How to avoid nutrient loss this fall during manure spreading

Chryseis Modderman and Melissa Wilson
University of Minnesota Extension
Manure spreading will soon be underway this fall.

Fall manure applications are right around the corner, so here are some reminders on best practices for manure applications and avoiding nutrient loss.

Application tips

Sample your manure and get it tested. Manure is a variable product, so knowing the nutrient levels in the manure is important. Don’t trust the “book value” manure nutrient tables. Those are just estimates and averages, and your manure almost certainly differs in nutrient content.

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 which nutrients are needed, while the manure test tells you how much of the nutrients you have. Both are an essential piece of the manure application puzzle.

Nitrogen calculations can be tricky. Did you know that 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. This will ensure you are applying a rate that will meet the nitrogen needs of the plant.

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, you need to subtract that N in your rate calculations for this year.

Avoiding nutrient loss tips

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 manure based on how much nitrogen you need. When applying at an N-based rate, most manure contains 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 (less than 50 degrees Fahrenheit) soils to apply manure. When applying manure in the fall, you have a long wait until spring when a crop will use those valuable nutrients, and you want those nutrients to stay in the soil and wait patiently. Nitrogen, unfortunately, has a knack for escaping into the environment, and nitrate is the most mobile form of N. Manure doesn’t contain significant amounts of nitrate, but the ammonium in manure (the plant-available form of N) can convert to nitrate through a process called nitrification. Nitrate is easily lost through leaching and denitrification (lost to the atmosphere as a gas), so we would like to keep the nitrogen in manure 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 temperatures. Therefore, we recommend waiting until soils are 50 degrees 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. Check the forecast and keep an eye on your soil saturation. The Minnesota Runoff Risk Advisory Forecast tool is designed to predict when runoff is likely in the next few days. You can even sign up for text or email alerts for your area.

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.