Manure Minute: Fall manure application tips
October 12, 2021 by Chryseis Modderman | University of Minnesota Extension and Melissa Wilson | University of Minnesota extension specialist
Fall manure applications are right around the corner (or, for some, are well underway), so here are some reminders on best practices to make accurate applications and avoid nutrient loss.
- Sample your manure and get it tested. Manure is a variable product so knowing the actual nutrients (not “book values”) in the manure is important for accurate application.
- Soil sample. While we’re on the subject of nutrient analysis sampling, you will also need to have a recent analysis of your soil. The soil test tells you what nutrients are needed, while the manure test tells you how much of the nutrients you have. Both are an essential piece of the accurate application puzzle.
- Nitrogen calculations can be tricky. Not all of the total nitrogen in manure is plant-available in the first year. Manure provides two forms of nitrogen: the inorganic N (immediately plant-available), and the organic N (not immediately plant-available). The organic fraction will need some time to break down (called mineralization) to become usable by the plant.
- When calculating a nitrogen-based application rate, use the total nitrogen multiplied by the availability factor. You can find the availability factor at your local extension manure application webpage.
- Don’t forget to credit all nitrogen sources. Was last year’s crop a legume? Was manure applied last year? Does your irrigation water contain nitrogen? Will you use a commercial starter fertilizer that contains N at planting? If you answered “yes” to any of those, subtract that N in your rate calculations for this year.
Tips for avoiding nutrient loss
- Avoid phosphorus buildup in your soils. Excess phosphorus in soil can lead to runoff and phosphorus pollution. To avoid P buildup, don’t blindly apply based on how much nitrogen you need. When applying at a N-based rate, most manure supplies too much P for the plant to use, causing it to build up in the soil over time.
- Instead, if your soil test shows high P levels already, consider applying at a P-based rate. That means looking at how much P your plants will need, and applying at a rate that provides that amount of P. Of course, this will likely underapply N, so you will need to supplement with another N source.
- Wait for cool (<50ºF) soils to apply manure. When applying manure in the fall, you’ve got a long wait until spring for a crop to use those valuable nutrients; and you want those nutrients to stay put in the soil and wait patiently. Nitrogen has a knack for escaping into the environment, and nitrate is the most mobile form of nitrogen. Manure doesn’t contain significant amounts of nitrate, but the ammonium in manure (the plant-available form) can convert to nitrate through a process called nitrification.
- Nitrate is easily lost through leaching and denitrification (lost as a gas), so we would like to keep manure nitrogen in the ammonium form, and not let it convert to nitrate. As long as the manure is incorporated into the soil, and not left on the surface, most of the ammonium will stay where you put it.
- How do we avoid this dreaded nitrification process? Apply to cool soils. Nitrification happens rapidly at high temperatures, but slows with cooler temps. Therefore, we recommend waiting until soils are 50⁰F or cooler to apply manure. Note that nitrification is not halted at cool temperature, just slowed. Even around freezing, the process continues very slowly.
- Don’t apply when runoff is likely. It should be common sense to not apply manure right before a big rainstorm or onto frozen or snow-covered soil. Check the forecast and keep an eye on your soil saturation.
- Incorporate manure into the soil. When there is not a crop to take up nitrogen, incorporating manure into the soil immediately after application is important for avoiding nitrogen loss to the atmosphere as a gas through a process called volatilization. If manure is left on the surface, nearly all of the immediately-plant-available nitrogen (ammonium) will be lost, though organic N will remain. •