By Kirsten Workman (as published in our Spring 2021 Newsletter)
While manure often gets a bad rap, it gives farmers who raise livestock a valuable resource to meet the nutrient needs of crops and supplies organic matter to the soil in a way unmatched by commercial fertilizers. Utilizing manure effectively maximizes fertility, reduces runoff, and economizes spreading costs. Manure’s nutrient content varies with on multiple factors such as moisture, livestock, bedding, storage and feeding strategies. You should analyze manure annually to value these nutrients, including nitrogen (N).
Nitrogen in manure comes in two forms: organic and inorganic. The organic is a slow release N, broken down by soil organisms into plant-available forms, a process called mineralization. Most mineralization happens in the first year of manure application, and will continue slowly for one or more additional years. The inorganic portion (expressed as ammonium-N or NH4+ on your test results) is readily plant available, acting like a fertilizer application. Like fertilizer, it can be prone to losses. Liquid manure has much more readily available N, meaning it has the benefit of being available but also must be managed accordingly. Well-timed and incorporated manure optimizes N for your crop and minimizes losses, both to the air and water.
Timing. Spreading manure on living plants is the simplest way to retain available N. Spreading in the fall instead of spring can drop the amount of N available to a crop by as much as 55%, depending on dry matter content and incorporation. Manure applied in the fall has far more opportunity for loss before utilization, than manure applied in May. Volatization (loss to the air), leaching and runoff are all more likely to occur when applying on bare soil between annual crops, or even when hay/pasture is dormant, particularly with rain and snowmelt. This is not only an environmental issue, but also a farm profitability issue. Purchased nitrogen is a significant cost, so the more efficient use of manure the better.
Incorporation. Getting manure below the soil surface and into the root zone is key to retaining N. The longer the delay in time between application and incorporation, the greater the loss of ammonia volatization (NH3+). There are several ways to incorporate, but timing is critical. Incorporation methods include tillage, injection with specialized equipment, and even gentle rainfall (as long as it does not cause runoff).
Other considerations for retaining nitrogen from manure include high soil organic matter and cation exchange capacity, both the pH of soil and manure below 7, little or no wind, cool temperatures (but not frozen soils), moist but not saturated soils, and applying to a living crop including a cover crop. This chart illustrates the relationship between broadcast, incorporation, and nutrient loss:
Highest Loss to Lowest Loss in order –
Broadcast on surface without incorporation Broadcast; Incorporation 1 to 7 days after application Broadcast; Incorporation < 24 hours after application; Immediate incorporation or injection.
Manure Math. Using the book value for liquid dairy manure 8,000 gallons applied in the spring, immediate incorporation = 100 lbs. of available N 8,000 gallons applied in the fall with no incorporation = 52 lbs. of available N for your crop.
Book value – See “Nutrient Recommendations for Field Crops in Vermont” (http://go.uvm.edu/nutrecs; PDF, revised 2018, University of Vermont). Book values for manure are on page 24, Tables 15 and 16. Manure N availability changes with timing and incorporation are on page 25, Tables 17 and 18.
Need to test your manure? Visit the UVM Agricultural and Environmental Testing lab website (http://pss.uvm.edu/ag_testing) for forms and sampling instructions. You can also call us at 802-388- 4969 if you would like a manure jar sampling kit. Kits will be left in a box outside our office doors and can be resubmitted there. You must let us know ahead of time when you plan to drop it off because manure samples have to be frozen in a timely manner to retain N content. The best time to sample manure is at or near spreading.
1. Nutrient Recommendations for Field Crops in Vermont (BR 1390.2). University of Vermont Extension, 2020. https://www.uvm.edu/sites/default/files/Northwest-Crops-and-Soils-Program/2021%20Events/NMP%20Class/NutrientRec_BR1390.3_Sept2020.pdf
2. Manure application methods and nitrogen losses. University of Minnesota Extension (2018). https://extension.umn.edu/manure-management/manure-application-methods-and-nitrogen-losses
3. Conserving Ammonia in Manure (CDLE Pub. 09-50). UMass Extension (2009). https://ag.umass.edu/crops-dairy-livestock-equine/fact-sheets/conserving-ammonia-in-manure
Taking a Manure Sample: https://www.uvm.edu/sites/default/files/media/Manure3.pdf
UVM Manure Sampling Analysis at the Agriculture and Environmental Testing Lab: https://pss.uvm.edu/ag_testing/
More information about manure sampling in Vermont for 2021: https://cvfc-vt.com/2021/03/nutrient-management-minute-manure-sampling/