by Daniel Andersen & Kris Kohl, Iowa State University Extension

Manure is a valuable source of nutrients; timing manure application correctly is crucial to maximize benefits and minimize environmental effects. A critical factor in this process is soil temperature, which directly influences how nitrogen (N) in manure behaves. This article explores why soil temperature is essential for fall manure application, why delaying application is recommended and how farmers can manage the challenges of waiting.

Why soil temperature matters

A primary nutrient farmers look to supply through manure is nitrogen, much of which is in the form of ammonium in liquid manures. Ammonium carries a positive charge, and Iowa soils do an excellent job of holding it. However, during warm temperatures, soil bacteria convert ammonium into nitrate, a process called nitrification. Once nitrogen is in nitrate form, it becomes more mobile in the soil, meaning it can leach into groundwater or be lost to runoff as water moves through the soil.

To prevent this loss, delaying manure application until soil temperatures drop below 50° F (10° C) is crucial. Consequently, microbial activity slows down significantly, reducing the nitrification rate. When the soil is too warm, applying manure can lead to substantial nitrogen loss before the crop can use it, reducing its value and potentially causing environmental harm.

In areas like the Midwest, where winter freezes halt microbial activity, targeting this 50° threshold is a standard recommendation. By waiting until the soil is sufficiently cool, farmers can retain more nitrogen in a form that will still be available for the crop in the spring.

The challenge of delaying fall manure application

While waiting for the right soil temperature can maximize nitrogen retention, it can be challenging to delay application, especially given the unpredictable nature of fall weather. As harvest concludes, many farmers are eager to clear their storage pits before winter. However, applying manure too early can result in nitrogen losses, creating a dilemma.

Some common challenges that arise when waiting for ideal conditions:

• Storage capacity. Many farmers need more manure storage capacity, making it difficult to hold off until soil temperatures fall. Farmers may feel pressure to apply manure sooner if pits or lagoons are full, even when conditions aren’t ideal.
• Weather risks. Fall can be an unpredictable time for weather. In some years, conditions can go from warm to frozen quickly, limiting the window for applying manure at the right temperature.
• Labor and time constraints. Fall is a busy time on the farm, with harvest wrapping up and winter preparations beginning. Labor availability is often limited, and with shorter days, the time available for manure application narrows. Balancing these demands can make waiting until soil temperatures drop below 50° a challenge.

Strategies to manage the challenges of waiting

Despite these challenges, there are strategies farmers can adopt to manage fall manure application timing better:

• Monitor soil temperature. Invest in a soil thermometer or use local agricultural extension resources to track soil temperatures. Regular monitoring ensures you can take advantage of the right conditions as soon as they arrive.
• Use nitrification inhibitors. For farmers who must apply manure before soil temperatures are ideal, nitrification inhibitors can help slow the conversion of ammonium to nitrate. These products extend the window before nitrogen becomes susceptible to leaching, offering some protection in warmer conditions. Typically, these products allow one to two weeks of reduced nitrification and need soil temperatures to trend towards 50° to be effective.
• Plant cover crops. Planting a cover crop can help capture and hold nutrients during winter, reducing the risk of nitrogen loss from fall-applied manure. Cover crops like cereal rye can absorb nitrate, keeping it in the field until the primary crop is planted in spring. If using cover crops, especially if manure was applied when the soil was still warm, planning for some starter nitrogen in the spring alleviates early-season nitrogen tie-up concerns.
• Consider split applications. In some cases, applying part of your manure in the fall may be advantageous, as well as reserving some for spring application or planning to use a starter nitrogen fertilizer. This approach can help manage storage needs while spreading the nutrient availability over time and help ensure sufficient nitrogen for crop production.

Conclusion

Fall manure application offers many benefits, but timing is critical to ensure you get the most out of your manure and protect the environment. By waiting until soil temperatures drop below 50°, farmers can maximize nitrogen retention and minimize losses. While waiting presents challenges — such as storage capacity, weather and labor constraints — there are strategies to help manage these issues. Using nitrification inhibitors, planting cover crops and considering split applications are all ways to balance the needs of the farm with the best practices for nutrient management. Taking the time to plan and ensure conditions are suitable for manure application will pay off in the long run, both in terms of crop yield and environmental stewardship.