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Manure Spreader Calibration To Deliver Appropriate Application Rate

by Barb Ogg, PhD, UNL Extension Educator

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Dewatered biosolids from the City of Lincoln's Theresa Street Wastewater Facility are 20% solids--a consistency that can be handled by many conventional manure spreaders. We have found that spreaders giving the most uniform applications are those that have some sort of pusher or auger that forces the material to the rear of the spreader. Chain-driven spreaders do not always work very well because the chains may slide under the biosolids when the spreader is not full. This can result in an uneven application. Some cooperating farmers have used side slinging spreaders which seem to work quite well.

Application Rates


Before a manure spreader can be calibrated, the applicator must know the application rate. The application rate is based on the agronomic nitrogen (N) needs of the next crop. The following four factors must be considered:

1. Potentially Mineralizable Nitrogen (or PMN). PMN is the predicted amount of nitrogen that will mineralize from organic N and be available for plant growth during the first growing season. For a given sludge, PMN depends on soil type, temperature and soil moisture, the type of biosolids and method of processing. Based on experimental evidence with anaerobically digested sewage sludge (Pierzynski 1994), 15% of the organic N should be available for plant uptake the first year. 

2. Ammonia available for plant growth. Volatilization is when the nitrogen in ammonia (NH3 ) is changed through a chemical process to atmospheric N and is lost. When manure is applied to the soil surface, studies have shown that much of the ammonia volatilizes within the first day. We estimate that only about one-third to one-half the ammonia present in biosolids as it leaves the treatment plant will be available for plant growth because, after being applied, the material typically lays on the ground a couple days before it is incorporated into the soil. 

Factors #1 and 2 can be used to calculate the amount of potentially available nitrogen per cubic yard of biosolids. Based on the amount of organic nitrogen in Lincoln's biosolids, there is approximately 3 lbs of N will be available for crop growth per cubic yard of wet material.

Other factors involved in determining application rates are: 

3. Nitrogen needs of the next crop. Corn, soybeans and alfalfa are the biggest N users. Smaller amounts of N are needed for sorghum and wheat. 

4. Residual nitrate present in the soil. Deep soil samples in the root zone (48-inches deep) are analyzed to determine residual nitrate.

Manure Spreader Calibration


Manure spreaders are calibrated by adjusting the delivery of biosolids applied to a specific area of land to make sure it approximates the given application rate. To calibrate a manure spreader, the following information is needed: 

A: Capacity of the spreader (make sure the units are expressed in cubic yards at level capacity) 

B: The application rate (cubic yards per acre)

C: Width of the spreader swath (ft)

Using A, B, and C, above, calculate D in the following equation. D is the distance to empty one level spreader load for the spreader capacity, application rate and application width, A, B, and C, above. The equation also includes a conversion, 43,560, the number of square feet in an acre. 

D = A ÷ B × 43,560 ÷ C



Hypothetical example: 

6 yd3 (Spreader capacity)   (43,560 ft2 / acre)
D =  ____________________________ x _________________
  30 yd3 per acre (Application rate)   12 ft (Spreader swath)
       
D =  726 ft   tractor

After you have calculated your D value, measure and flag this distance in the field. Choose a gear and tractor speed that you think will deliver the correct rate and spread a leveled full load of biosolids between the two flags. If your guess was fairly close, you should deliver the entire amount in the spreader between the two flags. If you run out before you reach the second flag, you are spreading too heavy a rate and you'll need to increase the tractor speed or decrease the gear speed to deliver less. If material is left in the spreader, you should slow the tractor or increase your gear speed to increase the rate.

This method of calibrating spreaders is not very sophisticated, but it works. The application rate is documented after the biosolids is applied by dividing the amount of material delivered to the field by the field acreage (cubic yards/acre). 


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PHOTO Credit: Vicki Jedlicka

Lincoln's biosolids recycling program is a joint collaboration between the City of Lincoln, Public Works and Utilities Department and University of Nebraska-Lincoln Extension in Lancaster County with assistance from the University of Nebraska Agronomy Department, Lincoln/Lancaster County Health Department and the Natural Resources Conservation Service.


Contact Information

University of Nebraska-Lincoln in Lancaster County
Barb Ogg or David Smith
Web site: lancaster.unl.edu
444 Cherrycreek Road, Suite A, Lincoln, NE 68528 | 402-441-7180