Tips for Fall Manure Applications

Authors: Chryseis Modderman, Extension Educator, Melissa Wilson, Extension Specialist, Annalisa Hultburg, Extension Educator

Fall manure applications are right around the corner, so here are some reminders on
best practices to make accurate applications and avoid nutrient loss.

Application tips 

• Sample your manure and get it tested. Manure is a variable product so knowing the actual nutrients in the manure is important for accurate application. Don’t trust the “book value” manure nutrient tables. Those are just estimates and averages, and your manure almost definitely differs in nutrient content. A basic manure test will tell you the N, P, and K content as well as the moisture content; add-on tests can tell you about micronutrients and secondary macronutrients.  You can learn more about accurate sampling by visiting our Manure sampling and nutrient analysis page.
• Test your soil. 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. 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. Instead, use the total nitrogen multiplied by the availability factor. This will ensure you are applying at a rate that will meet the nitrogen needs of the plant. Visit our Calculating manure application rates page to find the availability factor table and for more information on how to calculate rates.
• 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. Use the Guidelines for manure application rates and Commercial fruit and vegetable growing guides pages to help determine how much N credit you should be using.

Manure photo: Chryseis Modderman

 

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 (<50⁰F) 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, unfortunately, 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. Visit the Minnesota Runoff Risk Advisory Forecast and click on the 6” soil temperature tab to view soil temperatures in your area (it’s also a great tool for predicting runoff events).

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 MN Runoff Risk Advisory Forecast 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.

Food is safety considerations when using any manure-based compost

When you are using animal-based soil amendments like manure, it is important to remember that while it has important and beneficial microbes and soil-building properties, it can also contain harmful pathogens that can make people ill if it is not handled and applied properly. A good rule of thumb is to allow at least 120 days between application of the manure into the soil and the harvest of fresh produce. This gives many of the potentially harmful pathogens time to dry and die in the environment. A fall manure application will meet this time allowance. If you purchase your compost from a facility that treats the manure and it comes with a certificate of conformance and analysis, you would not need to use this application to harvest interval.