms: organic N and ammonium N. Both forms are relatively stable when first applied and tend to stay in place if incorporated into the soil. But once in the soil, microbes start working. Under aerobic conditions, microbes convert ammonium into nitrate—a form of nitrogen that is both mobile in the soil and vulnerable to loss through leaching or denitrification.

If manure is applied earlier in the fall, microbes have:

·         Warmer soils, which speed up the conversion of ammonium to nitrate, and

·         More time before crop uptake, which means more opportunity for nitrate to be lost.

Combine those two with fall and spring precipitation moving through the soil, and you’ve created a recipe for nitrogen loss.

Tracking Microbial Activity with μGDD

To make sense of this, researchers have developed a microbial growing degree day (μGDD) index, a simple way to compare how much microbial activity (and therefore nitrogen conversion) might occur depending on when you apply.

Microbial activity roughly doubles with every 18°F increase in soil temperature but drops to zero when soils freeze. Using that relationship, we can calculate μGDD to compare the relative risk of nitrogen conversion across application dates and locations.

The formula looks like this:

Where:

·         T_t is the daily average temperature of the soil

·         T_ref is a reference temperature, which I set at 32°F

·         Q₁₀ is 2, which is essentially saying activity doubles every 18°F temperature increase

This doesn’t tell us how many pounds of N are lost, that takes water movement, and it doesn’t even tell us how much of the nitrogen is converted into nitrate, but it does let us compare the relative risk of N loss at different application dates and across geographic locations.

What the Results Show

When we apply this μGDD approach, a clear pattern emerges:

Figure 1. Relative microbial activity as a function of manure application date. Delaying application until November significantly reduces the potential exposure to microbial degree days and the opportunity for conversion of nitrogen forms to nitrate

Earlier fall applications carry more microbial activity (and therefore risk). Well-timed applications in the fall (approximately November 1 are about double the risk of nitrification as compared to a spring application, but it is really the months of September and October where we start to see the relative risk of nitrification increasing rapidly. Risk does not equal loss, but it does show the opportunity for loss with weather patterns that don’t cooperate and cause wet soil conditions, water movement, and drainage.

The second point I want to make is there isn’t a one-size-fits all recommendation for Iowa. Northern Iowa behaves like “late” southern Iowa. In other words, applying in early October in northern Iowa may carry about the same microbial risk as applying 10–14 days later in southern Iowa. The map shown represents the relative range in microbial degree days a site in southern Iowa would have experienced relative to northern Iowa if they both applied at the same time.

Figure 2. Relative microbial activity as a function of location if applied on November 15^th. Other application dates looked similar. To obtain microbial activity risks similar to northern Iowa, southern Iowa must apply approximately two weeks later.

Putting It in Context

This finding reinforces what we’ve discussed in other articles:

·         Wait until soils are cool (below 50°F) before applying fall manure to slow microbial conversion.

·         Use nitrification inhibitors if you need to apply earlier; they can buy some time, though they won’t prevent all conversion.

·         Consider cover crops to capture nitrate if early application is unavoidable.

At the end of the day, earlier fall applications mean more time for microbes to work and more chance for N loss. The μGDD framework gives us a way to quantify that relative risk and better understand why timing matters. It isn’t the only factor, but it can help identify why the risk changes.