Worksheet for Calculating Biosolids Application Rates in Agriculture

Dan Sullivan and Craig Cogger
Oregon State University and Washington State University-Puyallup

Other considerations for calculations

• Small acreage sites without a reliable yield history. Some communities apply biosolids to small acreages managed by "hobby farmers." In many of these cases, there is no reliable yield history for the site, and the goal of management is not to make the highest economic returns. You can be sure of maintaining agronomic use of biosolids nitrogen on these sites by applying at a rate substantially below that estimated for maximum yield.

• Equipment limitations at low application rates. At some low-rainfall dryland cropping locations east of the Cascades, the agronomic rate calculated with the worksheet will be lower than can be spread with manure spreaders (usually about 3 dry tons per acre). At these locations, you may be able to apply the dewatered biosolids at the equipment limit, but check with your permitting agency for local requirements.

• Unavailable soil nitrate (dryland cropping, east of Cascades). Not all of the nitrate-N determined by testing dryland soils (line 1.6) is available to the crop, because chemical extraction of nitrate is more efficient than plant root extraction. This difference becomes significant when soil nitrate concentrations are low (less than 10 mg nitrate N/kg soil) and sampling is done to greater than two feet.
  
  Recent research has shown that the amount of "unavailable nitrate-N" increases with soil clay content. Use the following formula if you estimate "unavailable nitrate-N" based on soil clay content.

Unavailable nitrate-N (mg/kg) = percent clay x 0.1

• Denitrification and immobilization. Denitrification (the loss of nitrate as gaseous N₂ or N₂O) and immobilization (the loss of nitrate or ammonium by incorporation into organic compounds) can occur following biosolids application. At agricultural sites these losses usually are not included in biosolids loading rate calculations because university fertilizer guides account for average losses due to these processes. Check with your local permitting agency before including denitrification or immobilization losses in the loading rate calculations.

• Site Specific Inputs. Biosolids application rates can also be calculated using a more detailed N budget method found in Washington State Department of Ecology Publication Managing Nitrogen from Biosolids. The N budget method allows for more site-specific inputs into the calculation. It will be most valuable when budget components are based on actual site monitoring data. If you do not have detailed site nitrogen data, use the worksheet presented above.

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Worksheet for Calculating Biosolids Application Rates in Agriculture, PNW 511-W
by Dan Sullivan and Craig Cogger
(Oregon State University and Washington State University-Puyallup)
March 1, 2000